Et Tunc Nulla Erat XISynapsida (And Once There Was)
In this article, we are going to go back a bit in time from the previous article and pick up on synapsids, pelycosaurs and therapsids that eventually lead to mammals. Synapsida:We last left off with dinosaurs leading into birds. For synapsids, we’re going to have to hit reverse and dig backwards into time in order to tell their full story. So, where we left dinosaurs and birds at the end of the Cretaceous 66 mya, we’re sending the time machine back 308 mya to the Pennsylvanian in the latter portion of the Carboniferous Period.Carboniferous Period:By the time of the Carboniferous, the supercontinent, Pangaea had fully formed bringing all the current continents together into one large global landmass. As centered on the equator, most of Pangaea’s climate was tropical around the coastal borders while the interior was very hot and dry. Basal synapsids entered and exited the world during this time becoming extinct by the middle portion’s ending of the Permian. Pangaea allowed synapsids to cover most of the coastal landscapes and geographically were the most dispersed tetrapods of the time frame.
The synapsid fossil story, which by the way, is presently still being told today via extant mammals, begins toward the end of the Pennsylvanian during the latter half sub period of the Carboniferous. This was a time after the shallow seas had receded due to polar glaciation ultimately exposing land that created lush tropical forests filled with marshes and swamps. These low in bacteria swamps along with trees containing bark are primarily the source of all the North American and European coal deposits in pressure cooking the fallen flora from peat into modern time coal beds and natural gas reserves.
Artist: Dinoraul Carboniferous
During the ~ 60 million reign of the Carboniferous, the first half known as the Mississippian, atmospheric oxygen levels went up to 20% (about what it is today), but then rose up to its highest level ever at 35%. Animals took advantage of this with the largest of amphibians growing in size to that of modern crocodiles. In dominating the animal world back then, specific amphibians eventually gave rise to reptiles, transforming amniote tetrapods into terrestrial egg laying animals. The first known diapsid reptile was Hylonomus lyelli (Hi-lawn-e-muss=lye-l-e). By now, amniotes were taking advantage of what dry land offered.
Artist: Karen Carr Hylonomus
However, arthropods were also a big benefactor of the high oxygen levels, in particularly insects in speciation and size. While a lot of insects were plant eaters of the time period, the ravenous insectivorous dragonfly, Meganeura (Meg-ah-noor-ah) reached lengths of 70cm/28in. Scorpions grew to 1m/3.3ft. The millipede, Arthropleura (Are-thro-plur-ah) grew up to 2m/6.6ft. Many insect families evolved during this period and grew to gigantic sizes due to the high oxygen percent. Most insects, including Meganeura evolved a tracheal system for breathing in the oxygen.
Meganeura, while evolving to its gigantic size, was only able to fly due to high oxygen levels of 30%+. At today’s 20.95% oxygen level, Meganeura and Arthropleura could not survive and when oxygen levels began to decrease during their time, it’s most likely the reason for their extinction by the end of the Carboniferous.
Credit: RTL Klub Meganeura
Artist: Tim Bertelink Arthropleura
Although the technical age term of the Carboniferous is the ‘Age of Amphibians’, some play with the term in stating it was the ‘Age of Roaches’ as there were so many species of roaches then with one blattid reaching 8.9cm/3.5in in length. The photo below is a comparison between the 300 mya cockroach and a modern day North American cockroach.
Credit: U of Ohio Blattodean
Just into the Carboniferous, lush tropical forests formed composed of large seed ferns, Cordaites (conifer-like trees), Lepidodendron (scale trees that were tree-like and vascular), giant club mosses and tall horsetails. These vast prevailing forests are what took out carbon dioxide from the atmosphere replacing it with oxygen. However, the atmospheric gas replacement along with continental drift, is what kick-started a drier cooling off period.During the Pennsylvanian, the supercontinent, Pangaea was forming smashing Africa into what is now eastern North America. Also, the super continent Laurentia formed by smashing South America into what is now the southern and eastern half of the U.S. Driven by plate tectonics, this occurred when South America approached the southern portion of America pulled by the subduction of Laurentia marine sediment. This resulted in volcanism and orogeny, the process of mountain building. There were no temperate plants at the time, so the highland barren rock exposed to weathering eroded away sending its wash down into valleys and plains below. Today’s N. American Appalachian Mountains and Ouachita Marathon Belt Mountain complex are merely the igneous cores of once tall mountain ranges reaching heights of the Alps of over 3353m/11,000ft. One needs to erase the thought that if it looks like a reptilian lizard then it is a lizard. One has to have the understanding that in early life, no matter a direction a tetrapodal animal was trending in evolutionary terms, the basic body outline for a tetrapod was just underway in following that of the earliest of tetrapods in the amphibian body plan. This consisted of a head, a movable neck, a thorax (body trunk), tail and four limbs ending in feet and toes to hold it all up. For reptiles, they first departed on an evolutionary strategy to exist on dry land independent of being tied to water in reproductive and adulthood development. Still though, in rising from watery dependency, thirst was no commodity, it was a necessity. Coming from an ancestral water dependent lineage, the reptile body is a mean average of 72.3% water. So, it is necessary on dry land for reptiles and any other tetrapod to find water sources to ingest and accommodate that water average that extends into low and high extremities of 63.6% to 81.3% of body mass. Evolution may have taken varying turns for differing animal groups, but that water hold on their existence in a basic body plan still entails a prerequisite requirement.
The major clade, Synapsida (Syn-ap-si-da) of course is reptilian-like, but synapsids have a single skull opening called the temporal fenestra behind each eye socket. This differs from diapsids which have two temporal fenestrae behind each socket. Anapsids have none, but are the basal amniote that both synapsids and diapsids evolved from. Today’s anapsids are represented by the order, Testudines, which are tortoises, turtles and terrapins, but it appears that their anapsid skull structure is a reversion from an ancestral diapsid rather than from continuous anapsid descent.
Synapsida is commensurate to Theropsida (The-rop-si-da), which isn’t to be confused with Therapsida. Therapsids are a subordinate group of synapsids that include proto-mammals synomynous to what was once referred to as mammal-like reptiles. Mammals are the only extant synapsid group. All other tetrapod amniote animals such as reptiles and birds are grouped as sauropsids.One common link to all theropsids (synapsids) and sauropsids (diapsids) is the lateral pubic tubercle. This tubercle, as part of the pubic bone assemblage, is a small rounded forward projection. It is located on the upper border of the medial portion of the superior ramus of the pubis. The lateral pubic tubercle of early theropsid amniotes is homologous with those of the genus, Sphenodon, lizards and also with the mammalian pubic tubercle except that the mammalian tubercle is functionally replaced by a process of the pre-pubic bone, in which case it still exists in a similar anatomical analogy. The taxonomic clade, Sauropsida (Sau-rop-si-da) includes all extinct and extant reptiles, all extinct parareptiles and all extinct/extant birds. In pitting theropsids (synapsids) versus sauropsids (diapsids), the most important difference beside the skull holes (fenestrae) is the vascular and pulmonary systems.
Excluding turtles, sauropsids have a low concentration of urea in their blood plasma resulting in suppression or even a loss of the urea cycle. In fact sauropsids synthesize uric acid. Due to kidney function, theropsids have a much higher concentration of urea in which the urea cycle is critical in the conversion and elimination of toxic ammonia buildup.
Below is a Brown University animation of the evolving vascular systems from stem tetrapods to mammals:
Both theropsids and sauropsids achieved higher surface areas with lungs than did their amphibian forerunners, although in different types of lung capacities. Theropsids evolved an alveolar lung where tiny air sacs (alveoli) take in oxygen by expanding once air is breathed in, then during exhalation, deflate expelling carbon dioxide. Sauropsids evolved a more unidirectional flow of air with a faveolar pair of lungs. Faveoli lungs possess millions of tiny passages known as para-bronchi that sit at both ends of the dorsobronchi (singular: dorsobronchium ~ smaller tubes branching off from the mesobronchi) causing air flow to travel in the same direction from posterior to anterior, thus creating a constant flow of fresh air and constant expelling of spent air.
Credit: brtc.tamu.edu Alveolar/Faveolar lungs
However, when it comes to locomotion sauropsids, such as extant lizards and crocodilians still utilize side-to-side body movement as amphibians and fish do. This ceases ventilation from lung-to-lung and thus relegates them to short bursts of speed. Synapsids, with a dog or primate for example, developed a muscular diaphragm assisting in the inhalation and exhalation of air in and out of the lungs during sprints. Theropsids exhibit heterodont dentition in having differentiated teeth including molars, incisors and canines. Earlier synapsids had up to three enlarged canine pairs, but once therapsids made their appearance, it came down to only one pair in the upper jaw. Sauropsid teeth were primarily the same shape in possessing homodont dentition. As well, where sauropsid teeth could be continually replaced due to loss or excessive wear in being polyphyodont (continual teeth replacement), theropsids became diphyodont in initially rooting a deciduous (baby) set of teeth to later be replaced by a permanent (adult) set of teeth.
Heterodont vs Homodont Dentition
One big advantage is that theropsids developed parental care long ago, where the extent of most sauropsids was to lay eggs in a suitable spot then leave for the developing eggs and hatchlings to fend for themselves. A 305 mya Nova Scotia, Canada fossil find of a varanopid synapsid shows strong evidence of parental care. The 305 mya varanopid has been named, Dendromaia unamakiensis (Den-droh-my-uh=ooh-nam-uh-ken-sis) composed of an adult and an offspring preserved within a lithified lycopod tree stump in such a way as to be expressing parental care. Most likely the tree stump was an acting den for the pair and thus far is the earliest fossilized situation of vertebrate postnatal care.
Artist: Henry Sharpe Dendromaia
During the quick burial from flood sediment, the mother’s tail was wrapped around the baby that was nuzzled up against the mother’s left femur and pubic bone. The genus name, Dendromaia is Latin for: ‘the mother in the tree’.
Even though synapsid is synonymous to theropsid, synapsid is Greek meaning, ‘fused arch’, while theropsid is also Greek, but means, ‘beast face’. Essentially, theropsids are divided into two sub clades, which are the more basal synapsids, the pelycosaurs and the more evolved therapsids. Therapsids eventually evolve and lead to mammals. I would like to note here that the term, pelycosaur is gravitating towards an informal grouping composed primarily of the most basal synapsids. With this, Eupelycosauria is trending to be defined as a clade that includes most pelycosaurs along with therapsids and mammals. For now though, I am sticking to the pelycosaur/therapsid/mammal grouping with proto-mammals as a subset.
Artist: DiBgd pelycosaurs vs therapsids Artist: Roman Ugueto
Credit: WWW protomammals/extinct mammals Artist: Christian Jegou
Below is an approximate 10 minute video of synapsids if you’d like to watch:
Early-Day Synapsid Forerunners:
First off, a precursor to theropsids along with sauropsids is in the family, Gephyrostegidae, (Gaff-ah-row-stag-ah-day) such as Gephyrostegus (Gaff-ah-row-stay-gus) and Bruktererpeton (Bruke-tear-er-pah-ton). No more than 22cm/8.7in and 19cm/7.5in respectively in snout-to-vent length, these two small tetrapod reptiliomorphs shared a more common ancestor with amniotes than they did with amphibians, in which gave rise to synapsids and diapsids. Even though they were considered anamniotes in relying on water to maintain their eggs like fish and amphibians, there is much evidence in their fossils, such as a well ossified ankle assemblage that they were fully terrestrial in their adult stages. These two anamniotes lived within the Westphalian stage time frame of the Pennsylvanian, 315-307 mya.
Artist: David Peters Bruktererpeton
During the ~ 60 million reign of the Carboniferous, the first half known as the Mississippian, atmospheric oxygen levels went up to 20% (about what it is today), but then rose up to its highest level ever at 35%. Animals took advantage of this with the largest of amphibians growing in size to that of modern crocodiles. In dominating the animal world back then, specific amphibians eventually gave rise to reptiles, transforming amniote tetrapods into terrestrial egg laying animals. The first known diapsid reptile was Hylonomus lyelli (Hi-lawn-e-muss=lye-l-e). By now, amniotes were taking advantage of what dry land offered.
|Credit: RTL Klub Meganeura|
|Artist: David Peters Bruktererpeton|
|Artist: David Peters Gephyrostegus|
A latter occurring anamniote during the Early Permian, 295-270 mya was the 1m/3ft long, Tseajaia (Say-ha-hi-yah). It was a clawless anthracosaurian in that it was an amphibian with a reptile-like skeletal anatomy that was more in line with amphibian-like skull morphology, but as an advanced amphibian showing reptilian traits.
|Artist: Nobu Tamaru Tseajaia|
However, Tseajaia was terrestrial as an adult and an ancestral cousin to Diadectes (Di-ah-deck-tees), a reptiliomorph amniote that possessed a reptile-like skeleton and heterodont teeth as analogous to pelycosaurs with the front teeth possessing compressed crowns suggesting an herbivorous diet. This would make it one of the first tetrapodal herbivores. The eight front teeth were spatulate and peg-like, serving as incisors used to nip off mouthfuls of vegetation. The broad and blunt cheek teeth serving as molars showed wear through occlusion. Due to its dentition it was most likely omnivorous.
|Artist: Valdislav Egorov Diadectes|
But at, 1.5-3m/5-10ft in length, Diadectes was also the first large terrestrial tetrapod with splayed legs supporting a massive limb girdle to carry the body’s heavy vertebrae and rib carriage with the rib spines projecting outwards to the sides. The five species of Diadectes fossil remains have been found in the 290-272 mya Permian Wichita beds of Texas, USA.
Diadectes was one of the first fully terrestrial large tetrapods that were herbivorous. Features that link it to pelycosaurs was its shorter tail, heterodont dentition along with the capability to breathe while eating accommodated by a second partial hard palate. As a forerunner in herbivory it possessed a large intestine to digest fibrous plants, which in turn gave rise to a larger skeletal anatomy. The name, Diadectes literally means ‘crosswise-biter’.
As a sister group to Tseajaia and Diadectes, Limnoscelis completes the clade groups referred to as, Diadectomorpha. Limnoscelis roamed in what is now N. America during the Late Pennsylvanian to the Early Permian 306.5-295 mya. It possessed both ancient amphibian traits and derived reptilian morphologies.
Rather large for land tetrapods back then, Limnoscelis was 2.1m/7ft in length. All its teeth were conical shaped and labyrinthodont in form in having the enamel and dentin infolded. This dentition shows that it was a carnivore and most likely a piscivore as well. Like the other diadectomorphs, present in Limnoscelis fossils were a glenoid foramen on the scapula and similar pectoral and pelvic girdles.
|Artist: Dmitry Bogdanov Limnoscelis|
The more primitive and earliest theropsids are grouped as Pelycosauria (Pel-e-co-sau-ree-ah). The latter and more advanced group of theropsids is grouped under the heading, Therapsida (Thuh-rap-sah-duh).
Most pelycosaurs walked with bent limbs stretching outwards from the body while the more derived therapsids were beginning to shore the body upright with limbs being positioned underneath for body support during mobility. Unlike sauropsids, pelycosaurs did not possess epidermal scales, but instead possessed skin coverings of osteoderms, ventral scutes, naked glandular skin or a combination of all. In following Diadectes lead in achieving greater size of over 20 kg/44.1 lbs, three pelycosaur lineages independently had surpassed 20 kg by the end of the Carboniferous. Milosaurus in the as yet unranked pelycosaur clade, Haptodontiformes reached a mass of 42 kg/90.4 lbs.
Pelycosauria is considered to be paraphyletic in the exclusion of therapsids since it is a grouping that therapsids directly descended from, but not all. Some pelycosaur clades led to no descendants becoming extinct instead, such as caseids. With the trend to utilize the grouping Eupelycosauria, it makes the synapsid clade monophyletic as it includes therapsids and mammals along with pelycosaurs. For those interested, below is a cladogram of early eupleycosaurs.
I personally still prefer the pelycosaur paraphyletic separation grouping model, because pelycosaurs in being paraphyletic were intermediates between reptiles and mammals. This is why they were originally dubbed mammal-like reptiles. Due to herbivory adaptations, pelycosaurs were the first large terrestrial amniotes and were also the first to develop a more upright stance for greater mobility. They were the first group to substantially progress from belly crawling to upright running.
The oldest synapsid fossil thus far discovered and may even be a stem pelycosaur is, Protoclepsydrops (Pro-toe-klep-see-dropz) that lived anywhere between 318-315 mya. Typical of early terrestrial synapsid amniotes, its primitive vertebrae had tiny neural processes. It may have been an ophiacodontid, but unfortunately, found in what is now, Nova Scotia, Canada’s ‘Joggins Formation’ of siltstones and sandstones, the scant fossil consisting of vertebrae and humeri does not give enough information to definitively classify the fossil as a pelycosaur. The name means ‘first Clepsydrops’.
Pelycosaurs had a temporal range from the Pennsylvanian to the Permian 308-260.4 mya, consisting of six families in: Ophiacodontidae (O-fye-ah-co-dawn-t-day) from 308-280 mya, Varanopidae (Vah-ran-op-ah-day) 305-260 mya, Edaphosauridae (E-daff-o-sawr-ah day) 302-280mya, Sphenacodontidae (Sfa-nok-oh-don-tie-day) 300-272 mya, Caseidae (Kazz-e-day) 300-263 mya and Eothyrididae (E-o-thy-rid-ah-day) from 295-290 mya.
With a temporal range of 310-279 mya, ophiacodontids lived within the bounds of the Carboniferous coal swamps suggesting that they may have been semi-aquatic if not fully aquatic, however vertebral morphometric evidence bears out that they were terrestrial laying eggs on land but dependent on the swamp ecology. As such, they most certainly would’ve frequented waters to capture slimy prey like salamanders and fish with their small pointed teeth. Once the Permian arrived drying out the swamplands, by the middle of the period all ophiacodontids became extinct.
Ophiacodontid fossils have been found in the USA from the states of Texas, New Mexico, Kansas, Arizona, Colorado, Utah, Ohio and Oklahoma, while the earlier smaller species came from Nova Scotia, Canada, France and England.
Early ophiacodontids’ body form resembled the varanopids, but there was a tendency in later ophiacodontids, in particular the larger species, to elongate their massive skulls. The larger ones possessed massive shoulder girdles most likely to support their large heads with muscle attachments. As well, their hind limbs were longer than the forelimbs. In being small, pointed and sharp, most of the teeth were similarly shaped, but were also showing variation in size. This, along with the deep skull anchoring strong jaw muscles to open the jaws wide and snap shut and legs shifting more under the body weight supportive of an upright position, are all indicative of characteristics leading to mammals.
Ophiacodontids were the first, therefore most primitive and oldest family of pelycosaurs and most likely are basal and ancestral to all other pelycosaurs, along with therapsids and mammals as well. All ophiacodontids were carnivores and at least some may have also included a piscivorous diet; even fewer may have supplemented their diet with soft vegetation.
|Artist: Smokey Bjb Echinerpeton|
One of the oldest known synapsids is Echinerpeton (E-kin-er-pah-tawn) from 308 mya during the Pennsylvanian at the end of the Carboniferous Period in what is now Nova Scotia, Canada. The ‘Morien Group’ is the geologic stratigraphy the fossils were found in composed of sub arkose (feldspar containing) and arenite (clastic rock) sandstones along with siltstones that were randomly conglomerate sediment. Echinerpeton was ~30cm/11.8in long from snout to vent and most likely was an insectivore.
Even though there are six fairly complete fossil specimens, due to its early age and unresolved polytomy relationships, Echinerpeton was very difficult in classifying in whether it was simply a basal pelycosaur or was representative of an early day synapsid family. It has now been determined that it was one of the first ophiacodontids.
The short neural dorsal spines on the back did allude to the fact that it may have been a sphenacodont. But in having an ilium that is narrow and backward-pointing similar to early reptiles, where sphenacodontids’ ilium were widened at their front to support the hip’s connection with the sacral vertebrae, placed Echinerpeton as an ophiacodontid. This and other anatomical distinctions entrenched it within the family, Ophiacodontidae. Echinerpeton was ~30cm/11.8in long from snout to vent and most likely was an insectivore, even though the mouth was filled with similar sized long teeth with the longer maxillary teeth in front of the orbit.
|Artist: Nobu Tamura Archaeothyris|
Looking much like today’s extant smaller lizards, Archaeothyris dentition with large canines, suggest it may have been carnivorous feeding on smaller animals, but assuredly was an insectivore as well. At 50 centimeters, during its time it would have been one of the larger carnivores snapping up smaller reptilians and arthropods that were much larger then than today due to the atmospheric makeup of higher oxygen levels. It lived in the warm Upper Carboniferous forests where giant scale trees, such as Lepidodendron grew up to 50m/164ft tall.
Anatomically, Archaeothyris had ribs much longer than its contemporaries making for a deeper torso. The fore and hind limb bone elements were relatively the same size supported in the forelimbs by a tall and slender spatula with medially long clavicles. Archaeothyris had strong muscular jaws that opened much wider than other current synapsid and sauropsid jaws couldn’t manage, allowing it to consume larger prey. The ventrally convex maxilla was concave below the orbit descending posteriorly. With this combination, a produced pull of the jaw muscles was possible, which probably meant Archaeothyris could cull and pull prey with its canine teeth.
|Artist: Nobu Tamura Varanosaurus|
With total body length on average 1.25m/4.1ft, Varanosaurus (Vah-ran-o-sawr-us) looked as its name implies, much like an extant monitor lizard in the genus, Varanus. It lived in what is now the southern USA.
With an elongated skull ending in a pointed snout and a mouth full of sharp teeth including two pseudo-canines, Varanosaurus’ size and dentition indicate that it was an active predator in seeking out smaller prey. It lived during the Permian 280 mya and as one of the last of the ophiacodontids, was also prey to Dimetrodon limbatus.
There are six species under the genera, Ophiacodon (Oaf-fye-ah-koh-don), which are: O. hilli with an age range of 295-279.2 mya; O. major from 290.1-279.2 mya; O. mirus from 295-279.5 mya; O. navajovicus from 298.9-295 mya; O. retroversus from 295-29.5 mya and O. uniformis from 290.1-279.5 mya.
|Artist: Ntvtiko O. grandis|
Ophiacodon species ranged in size from 1.5-3.6m/4.9-11.8ft long, with weights of 25.9-230kg /56-507lbs. The ventrally convex maxilla (upper jawbone) was also notched beneath the naris (nasal assemblage) giving space for the upwardly curving mandible (lower jawbone). The term ‘ophiacodon’ is Greek meaning, ‘snake tooth’.
|Artist: Nobu Tamura O. mirus|
I would like to end here with ophiacodontids in that the genera, Stereophallodon, (Steer-e-o-phal-lo-don), Stereorachis (Steer-e-o-rei-kis), Baldwinonus (Bald-win-o-nuss) and Clepsydrops (Klep-see-dropz) were once all originally considered nomen dubium synapsids, but after intense further phylogeny studies they’ve been placed into the family of Ophiacodontidae.
By the time the Permian rolled around 299 mya, there were very few ophiacodontids and by 279 mya during the Permian, there were none. By the end of the Permian 252 mya, all the basal pelycosaurs had vanished. Into the Early Permian, the climate was getting warmer with the extremely seasonal Pangaea supercontinent drying up the Carboniferous swamps and tropical rainforests that ophiacodontids depended on. Grasses had not yet evolved, so the main terrain wasn’t savannahs, but deserts. Other primitive pelycosaurs, like: varanopids, edaphosaurids, sphenacodontids and caseids to later evolve during the Permian, were more acclimated to the changing climate and topography, but these more basal pelycosaurs also died out towards the end of the Permian, giving way to the more advanced therapsid synapsids, which virtually ruled the land throughout the Permian.
|Artist: Julius Csotonyi The Permian Period|
A group, I have not mentioned were the parareptiles that were primitive anapsid reptiles suddenly appearing 306 mya with most dying off by the time of the Permian/Triassic extinction, although a few like the herbivorous procolophonians lived down to 201.3 mya. Sauropsids during the Permian such as, araeoscelids and tangasaurids were basically small and lizard-like. Amphibians suffer great losses in species diversity. Amphibians lost so much in speciation that today amphibians known as lissamphibians represent no more than 0.1% of all Earths amphibian life. Salamanders and frogs are from the same lineage that split off during the Jurassic while the most primitive extant amphibian are the caecilians, but since they evolved into a burrowing animal, they have lost a lot of the remnants (such as limbs and eyes) of their earlier predecessors.
The large Carboniferous insect species all died out as once the innumerable plants died off, oxygen levels dropped making the larger insects with inadequate respiratory systems unable to obtain enough oxygen to send into deeper tissues. However, many families of insects that evolved during the Permian’s seasonal periods are extant today such as: crickets, mayflies, and primitive groups of coleopterans, like the tiger beetles.
Plants that took over dominance during the Permian in replacing the rain forests were lycopods, horsetails, ferns and seed ferns. Gymnosperms (seed bearing plants) first appear with the strategy of insulating and protecting the plant embryo with the encapsulating seed sheath. These plants were found along waterways such as rivers, streams and lakes.
Laurasia to the north and Gondwana to the south begin to pull apart dissecting Pangaea and forming the Tethys Sea. At this time, ferns suffer more, while seed plants like conifer species, gingkoes and cycad species appear.
There are numerous genera species members of varanopids with two subfamilies in Mycterosaurinae (Mike-ter-o-sawr-uh-nay) consisting of five species and Varanopinae (Vah-ran-op-uh-nay) with seven species. Cabarzia (Caw-bar-zee-uh) and Tambacarnifex (Tam-bah-car-nee-fex) do not appear on the above cladogram, but Cabarzia is a mycterosaurdontine under Mycterosaurinae, while Tambacarnifex is a varanodontine under Varanopinae. There are currently sixteen known genera and eighteen species. There are five basal varanopids in: Apsisaurus (App-see-sawr-us); Archaeovenator (R-kay-o-ven-nay-tor); Pyozia (Pi-o-zeh-ah); Dendromaia (Den-droh-my-uh) and the recently discovered Ascendonanus ((Ah-chin-doe-nah-nuss)), in which is also not on the cladogram. I would like to note here, there is debate on whether to pull Ruthiromia (Ruth-i-roam-e-ah) from being a varanopid and designate it as an ophiacodontid. As earlier mentioned, Varanopidae is named so because the varanopid family members superficially look similar to monitor lizards of the family, Varanidae. Varanopids ranged from ~ 309 mya to 260 mya, while in size, ranged from 25cm/9.8in with Dendromaia and at 2.5m/8.2ft with Watongia (Wah-tawn-gee-ah). With a wide distribution, varanopid fossils have been found in N. America, S. Africa and northern Russia.
The long jaws of varanopids were filled with sharp teeth that varied in size, although they were primitive to mammalian heterodont dentition standards. The larger varanopids were carnivorous while those that were reduced in size were insectivores. All varanopids, with ossified feet and long tails, no matter the size were agile creatures as compared to the contemporary fauna. Even though the larger varanopids were carnivorous, at times they played the role of prey instead of predator as exampled below in the 1.3m/4.3ft long Varanodon (Vah-ran-o-don) illustration being chased by the primitive temnospondyl amphibian, Nooxobeia (Noox-o-be-ah). Even though the amphibian had elongated legs for terrestrial mobility, the one advantage Varanodon had over the larger Nooxobeia was its more agile swiftness.
|Artist: SmokeyBjb Varanodon chased by Nooxobeia|
There are basically six main varanopid autapomorphies which are:
1. elongated quadratojugals (cranial skull bone located rear lower corner)
2. anterodorsal sloping (anatomically in front and toward the back)
3. anterodorsal sloping of quadrate
4. parasphenoid (unpaired dermal bone at midline roof of mouth) dentition
5. elongate hyoids
6. plate-like interclavicle heads
As far as the varanopids go, at 305-309 mya Dendromaia is the oldest while the youngest, Heleosaurus (Uh-lee-o-sawr-us) is from 260 mya. Varanopid fossils have been found throughout the southern central and eastern half of the USA, Nova Scotia, Canada, South Africa, Germany and Russia. Note that in older literature at times you will find the spelling of Varanopidae as: Varanopsidae, or even Varanopseidae.
|Artist: Fredrick Spindler Ascendonanus|
The basal varanopid, Ascendonanus lived 290-291 mya during the Permian of what is now the state of Saxony, Germany in the fossil forest near Chemnitz. Five individuals were fossilized around a fossilized tree trunk. At 40cm/15.75in, it was rather small and with the slender pointed teeth fashioned with no flattened or serrated structuring, Ascendonanus was insectivorous preying on insects and other invertebrates.
The term, Ascendonanus is Latin meaning, ‘climbing dwarf’ and is for good reason. This 40cm/15.75in long varanopid had limbs adapted for an arboreal lifestyle and is the earliest known tetrapod that possessed tree climbing adaptations. The forelimbs are almost as long as the hind limbs ending in enlarged feet elements with a longer fourth digit on the manus (terminal segment of forelimb) and pes (terminal segment of hind limb). Unlike other varanopids, Ascendonanus’ feet ended in very strong recurved claws for clinging.
The tail was longer than in other synapsids and most likely served as a counter balance in climbing. The centra of the vertebrae each had a ridge on the underside for processes attachment. Along with this and enlarged blades on the ilium of the pelvis are distinct characteristics of varanopids.
Eyelid ossicles are evident while the very large orbits (eye sockets) supported sclerotic rings that are not ossified, but rather are more cartilaginous. However, the round body ossicles (dermal bones) embedded in the skin are ossified. The near complete fossil skeletons also had preserved integument with soft skin tissue along with full body scale impressions. The fossilized skulls have a single lateral temporal opening (fenestra) clearly indicating Ascendonanus as a synapsid.
|Ascendonanus exquisite fossil|
I’d like to end Ascendonanus discussing a little bit of taphonomy on how its exquisite fossilization occurred. The city of Chemnitz sits atop an ancient Permian petrified tropical forest that consisted of Calamites species (tree-like horsetails), conifers, tree ferns and Cordaitales (extinct woody plants that gave rise to gingkoes and cycads). This was home to Ascendonanus. How the forest became fossilized and its inhabitants became fossilized, including Ascendonanus was due to a volcanic explosion, but a particular one.
291 mya, the Zeisigwald Volcano erupted mildly sending volcanic pyroclastic ash onto the countryside. Since there is no evidence of trees being scorched the temperature of the ash was essentially cool being no more than 280° C/536° F. The blast and coolness was a result of the rising rhyolitic magma coming into contact with groundwater. Whether it was from the concussion of the initial blast, or from inhalation of the released obnoxious gases, the five Ascendonanus individuals were knocked off the treetop falling to the ground around the trunk base where they were instantly buried by the fine ash, cutting off decay while preserving the specimens of animals and plants.
The initial blast in knocking off the varanopids, also knocked off small tree limbs and leaves burying them up to 53cm/21in. Subsequent more violent blasts followed covering up the initial ash in tephra (from dust to boulder sized fragmented material). These successive blasts knocked the treetops off leaving only a meter (3.3 feet) to 3 meters (9.9 feet) of tree trunks standing. In addition, the silicic acid composing the tephra sealed the flora and fauna ensuring intact petrification and fossilization.
|Artist: John Sibbick Heleosaurus|
Heleosaurus was a small mycterosaurine varanopid that lived 260 mya in what is now, South Africa from the upper ‘Abrahamskraal Formation’ composed within the ‘Karro Supergroup’. All this rock strata is a component of the ‘Karoo Basin’. Two fossil sites were discovered; one holotype and one with another consisting of an aggregation of five individuals in situ within lithological strata consisting of a fine-grained, greenish-grey mudstone leveled to the bedding plane.
Between the two fossil sites, the triangular skulls are very much the same with a long narrow rectangular process of the naris along the antorbital region forming a straight dorsal border with the external naris. Also shared between the two is where the premaxilla meets up with the naris, there is a straight suture instead of the typical varanopid V structure, while the jugal (bony cheek arch) and quadratojugal (bone forming rear lower corner of the skull) have a pinched-like tubercular ornamentation on the lateral sides. Other varanopid autapomorphies were the anterodorsal sloping of the posterior margin of the quadrate and the presence of elongate hyoids that extend posteriorly beyond the skull.
The teeth tips are recurved and additionally, the edges of the marginal teeth bear serrations. These dental features are found in some basal archosauromorphs, but in strong contrast as seen in other varanopids, the teeth are shallowly implanted on the alveolar shelf. Heleosaurus also possessed a pair of canines much larger than the other dentition. Determination of the size was in comparing the two adults, with one as the holotype and the other adult in the aggregate, as 50cm/19.7in.
|Artist: David Peters Heleosaurus skeletal|
As far as the body goes, the pelvic girdles in both fossils show the typical synapsid elongated ischium as well as a blade like distal shape and a well-developed pubic foramen. The elongated ilium rises anteriorly above the acetabulum. The pubis does not fuse to the ilium, instead twisting 90 degrees to the iliac blade. The slender femurs, preserved in both fossils are elongated bones with a sigmoidal curve and a proximal end that turns up while the distal end curves down. With a femur shaft diameter approximating 10% of its total length, a trochanter (bony protuberances on femur end) is widely separated from the head. Overall the femur is almost identical to that of Mycterosaurus. The elongated ischium (curved bone forming a base for both halves of pelvis) is typical varanopid. The adult in the aggregate fossil had preserved cervical osteoderms.
It has been interpreted that the fossil with the five individuals having one adult and four subadults was a family. This bears out that parental care from Dendromaia of 305 million years ago and to Heleosaurus of 260 million years ago had become an evolved feature that was to become the pinnacle strategy for species survival throughout theropsid natural history where extant mammals still today carry on with the parent/child bond.
One thing intriguing about this fossil family is exactly how they died. Throughout my research, I never could find a definable reason. It was not due to a flood washing them up on a bank for the bodies would have been in disarray twisted and contorted pointing in every direction. This was not the case. The bodies were huddled together right-side up with all pointing forward in one direction. It’s as if they were sleeping together caught in this pose just prior to dying. It’s also not due to a volcanic explosion as is the case for the Chemnitz Ascendonanus fossils, as there is no evidence whatsoever of volcanic debris in the fossil area. There isn’t any evidence that they were in a burrow that caved in burying them in situ. The only scenario I can come up with is that a distant volcano (as there was volcanism at the time in other ‘Karoo Basin’ regions) was belching out obnoxious heavy gases that traveled and eventually overwhelmed them as they slept; even perhaps a sudden cold spell caught and stilled them where they laid.
|Credit: J. Botha-Brink/S.P. Modesto Heleosaurus family in death|
To let you know here, the aggregated fossil with the five individuals is now being claimed (Frederik Spindler et al 2018) to be a new varanopid species holotype and named it, Microvaranops (My-cro-va-ran-ops). However, with the close anatomical features of both fossils, with intent and purpose I’m still going to treat both fossil finds as one and the same in being, Heleosaurus.
Heleosaurus is a sister taxon to Elliotsmithia (L-lee-ot-smif-e-ah), which was also a small varanopid living in the ‘Karoo Basin’ around the same time. Therapsids were dominant during this time period and were the top predator. However, these two small mycterosaurines were not excluded ecologically by the much larger therapsids or restricted to equatorial regions during the Late Permian. As insectivores they consumed invertebrates and may have occasioned a carnivorous diet picking off small vertebrates. So, they filled in a niche by being small.
|Artist: Maastrichiang Guy Varanops|
With a head to body length of 1.2m/3.9ft and a tail even a bit longer, Varanops (Vah-ran-nops) was one of the larger varanopids. Only the varanopid, Watongia was larger at 2.5m/8.2ft, in which both are varanodontines belonging to the subfamily, Varanopinae. Below is a comparison in size of the pelycosaurs: Cotylorhynchus in the background, Ophiacodon in the middle and with Varanops, the smallest one in front.
Varanops fossil finds have been discovered in Texas and Oklahoma, USA. A large number of Varanops fossils have been found in the ‘Cacops Bone Bed’ of Baylor County, Texas. However, a particular find in Taylor County, Texas just southwest of Abliene showed a well preserved large adult Varanops species with teeth marks that stood out with defined detail. As an example of a fully grown adult in showing a high level of ossification at the shoulder girdle, the manus (terminal segment of a forelimb as in hand and wrist) and the pes (terminal segment of feet) displayed remarkable evidence of being scavenged long after death. Along with the teeth marks was one tooth left by the scavenger as lodged between the proximal ends of the left radius (lower outside arm bone) and ulna (lower inside arm bone).
|Varanops fossil showing teeth marks|
The tooth belonged to a terrestrial amphibian, which was most likely a dissorophoid temnospondyl due to the tooth’s morphology and the part and counterpart of the bite marks. This dental pattern is entirely consistent with the bite marks of a fully terrestrial temnospondyl with upper larger bite marks on the bone’s top surface and numerous smaller bite marks on the undersurface.
The presence of extensive surface cracking of the long bones parallel to fiber structure, and some mosaic cracking occurring on the surface of the bones gives credence to the fact that these bone surface textures were not produced by being buried immediately after death. Instead, they were exposed for some time permitting weathering of the bones before burial. This is how paleontologists can account for the fact of scavenging occurring on this individual fossil rather than merely a prey kill by a more dominant predator.
In Varanops, the cervical ribs were not too sturdy and didn’t enter the torso. The dorsal ribs were much larger enclosing the torso. Transverse processes appeared on each vertebra. The long ilium was robust with the tibia and femur shorter than the forelimb’s longer radius and ulna.
Varanops, with its long sturdy legs and solid torso was agile enough to chase down prey as well as a capable enough carnivore to go after quarry as large as itself. Once prey was downed, it most likely would use its large incisors to nip off meat as the smaller front teeth gripped while using its laterally compressed and recurved back teeth for slicing. Built for open ground, it most likely used a burst of speed to attack prey or flee a predator. The rising of therapsids most likely began outcompeting Varanops within its niche having made it become extinct before the end of the Permian.
|Artist: Henry Sharpe Tambacarnifex|
Closely related and a sister taxon to Varanops was Tambacarnifex that had a temporal range from 290.1 to 279.5 mya. The fossil find coming from central Germany’s Permian ‘Tambach Formation’, the name means, ‘Tambach butcher’. It is the only varanodontine found outside N. America. From the region and time frame Tambacarnifex came from, herbivores far outnumbered predators making the ~1.5m/4.9ft long, Tambacarnifex the apex predator of its Early Permian paleoecosystem.
Edaphosaurids are a sister group to sphenacodontids and ranged from 302 mya to 280 mya. Thus far, nine genera with fifteen species have been found with the genus, Edaphosaurus (Eh-duh-fuh-saw-rus) having four species while the genus, Ianthasaurus (E-un-thuh-sawr-us) has two. The other genera are: Bohemiclavulus (Bo-hem-e-clav-ull-us), Gordodon (Gor-doe-don), Glaucosaurus (Glaw-ko-sawr-us), Lupeosaurus (Loop-e-o-sawr-us) and Xyrospondylus (Zye-ros-pon-dye-luss). Remigiomontanus (Ree-mige-e-o-mon-tan-nus), a southwestern Germany fossil find has just recently been found and even more recently described as a new edaphosaurid genus. Autapomorphy (unique derived trait features) analyses makes it appear to be an intermediate between Ianthasaurus and Edaphosaurus. Finally, the remains found in the Czech Republic of an Early Permian edaphosaurid is tentatively named, Ramodendron (Ram-o-den-drawn), but is considered a dubious monotypic genus as only fragments of the spinous processes were found. Originally fossil AMNH 4060 species named, E. credneri has been indeterminately now claimed to be a juvenile Bohemiclavulus.
|Artist: F. Spindler Edaphodsaurid species clade|
Found in N. America and European strata, edaphosaurids, along with diadectids were one of the earliest herbivorous tetrapods and were the first of amniote herbivores. Edaphosaurids ranged in size from 0.5m/1.6ft to 3.5m/11.5ft in total length. In relationship to the body the head was small. The body was squat and a bit robust to say the least, due to a lot of large intestines required to digest the rough plants then available for consumption. As far as most edaphosaur species’ primitive conical peg-like teeth, there were also tooth plates known as dental pads on the palate and on the inside of the lower jaws. Due to the dental pads, the term edaphosaurid means, ‘pavement lizard’.
|Edaphosaurid sail crossbars|
The most characteristic feature however was the edaphosaurid sail enmeshed with thin skinned webbing. Supported by the extensively long vertebrae neural spines, it appears similarly to sphenacodontids. However, the sails of edaphosaurids had lateral extensions emanating from the spines known as crossbars or transverse tubercles; sphenacodontids did not possess crossbars in their sails. The crossbars became shorter as the ascended higher up the sail.
Of course, sails are not unique to synapsids as later, diapsids in the Triassic ctenosauriscid archosaur, Arizonasaurus (from 243 mya) and the Cretaceous theropod dinosaur, Spinosaurus (from 112.03 to 93.5 mya) would evolve sails. Sails appear to have been a thermoregulatory devise that will further be discussed under sphenacodonts.
Besides the sails, edaphosaurids also shared other morphological and anatomical features with sphenacodontids, such as: a lateral lappet (projection) of the frontal usually reaching the orbit; ventral border of skull is emarginated; pre-articulation is twisted posteriorly as to underlie the pterygoid process (bone where sphenoid wings unite with body) of the articular (cartilaginous joints); jaw articulation below the tooth row and rear of dentary (anterior bone of the lower jaw bearing teeth) displayed well-developed coronoid (hooked projection of bone) eminence.
|Artist: Alexey Kopnzelko rigid edaphosaurid sail|
Although the sails did not contribute much to mechanical stability or mass, they did however contribute to surface area and did not act as a pillow on a clothesline flapping in the wind, as it was a crossbar sturdily bound structure with the neural spines arranged parallel to each other while aligned in a straight line. As well, in a Lupeosaurus fossil specimen was the break of a spine during life that had healed; therefore the spine was rigidly held in life for the broken ends to have annealed.
|Artist: DiBgd Ianthasaurus spp.|
The two species of Ianthasaurus (Iantha river lizard) were small with I. hardestii (har-des-tye) possessing an 8cm/3.2in skull and was no more than 75cm/30in in total length. I. mirabilis (meer-ab-buh-liss) was even smaller. Again, I. credneri (kred-nair-ee) is now, as mentioned earlier, being entertained as a juvenile Bohemiclavulus. Due to their smallness and being lightly built, ianthasaurs were most likely more agile than the other larger edaphosaurids. The skulls were very similar to their sphenacodontid cousin, Haptodus (Hap-toe-duss). I. hardestii occurred 302 mya, while I. mirabilis was from ~ 304.1 mya.
The skulls of the ianthasaurs were prefrontal triangular in shape with long nasal bones much unlike the more massive supraorbital shelf (bony eyebrow) of the edaphosaurs. Also, the teeth were much more sharply pointed than edaphosaur dentition along with being slightly recurved. Ianthasaurs also did not possess dental plates. With this dental configuration, ianthasaurs were insectivores but most likely also greatly supplemented the diet vying as an herbivore as well.
Edaphosaurus species’ temporal range temporal range was from 303.4-272.5 mya. Holding a rather small and semi-triangular head, the cervical vertebrae were reduced in length, while the dorsal vertebrae were massive in anchoring the sail spines. The tail was long, the limbs were short and robust in supporting the stout body and the ribs form a wide ribcage.
The reason for the body girth was due to the diet in which required a capacious gut to digest plant roughage and cellulose. With a deep lower jaw likely having powerful muscles, along with marginal serrated tipped teeth, favored biting mechanics to crop and shear off low growing plants. To finish off the roughage before swallowing, the dense batteries of peg-like teeth were used for grinding and mashing the tough fibrous plants that grew during the time period. The jaw structure supported propalinal (front to back repetition) movement. They also had palatal teeth for further mastification of food material. Edaphosaurs most likely evolved from much smaller insectivores like ianthasaurs, a basal edaphosaurid.
Again, the sail was most likely for thermoregulation, but with size, coloration and specific species symmetry, it may have also served as species recognition and/or mating. If we truly knew the sail coloration, then we could further produce plausible theories in what its intentional purposes were for. Such as in the illustrations just above, if the drab brown coloration is accurate most likely the sail would have been for thermoregulation. But as in the other illustration, if it was multicolored, then it most likely could have solely been for species recognition and mating purposes, or as both in recognition and thermoregulation.
The sphenacodont clade becomes a bit more complex in that the basal stem-based groups physically vary from the later evolved groups. This base group constitutes a transitional evolutionary series from early pelycosaurs to ancestral therapsids in which is the lineage that led to mammals. Therefore, in making up the clade the class, Sphenacodontia includes all non-sphenacodontids, sphenacodontids and sphenacodontines that eventually lead to therapsids.
Sphenacodont group divisions are divided into the node-based superfamily clade of, Sphenacodontoidea while ending in the clade members of the family, Sphenacodontidae. This includes the subfamily clade of, Sphenacodontinae that consists of the more famed synapsid, Dimetrodon.
As well, there is the mirorder (disordered) clade of, Sphenacomorpha containing all sphenacodont species including those that are incertae sedis in that their broader relationships are currently unknown and in need of further definition and evaluation, or they may simply be an early sphenacodont speciation dead-end. Mirorder is a fairly new taxonomic ranking that is below grandorder (large) but above order. If you view a Eupelycosauria cladogram, it takes the place of Sphenacomorpha along with the previous synapsid groups we have discussed.
|P/T Synapsida Cladogram|
The temporal range for sphenacodonts was from 305.9 mya to the present mammalian populations. They ranged from N. America and Europe. The major features shared by sphenacodonts is the thickening of the maxilla (upper jaw) bone to accommodate the large caniniform (front) teeth, while the premaxillary upper front teeth are set in deep sockets. This is a new feature as all other sister groups and more primitive synapsids had teeth that were set in shallow sockets.
|Synapsid extant jackal teeth|
Deriving from the most primitive sphenacodonts, sphenacodontoids are defined to include the most recent common ancestor of the family, Sphenacodontidae and the therapsids along with therapsid descendants which includes mammals. The sphenacodontoids special characteristics were further specializations of skull proportions and dentition. Sphenacodontoids most likely evolved from the basal sphenacodont genus, Haptodus (Hap-toe-duss) through transitional stages. Haptodus along with its former species members shared many anatomical skull and skeletal features with sphenacodontids.
|Artist: DiBgd H.garnettensis|
With a temporal range of 305-303.9 mya, H. garnettensis (gar-nuh-ten-sis) is the only Haptodus species left found in what is now Kansas, USA. It was small being around 70cm/27.6in. The term Haptodus means, ‘gentle/soft teeth’ because the teeth were small, but no less the irregularly sized teeth were sharp. With this dentition and smaller size, it likely was insectivorous.
Haptodus had anatomical structures of both the more primitive synapsids in ophiacodontids and varanopids while also carrying features of the more derived sphenacodonts and dimetrodons. One primitive trait was a more distal position of the foramen accommodating the perforating artery and the morphology of the first distal tarsal that was not found in later more derived sphenacodonts. The more evolved traits not found in the more primitive theropsids were the narrowing of the tarsus and the medial centrale (a hand bone) that had shifted to the medial side of the first distal tarsal. Nonetheless, Haptodus along with its former species members shared many anatomical skull and skeletal features with sphenacodontids.
|Artist: DiBgd Ianthodon|
Currently, the most basal sphenacodont leading to sphenacodontoids is Ianthodon (I-an-tho-don) that had a temporal range of 305.9-303.4 mya living in what is now Kansas, USA. One of its two fossil sites was found in the same strata as Haptodus and Pantelosaurus. At ~ 10cm/3.9in, the Ianthodon skull was slightly more slender than the Haptodus skull, while possessing fewer teeth with 20 in the maxilla as opposed to 23 in Haptodus and 21 dentary teeth rather than the 24 in Haptodus.
|Ianthodon (A) lft-(B) rt maxilla|
Through plicidentine studies and analysis (form of dentine showing sinuous lines of structure in a transverse section of the tooth) to evaluate teeth root depths, Ianthodon shows that, along with other basal sphenacodonts that a large basal clade, which includes the more derived edaphosaurids and shpenacosaurids had already begun evolving and diversifying during the latter part of the Pennsylvanian. The dentition rooting of Ianthodon had already begun showing ratios of root versus total tooth length smaller than 41%. This ratio is indicative of the more advanced sphenacodontids, except Dimetrodon grandis in which possessed longer tooth roots of 50-57% in relation to total tooth length.
|Artist: ДиБгд Cutleria|
Cutleria wilmarthi, (formerly Haptodus wilmarthi) is pronounced: Cut-ler-e-ah=wil-mar-thy and is a basal derived sphenacodontoid while a sister taxon to Sphenacodontoidea. But it as well, is one of the basal most sphenacodontid. In continuing, it also has many similarities with the other more primitive sphenacodonts, such as Haptodus. Its holotype fossil was discovered near Placerville, Colorado, USA in the ~286 mya portion of the ‘Cutler Formation.’ Cutleria in total length was ~ 1.1m/3.6ft.
Sphenacodontidae, is a paraphyletic family of smaller more primitive sphenacodontids along with the larger more derived sphenacodontines from the subfamily, Sphenacodontinae. The term means, ‘wedged point tooth family’ due to the fact that the massive jaws contained long canines, dagger-like incisors and cutting cheek teeth all wedged in together. The temporal range was from 300-272 mya with the earlier more primitive forms having lengths between .6-1m/2-3.3ft while the later more derived forms grew progressively larger reaching lengths of 3m/9.9ft. All sphenacodontids were carnivorous. Fossil finds have been found in N. America’s and Europe’s Permian equatorial Pangaea. The sister group to sphenacodontids is Therapsida.
The basal genera sphenacodontids are: Ctenorhachis (Sten-o-rak-is), Macromerion (Mac-row-mare-e-un), Secodontosaurus (Sec-o-don-toe-sawr-us), Steppesaurus (Steps-sawr-us) and Tappenosaurus (Tap-pen-o-sawr-us). Tappensaurus was the largest of all sphenacodontids reaching a length of 5.5m/18ft, therefore was the largest apex predator during the Permian.
|Artist: Smokeybjb Ctenorhachis|
Although Ctenorhachis, did not have the famous sailfin that Dimetrodon spp. had, however, it did possess articulate vertebrae in having enlarged blade-like neural spines that formed a crest. In fact, the name Ctenorhachis is Greek meaning, ‘comb spine’. As well, the pelvis is almost identical to Dimetrodon. Ctenorhachis had deep narrow jaws holding dagger-like carnivorous teeth. Its fossil remains come from Texas and was laid down during the Artinskian Stage of the Permian 290.1-283.5 mya.
|Artist: Masato Hattori Secodontosaurus|
Up to 2.7m/8.9ft in total length, Secodontosaurus (Sah-ko-don-toe-sawr-us) is the only basal sphenacodontid that had a sailfin, although it is not known whether Macromerion had one or not as its Czech Republic fossil was incomplete in evidence. Occurring between 285-272 mya, Secodontosaurus fossil remains were found in the ‘Arroyo Formation’ and ‘Belle Plains Formation’ of Texas. The name means ‘cutting tooth lizard’. Even though it is a basal sphenacodontid, Secodontosaurus shares a more recent common ancestor with the sphenacodontine, Dimetrodon than do two other sphenacodontines in, Ctenospodylus and Sphenacodon.
|Secodontosaurus fossilized skull|
Secodontosaurus’ skull was different than most sphenacodontids in that the jaws were elongated and tapered with the mouth possessing prominent canine-like teeth in the front (known as caniniforms) with smaller slicing teeth in the back of the jaws. The reason for this is twofold: either it filled a terrestrial niche in capturing smaller prey that hid in rock crevices or in burrows, or it filled an aquatic niche in frequenting shallow waters to trap and snag fish with its long snout and sharp teeth.
The sphenacodontines in the subfamily, Sphenacodontinae were the most derived sphenacodontids, having straightened legs more upright beneath the body. With five genera in this subfamily, they were: Cryptovenator (Krip-toe-vah-nay-tor) from 300 mya, Ctenospondylus (Sten-o-spawn-dill-us) from 290.1-279.5 mya, Dimetrodon (Dye-me-tro-don) from 280-260 mya; Neosaurus (Nee-o-sawr-us) from ~ 290 mya and Sphenacodon (Sfee-nac-o-don) from 300-280 mya.
|Artist: J. Schindler Cryptovenator|
Along with Cryptovenator, Ctenospondylus and possibly Neosaurus, Sphencodon had more of a raised blade-like neural spine crest other than the large neural spine sailfins like the Dimetrodon spp. sported. These four ranged in size from 2-3m/6.6-9.9ft in length. Except for the neosaur fossil, with its teardrop shaped teeth, in which came from the ‘La Serre Horst’ of the Jura region in France, the three other crested sphenacodontines came from the USA.
These four sphenacodontines lived in the remaining Carboniferous wooded swamplands of the Early Permian Euramerica Pangaea supercontinent until the swamps dried up during the latter half of the Permian, in which they then became extinct. They all had long tails and short legs with the hind limbs shortest making the body a bit squatty.
|Artist: James Kuether Sphenacodon|
Sphenacodon, had a temporal range of 296.4-280 mya with its fossil remains coming from the USA. There are currently two species in: S. ferocior (fuh-row-c-or) and S. ferox (feh-rox), which both ranged in what is now, New Mexico, while in addition, S. ferocior fossil remains having also been found near the Utah/Arizona border. Both species lived during the same time from 296.4 mya, but where S. ferox died out by 295 mya, S. ferocior did not go extinct until 268 mya.
|Artist: dmitrchel Lft-S. ferox Rt-S. ferocir|
Like Dimetrodon, Sphenacodon had powerful jaw muscles and strong epaxial (positioned on dorsal side of an axis) muscles along the base of the raised neural spines that aided in stiffening and strengthening the backbone for walking and for lunging at prey by restricting side-to-side flexing motion. These epaxial muscles, being dorsally arranged were supportively attached to the enlarged neural spines giving the ability to lash out with powerful brute force strikes toward prey or foe.
|S. ferox fossil|
With typical sphenacodontine dentition of sharp pointed incisors, large stabbing caninforms and smaller slicing postcaninforms (back teeth), Sphenacodon could easily capture prey with the front teeth, then shear the flesh with the rear teeth in cutting the clumps into smaller swallowing lump sizes.
|Artist: Mark Stevenson Dimetrodon|
The famed sphenacodontine, Dimetrodon genus has always been confused in being a dinosaur. I recall as but a child, purchasing a bag of dinosaurs having Dimetrodon always in it. In all actuality T. rex is closer in relations to birds than to Dimetrodon, while Dimetrodon as an evolutionary cousin is nestled snugly in the tree along with humans. In the scheme of nature, dimetrodons occurred 50 million years earlier than the first dinosaur appearance, whereas its inherited legacy continued on down the synapsid line as a distant relative to Homo sapiens. Dimetrodon infers: ‘two measures tooth’.
One of the transitional features of Dimetrodon is the reflected lamina, which is a ridge in the back of the jaw. It’s found on the articular bone connected to the quadrate bone of the skull forming the jaw joint. In latter mammals, the articular and quadrate separated from the jaw joint while the articular developed into the malleus bone of the middle ear. For all current living mammals, including humans, this same reflected lamina became part of a ring called the tympanic annulus that supports the ear drum.
Another trait concerning endotherms is their capability of generating internal heat. Although it was small when compared to later more derived synapsids, dimetrodons possessed nasoturbinals which are ridges on the inner surface of the nasal cavity of the skull. Nasoturbinals in dimetrodons most likely supported cartilage increasing the area of the olfactory epithelium; the layer that detected odors. For later synapsids, the nasoturbinals increased in size that supported mucous membranes in moistening and warming inhaled air. This technique is evident in extant endothermic animals; thus, but yet another transitional feature.
|Dimetrodon skull fossil|
The tail possessing 50 vertebrae was as long as the body, while the skull of Dimetrodon species was tall, compressed laterally and slightly arched. Consisting of canines and pairs of caniniforms, the jaws were filled with very large teeth of varying sizes that protruded out of the closed mouth. The teeth were rooted in dentary bone, a precursor to mammalian dentition. The teeth were widest at the midsection giving the appearance of a teardrop just like in Neosaurus. Dimetrodon species were strictly carnivorous consuming tetrapods up to their size with the smaller species possibly dieting as a piscivore in dining on fish.
|Artist: DiBgd Dimetrodon spp.|
Most dimetrodons ranged in size between 1.7-3.8 m/6-12.5ft with the smallest in D. teutonis at 60cm/24in to the largest in D. angelensis at 4m/13.1ft. Species of Dimetrodon radiated into the arid environs of the Permian able to withstand the drier and hotter conditions and with a temporal range of 295-272 mya, became the apex predator over all of the Permian Period.
The neural spine sail in Dimetrodon species has been determined (through engineered computer generated analyses) that it was efficient enough in surface area versus body mass to release excess heat during daily hours, but also allowing Dimetrodon to retain a higher body temperature at night. The sail also regulated body temperature during different seasons, concluding that the sail was beneficial for capturing and releasing heat at all times in the year. This ability made Dimetrodon poikilothermic, albeit most likely at a lower internal body temperature than most homeotherms. Some current ectotherms remain in temperature-constant environments to the point that they are actually able to maintain a constant internal temperature as if being homeothermic. It is this distinction that often makes the term poikilotherm more useful than strictly stating an animal as being ectothermic.
As pointed out earlier, other animal species supported neural spine sailfins, even the 1m/3.3ft temnospondyl amphibian, Platyhystrix rugosus sported one. Its fossils have been found in the same Permian strata as sail-finned pelycosaur species, so it just might have convergently evolved a sailfin for the same environmental reasons. Although it was terrestrial as an adult it could’ve been semiaquatic lying in shallow water with the body and lower head submerged waiting for aquatic prey as the exposed sailfin to sun rays would’ve soaked up heat.
With the exception of D. borealis found in Canada’s Prince Edward Island and D. teutonis from Germany, all other dimetrodons were discovered in the USA. Also, of those from the United States, except for D. occidentalis from Arizona, New Mexico and Utah, all the other dimetrodon fossils were found in the ferric oxide rich sandstones in the, ‘Red Beds’ of Texas and Oklahoma. In the Dimetrodon genus, there are currently 14 species and they are in alphabetical order: D. angelensis, D. borealis, D. booneorum, D. dollovianus, D. gigahomogenes, D. grandis, D. kempae, D. limbatus, D. loomisi, D. macrospondylus, D. milleri, D. natalis, D. occidentalis and D. teutonis.
|Artist: Max Bellomio D. grandis|
D. grandis and other larger more derived dimetrodons gave out one nasty bite. Older and smaller dimetrodons like, D. milleri had teeth with straight cutting edges that pierced and cut, but wasn’t particularly good at slicing. Later, as the more derived, D. limbatus came along, the teeth had evolved small serrations in the enamel giving saw-like dentition. By the time D. grandis arrived on the scene, the teeth had further evolved prominent denticles along the slicing surface creating a serrated edge that predatory dinosaurs would mimic 40 million years later. This bite force would tear into flesh and shred meat. Evolving tooth structure occurred without any significant evolution in skull morphology, indicating changes in feeding style and trophic interactions.
|D. grandis tooth|
The actual sailfin symmetry of dimetrodons has been debated for a while. Some illustrations have the sail really curved while others express it in a more angular fashion. Also, some paleo-artists have the tips of the spines tuck inside the sail while some have the tips protruding outside the sail. We do know the neural spines were mostly covered by skin membrane as fossil finds show some broken spines that had healed. For this to have occurred, the broken spine had to be held in place. Also the tips of the spines were frequently bent or at times completely broken off suggesting the spine tip protruded out of the membrane sail. So, with this information and fossil evidence of D. grandis paleo-artist, Scott Hartman came up with the skeletal anatomy found below.
|Artist: Scott Hartman D. grandis skeletal|
|Edaphosaurid-Dimetrodon skull comparison|
If you might recall previous comments under Pelycosauria, except for caseasaurs, all synapsids are lumped together under the order: Eupelycosauria. However, I prefer utilizing nomenclature under the older form of primitive synapsids being classified under Pelycosauria, while the more derived synapsids are classified under Therapsida, which eventually leads to species under Mammalia. I feel that this gives more credence to caseasaurs as they have earned the rights. It is true that caseasaurs left no lineage and all became extinct leaving no heir apparent descendants. Therefore, they are a dead-end and did not contribute to the eventual mammalian group. But, caseids were one of the first tetrapodal terrestrial herbivores filling important niches for themselves and the floral biomes as well as for carnivores as a food resource. So, maybe Caseasauria (Kazz-e-ah-sawr-e-ah) and Eupelycosauria (U-pale-e-ko-sawr-e-ah) are the two main trending clades of early synapsids, nonetheless I’m listing them with the rest of the pelycosaurs under Pelycosauria as a sister group to all other pelycosaurs.
|Artist: Nobu Tamura Caseasauria spp.|
There are numerous features the caseasaurs share with other pelycosaurs, which the more derived therapsids didn’t retain. Some of these anatomical features are: a relatively small temporal fenestra, the absence of canine teeth in the upper and lower jaws in most caseasaurs, while the mandible exhibits no reflected lamina of the angular bone. For the most part, with barrel-shaped ribs the body was squat and short held up by relatively short but heavily built limbs which were attached to massive bone girdles. This suggests that the stride was sprawling and lumbering. The whole large body arrangement supported in comparison a tiny head.
Caseasaur fossils have only been found in the Late Carboniferous to Mid-Late Permian Periods in having a temporal range of 300-254 mya. Although they were common during the Early Permian, it appears that the caseasaur decline and eventual extinction was due to being outcompeted by the more derived herbivorous therapsids and the carnivorous theriodont therapsids that may have hunted in packs. The bulk of caseasaurs were terrestrial herbivores, but some may have been omnivores, while a few may have been semiaquatic herbivores. The earliest ones, the eothyrids, were insectivores, or maybe even omnivores. Herbivorous caseasaurs were able to extract plant nutrients from high fibrous plants of the drying climes due to the large ribcages making room for an extensive digestive system to breakdown cellulose. All caseasaurs shared a morphological specialized feature of the snout and its external nares (nostrils).
Caseasauria is composed of the basal genus, Eocasea (E-oh-cass-e-ah); the primitive family, Eothyrididae (E-o-thy-ride-uh-day) with three genera; and the family, Caseidae (Case-e-day) with 16 genera and 22 species. Caseasaur fossils come from the USA (primarily from Texas), Southern France, Germany, the ‘Cala del Vino Formation’ of NW Sardinia, European Russia and with one from Poland.
|Artist: Danielle Dufault Eocasea|
The evolving and herbivorous adaptation of caseasaurs actually originates from a carnivorous synapsid 300 mya in, Eocasea (E-oh-cass-e-ah). This 20cm/7.9in very primitive caseasaur with simple tubular cone-shaped teeth dieted on insects and small vertebrates. After bifurcating ~315 mya, in whether mentioning theropsids or sauropsids, Eocasea represents, thus far, the earliest known transition from an amniote carnivory diet to an herbivory one. Once the ability to feed on plants occurred, the threshold of Eocasea going from a carnivorous to herbivorous diet evolved several times throughout synapsid evolution; even within the lifespan of a caseasaur species in, Martensius that will be discussed later. In fact, theropsids (synapsids) achieved this dieting conversion 30 million years before sauropsids (reptiles) managed to achieve the conversion. The ecological terrestrial system that evolved from this was a balanced few predator to numerous prey ratio.
|Credit: R. R. Reisz/Jörg Fröbisch Eocasea fossil|
Eocasea fossil remains come from the Late Pennsylvanian in what is now the ‘Calhoun Shale’ stratum of Kansas, USA. The fossil find provides unequivocal caseasaur physiological traits such as: a large lateral temporal fenestra bordered by a posteroventrally (situated posteriorly and ventrally) narrow squamosal, a large postorbital with a wide dorsal surface contributing significantly to the skull region and the posterolateral (posteriorly situated and positioned laterally) wing of the parietal bone being broad carrying a large and wide supratemporal (upper portion of temporal skull bone) in a shallow groove on its dorsal surface. In being a primitive basal caseasaur, it still retained reptilian features such as the conical teeth whereas more derived caseasaurs possessed leaf-shaped teeth for shearing plant material. Also, it didn’t have the barrel-shaped ribcage, but more of the slender reptilian styled ribcage. Eocasea means, ‘dawn casea’ due to its very close relations to Casea spp. as evidenced in its sacral and pelvic girdle elements.
Eothyrididae includes the three primitive caseasaur genera in: Eothyris (E-oh-thy-riss) with a temporal range from ~ 290.1-283.5 mya; Oedaleops (Add-dal-e-ops) with a temporal range from ~ 293-290.1 mya and Vaughnictis (Von-nic-tiss) with a temporal range of ~ 296.4 mya. Eothyris means: ‘dawn opening’, while Oedaleops means: ‘swollen head’ and Vaughnictis is named after the late paleontologist, Peter Vaughn who first described it in 1965. Eothyridid fossils are from the USA Permian in Texas and New Mexico.
|Artist: Nobu Tamura Eothyris|
All three eothyridids were small being anywhere between < 1m/<3.3ft in total length with Eothyris in snout to vent at 30cm/11.9in, Oedalops at 25cm/9.8in and Vaughnictis at 0.7m/2.3ft snout to vent. Eothyridid skulls were no more than 6cm/2.4in with Eothyris’ and Oedalops’ skulls broadened much like the more derived caseasaurids, while the Vaughnictis skull was more slender tapering to the snout. However, all the skulls were not domed, but depressed.
|Artist: Mark Witton Oedalops|
The family, Eothyrididae is greatly supported, with nine similar dental and cranial features. Eothyridid dentition consisted of slightly recurved coronoid teeth while the premaxilla jawbone held three small precanine teeth (in front of larger canine teeth). The pair of canines was very large giving the snout a swollen appearance. In addition, Vaughnictis had rather small palatal teeth (teeth on the roof of mouth) giving a shagreen field (rough granulated surface). With these teeth arrangements and leg mobility, Eothyris and Oedalops lived as an agile insectivore, where Vaughnictis chose another ecological niche in collecting invertebrates by ambush in being less robust in maneuverability hampered with limited agile limbs.
Caseidae: Caseids are a sister group to eothyridids, therefore both groups are paraphyletic in being the closest clades to the last common synapsid ancestor. Their fossils have been found in Texas and Oklahoma USA, Europe, and European Russia. There are 16 genera with 23 species of caseids. The genera are:
1. Callibrachion (Cow-lee-brake-e-un) from 290.1-283 mya;
2. Datheosaurus (Dath-e-o-sawr-us) from 301.2-298.9 mya;
3. Trichasaurus (Tri-ka-sawr-us) from 279-272.5 mya;
4. Phreatophasma (Frayt-o-fazz-mah) from 270 mya;
5. Oromycter (Or-o-mick-tur) from 290.1-260 mya;
6. Ruthenosaurus (Ru-then-o-sawr-us) from 290.1-251.9 mya;
7. Martensius (Mar-ten-see-us) from 290-283 mya;
8. Euromycter (Euro-mick-tur) from 290-254 mya;
9. Ennatosaurus (In-nat-o-sawr-us) from 265-254 mya;
10. Cotylorhynchus (Cot-til-o-rink-us) from 279.5-265 mya;
11. Caseopsis (Case-op-sis) from 279.5-268 mya;
12. Caseoides (Case-oi-dees) from 279.5-268 mya;
13. Casea (Case-e-ah) from 290.1-272.95 mya;
14. Arisierpeton (R-is-zeer-pee-ton) from 290.1-283.5 mya;
15. Angelosaurus (An-gel-o-sawr-us) from 272.5-268 mya;
16. Alierasaurus (Eel-ur-rah-saurus) from ~ 285-272.5 mya.
|Credit: picuki.com Cotylorhyncus|
The head of caseids were disproportionately small when compared to the massive pudgy body as exemplified in the illustration above of the Cotylorhyncus body/ head comparison. Also as illustrated below with the Ennatosaurus head, the appearance reminds one of a turtle head. With that said, the skull had large temporal openings (fenestra) as opposed to the anapsid turtle, huge enormous nares, a large pineal opening (third eye) and an upper jaw that dramatically overhung the tooth row forming a forward projecting rostrum (part of the cranium holding in place the teeth, palate and nasal cavity). As in the other herbivorous synapsid groups like edaphosaurids, the caseid teeth were fairly uniform, except that caseids had a general reduction in the number of marginal and cheek teeth. Just as edaphosaurids, caseids also possessed the small palatal teeth on the roof of the mouth functioning to further shred food in opposition with the tongue.
|Artist: Fabio Manucci Ennatosaurus|
|Plant leaf evolution|
The widened ribs, besides protecting the large digestive tract, also might have aided at least some of the caseids in being semi-aquatic. Recent studies on the ribcage show that it may have supported a diaphragm to assist in limited costal ventilation buttressed by the abdominal musculature attached to the ribcage.
|Artist: Ntvtiho Ennatosaurus in foreground|
One caseid in Ennatosaurus had its fossil remains found in European Russia that included several juveniles and one adult skull. The juveniles were about the size of a housecat and with estimates measured from the skull, adults likely reached 6.1m/20ft in total length. A main significance of the fossil find is that it details a quick burial site within a single event killing the individuals together in which is highly likely a frozen moment in time scenario of parental care watching over the young.
|Artist: Stephanie Dziezyk Ennatosaurus swam|
Another aspect is that the fossil find was in sandstone strata that once was a beach on an isolated island. No other tetrapod fossils have been found in this strata bed, inferring that Ennatosaurus took the short swim to the island from the main coastline and even swam along the beach’s shoreline grazing on delicate aquatic plants. The caseid broad front forefeet for sure were used for digging up edible roots and rhizomes, but the limbs could also have been used for paddling on the surface or as submerged.
The most basal early caseids were not much larger than 1 meter/3.3 feet, while the more derived later caseids reached upwards to 5.5m/18.1ft in length. They had massive limbs to support the weight with Angelosaurus weighing up to 300kg/661.4lbs. Cotylorhynchus weighed up to 2 metric tons/4409.25pounds. About the only other anatomical feature that was notably small on the more derived caseids, besides their head, were the disproportionately small vertebrae.
|Artist: DiBgd Lft: Datheosaurus Rt: Callibrachion|
Through cladistic analysis, it was determined that Callibrachion and Datheosaurus are basal caseids. These two caseids reflect an earlier evolutionary caseid stage when spatulate teeth and broadened phalanges (finger/toe bones) were lacking while found as common in later caseids. Both are similar in size and in being basal primitive caseids were much smaller than the more derived with Callibrachion at ~ 1.4m/4.6ft in total length and with Datheosaurus at ~ 1.1m/3.6ft.
The Callibrachion fossil was found in the black shales of Autun of the ‘Upper Millery Formation’ near Margenne, France, while the Datheosaurus fossil was discovered in the ‘Upper Sandstone Bank’ composed of reddish brown sandstone and clay interbedded with rhyolitic conglomerate near, Nowa Ruda, Poland.
In both the species taxa, the fossils exhibit a short skull with a small facial region, which in not having a long snout excludes them from other synapsid groups and firmly into the caseid clade. As other caseasaurs, the skull also possessed a low and broad symmetry, while Callibrachion in addition had reduced marginal dentition caseasaur characteristics. Datheosaurus had the unique caseid character of a rather large pineal foramen, which is a bony correlate where the pineal eye was housed for the purpose of not seeing but to regulate body temperature.
|Artist: Dinoraul Cotylorhynchus|
In the genus, Cotylorhynchus there were three species in: C. bransoni with its fossils found in central northwest Oklahoma, C. hancocki found in the Texas northern counties of Hardeman and Knox while C. romeri was found in central Oklahoma of Kingfisher and Blaine Counties. Cotylorhynchus is a sister group to the contemporary caseid genus, Angelosaurus which also had three species found in the USA in the ‘San Angelo Formation’ of Texas and in Oklahoma. The three species ranged in size from: C. romeri ~ 3m/9.9ft, C. hancocki ~ 3.9/11.5ft and C. bransoni ~ 6m/19.8ft.
|Artist: Ntvtiho C. bransoni|
|Artist: Liam C. hancocki|
|Artist: Walter Myers C. romeri|
The scapulocoracoid (a pectoral girdle unit containing the coracoid and scapula) was massive while the humeri’s (singular: humerus ~ upper arm bone) flared ends were as large as the length of the bone. The manus (hands) as the distal end of a forelimb had paddle-like features giving a capability to swim turtle-like. Also, the manus digits possessed a considerable range of motion with large retractor processes on the ventral surfaces of the distal unguals (where nails, claws or hooves are rooted affecting their action) that allowed the flexing of claws with powerful motions. As well, the articulatory surfaces of the manus phalanges (digital finger/toe bones of hands/feet) were oblique to the bones’ long axis rather than perpendicular allowing much greater surface area for the flexor muscles. This enabled a defense trajectory in predatory protection and for stronger and more dexterous digging capabilities.
The undersized skull did house large synapsid fenestrae and nostrils. The extensive nostrils probably enhanced smell allowing Cotylorhynchus to detect underground plant parts and sniff out approaching predators. The peg teeth were iguana-like with the posterior marginal teeth supporting a longitudinal row of cusps. The maxillae (upper jaws) were situated as to make the snout overhang the front upper row of teeth forming a projected rostrum (the cranium bone supporting the teeth, palate and nasal cavity of the snout). In fact the name Cotylorhynchus means: ‘cup snout’.
C. romeri was the first Cotylorhynchus to appear 279.5-272.5 mya. Due to very similar morphological and anatomical features, it appears that the bit larger C. hancocki (3.9m/11.5ft) is a descendent of C. romeri who lived contemporaneously with the 6m/19.8ft C. bransoni 272.5-265 mya. Increasing size seems to be a derivative trait in the evolution of Cotylorhynchus and is most likely due to predation of larger carnivores. Increase in size, in particularly in size larger than the contemporaneous predator, gave prey animals an advantage in more strength.
Four fossils of the caseid Martensius were discovered from 1995 through 2006 amid excavation from the ‘Tambach Sandstone Formation’ at Central Germany’s ‘Bromacker Quarry’ in the Thuringian Forest. The fossils have been dated to ~ 290 mya with one of the four representing a juvenile. The juvenile fossil is very important because, although the anatomy was virtually the same, the dental structure of the young juvenile was different from the grown adults.
The adult Martensius teeth differ from the juvenile’s in that the 18 conical isodont (alike dentition) teeth in each maxilla (upper jaw) gradually increase in size serially then begin gradually decreasing until abruptly decreasing greatly in size in with the last 4 being extremely small. Each mandible (lower jaw) possesses 31 teeth shaped similarly as the maxilla teeth, but is essentially the same in size until the last twelve teeth exhibit an abrupt serial reduction to being barely visible with the last four posterior teeth.
|Artist: McAfee Martensius|
The juvenile teeth are similar in shape to the adults, but the arrangement has the anterior four teeth increasing serially in size posteriorly to the largest of the entire series. These are followed by four much smaller teeth of sub equal size, whereas the remaining three teeth decrease serially in size posteriorly from a size nearly equal to the largest of the series. This is dentition suited for an insectivore. As for the adult dentition, it is best suited for an omnivore, but the skeletal anatomy highly suggests trending as an herbivore.
The morphological and physiological skeletal anatomy bears out the fact that it was a basal caseid. At only ~ 63cm/2.1ft as for its small size, Martensius still displays caseid features such as flared ribs and large feet equipped for digging. The Martensius dentition bears out the fact it was a basal caseid in a transitional stage to the more derived caseid evolution in going from an insectivore then through an omnivore stage to a wholly herbivore lifestyle that allowed caseids to disperse and radiate out into new terrain abundant in plant food sources.
The research paleontologists theorize is that the juvenile insect diet promoted bacterial action that would benefit the adult omnivore diet that was trending more towards the herbivorous diet in that the introduced bacteria aided in the digestion of high fibrous plant material. The maxillary teeth in derived caseids were straight in position and more spatulate (leaf-shaped), while in contrast, adult Martensius dentition was slightly recurved and narrowly triangular in its transitioning.
This three stage diet plan and suited dental structure accommodation in going from an insectivore like the primitive basalmost caseasaur, Eocasea to an adult Martensius omnivore/herbivore to a fully herbivore derived latter caseid, like Cotylorynchus allowed the caseid family tree to flourish in the early and middle Permian. This process is called ontogeny by the researchers. Ontogenetic behavior can be confusing especially with phylogenetic behavior. So, we’ll attempt to separate the two out and give a better understanding to ontogeny.
Ontogeny is the biological events that occur from an animal’s embryonic state to its full adult state, while phylogeny deals more with the evolution of a genetically related group of organisms as distinguished from the development of the individual organism within the group. In dealing more from egg to adult with the physical and psychological aspects of an individual organism’s conduct, ontogenetic behavior highlights the individual lifespan, whereas phylogenetic behavior refers to the evolutionary history of a species.
Now, with that said, if I’m asked, (in which I haven’t been), I feel that when it comes to Martensius and its representation both as an individual species evolvement contributor pushing the caseid family to herbivory and secondly, its inner individual evolvement from an insectivore to omnivore/herbivore in its own lifespan, then both ontogeny and phylogeny would apply concerning Martensius.
Before we venture on into therapsids, I’d like to take a moment or two and interrupt here in relaying a tiny bit of an introduction to parareptilians. As you can see in the above cladogram, there were two reptilian groups…the diapsid clade Eureptilia and the anapsid clade Parareptilia. Just as soon as they appeared in the Pennsylvanian 307 mya, parareptiles disappeared just as quickly 201.3 mya at the end of the Triassic during the Tr-J extinction event. They may not have left any genetic lineage, but for sure during the Permian and Triassic, they did affect the evolution of other animal clades that did leave their genetic print in latter more derived animal clades. Turtles are the only extant anapsid group but through mitochondrial analyses, along with molecular and fossil evidence, it has been shown that the turtle lineage came from diapsids in reverting back to an anapsid skull; turtles did not come from any extinct parareptilian.
Excluding the absence of a temporal fenestra, there are six parareptilian categorical autapomorphies (derived traits that are distinct features of a specific taxon) that confirms Parareptilia as a clade, which are: the absence of a lacrimal-narial (lacrimal-facial bone / narial-nostrils) contact, the absence of a caniniform region, a shortened postorbital region, a single median embayment of the posterior margin of the skull roof, the absence of a supraglenoid foramen (region of the scapula transverse scapular ligament where foramen perforates the supraglenoid buttress; Latin: supra~above/glenoid~cavity), and the absence of a subtemporal process (bone below temporal) of the jugal (facial bone connected primarily to quadratojugal and maxilla bones).
Again, parareptilian skulls were absent of any temporal fenestrae with no caninform dentition within the jaws. In addition, they had a jaw articulation at the level of or slightly posterior to the occiput (the backside of skull) and a large anterior foramen on the maxilla just below the naris.
The oldest known parareptilian is Erpetonyx (Er-pah-ton-nicks) that lived during the Pennsylvanian 303.7-298.9 mya in what is now Prince Edward Island, Canada of the ‘Edgemont Bay Formation’ exhibiting subtropical swampland strata. Still though, it is millions of years younger than Hylonomus, the oldest known eureptilian living 312 mya.
Approximately 22.5cm/8.9in in length, Erpetonyx was small and judging by the dentition, was a carnivore/insectivore. As an anapsid, it had other parareptilian features such as similar forelimbs where the humerus (upper arm bone) was a bit longer than the ulna and radius (lower arm bones) while the toe claws were longer than the other phalanges (toe bones). The unique features were in having 29 presacral (pre-hip) vertebrae where most parareptilians had 26. The sharp conical teeth were recurved decreasing in size posteriorly and were in conjunction with denticles (tiny teeth) on the palate (roof of mouth) going all the way back to the jaw joint of the quadrate bones. Some broken teeth revealed dentin (calcified tissue) which was inherited from its amphibian ancestors. Although the temporal fenestra was closed, there was a large skull opening in the front edge of the base of the braincase in making room for the hyomandibular branch of the facial nerve.
Well, Erpetonyx was once the oldest parareptilian, but that title has been handed over to the recent discovery and 2019 evaluation of, Carbonodraco. Found in an Ohio, USA coal dump from an abandoned coal mine, the fossil was literally encased in a lump of cannel coal (impure coal). This coal is representative of a once steamy giant club moss swamp that this 25cm/9.8in small parareptilian scampered in 310-306 mya chasing after insects. The name, Carbonodraco refers to ‘coal dragon’.
There are three orders of parareptilians and they are: Procolophonomorpha (Pro-call-o-fawn-o-mor-fah) from 306-201.3 mya; Mesosauria (Mezz-o-sawr-e-ah) from 299-270.6 mya and Millerosauria (Mill-lur-o-sawr-e-ah) from 265.8-251 mya. There were no aerial eureptilians, but as tetrapods, they had conquered the Permian landscape and invaded coastal waters.
Procolophonomorpha with the subgroup nodes, Australothyris (Ah-strail-o-thigh-riss) and Ankyramorpha (Ain-kyr-ah-mor-pha) is the most diverse group of parareptilians and was around for 104.7 million years. Having only one species in Australophyris smithi, the subgroup, Australophyris, as named after the genus is holotypic. The subgroup, Ankyramorpha is further divided into the superfamily, Lanthanosuchoidea (Lan-than-nosh-shush-oi-de-ah) and the suborder, Procolophonia (Pro-call-o-fawn-e-ah) where they themselves are further divided into suborders, families and subfamilies eventually leading to 63 genera. The compiled parareptilian phylogeny cladogram below shows the only significant increase in diversification rate being that of the more derived procolophonids, indicated by the significant p-value of 0.01 which is credited to the research of Linda A. Tsuji and Johannes Müller.
Australothyris is the most basal procolophonomorph and helps support the concept that parareptiles originated in Gondwana in what is now, South Africa. As in all parareptilians, it was an anapsid with no temporal openings, but due to the large temporal, it is easily identified that there once was a temporal fenestra that was resealed. Procolophonomorphs are a sister taxon to millerosaurs in which suggests their fenestrae were also resealed.
This of course alludes to the fact that common ancestors to procolophonomorphs had fenestrae and what this further implies is that parareptilians did not directly branch off from the early day temnospondyl and lepospondyl amphibians which indeed were anapsids that first show up around 370 mya evolving from multi-jointed lobe-finned fish. So indeed, there must be a section of reptiliomorph anapsids that directly derived from amphibians that we are missing hitherto in the fossil record.
|Artist: Mikail Shekhanov Deltavjatia|
The ankyramorph, Deltavjatia (Dale-tah-vee-yah-tee-ah) was in the lineage line of a procolophonomorph pareiasaur that lived 260 mya. As in all other pareiasaurs, this 1.5m/5ft in length parareptile was an herbivore. Its fossils were found in Russia’s ‘Urpalov Formation’ by the banks of the Vyatka River near the port city of Kotelnich. With its bulbous head, Deltavjatia had common pareiasaur traits, but also had some primitive features such as histological ontogenetic changes in long-bone and rib microanatomy.
|Artist: Vladislav Egorov Elginia|
With a temporal range of 254-252 mya, another pareiasaur was Elginia (L-gin-e-ah) that lived towards the end of the Permian and the ‘Great Dying’ extinction event that ended the Permian. With the Permian climate heating up and getting drier, it probably put stress on the plants growing in what is now Scotland that Elginia depended on for survival. Found in the ‘Hopeman Sandstone Formation’ alludes to a very dry environment. As far as lengths go, Elginia was much smaller than other pareiasaurs in reaching a length of only 60cm/2ft. It’s surmised that the skull horns about its head with the two longest protruding forwards in front of the skull were more for display than for defense.
|Artist: Juan Cisneros Procolophon|
The last ankyramorph genus to be discussed is the procolophonid, Procolophon (Pro-call-o-fawn) that had ten species and was spread out in lands 251.3-247.2 mya in what is now: Antarctica, Brazil and South Africa. Reaching a length of only 30cm/1ft, it was even smaller than Elginia. That is an unusual small size, but it wasn’t a pareiasaur like Elginia was. It doesn’t take much of an imagination, but surely Elginia looked much like the extant, Phrynosoma (horned toad, or horny toad as we call ‘em in Texas). Procolophonids first appeared in the Permian, but living 249.7-237 mya, Procolophon lived well past the extinction period. Procolophon is classified under the subfamily, Procolophoninae where there are (including Procolophon) eight genera.
Thus far through fossil evidence, mesosaurs were the first of tetrapods to revert back from land into an aquatic environment whether in freshwater or near shore marine. With long pointed homodont dentition, they would snag fish and aquatic crustaceans. There was debate on whether they were fully aquatic or semi-aquatic. The fully aquatic argument’s ammunition was that mesosaurs were putatively ovoviviparous (bearing live young after fertilized egg development hatches inside the female’s body) and possessed several skeletal characters such as a long laterally compressed tail, long limbs and the pes (foot) was larger than the manus (hand). However, on the flipside of the argument, most semiaquatic basal tetrapods also had some of these features and studies show that the variation of the vertebral centrum length along the axial skeleton of mesosaurs fits better with a semi-aquatic morphometric pattern. So, the semi-aquatic side won out, but not completely.
Again, through fossil evidence, most lacustrine (lake) or coastal marine fossils were of juveniles and young adults, while any land sediment fossils were only of adults showing degradation due to atmospheric weathering. So, yes mesosaurs gave live birth in water and is where young mesosaurs mainly stayed, but the adults did venture forth onto shore land supported by strongly ossified epiphyses (bone that ossifies separately later becoming ankylosed) and tarsi (any of the numerous small foot bones between the tibia and fibula and metatarsus).
|Artist: Clavatti Mesosaurus|
There were three genera of mesosaurs in: Brazilosaurus (Bra-zill-o-sawr-us) from 284-279.5 mya, Mesosaurus (Mezz-o-sawr-us) from 299-280 mya and Stereosternum (Steer-e-o-stir-num) from 299-280 mya. They were not huge animals with Mesosaurus being the largest at 0.9m/3ft, while Stereosternum reached 0.8m/2.6ft and Brazilosaurus was at best only 42.5 cm/16.7in in total length.
|Artist: Julia d'Oliveira Stereosternum|
Besides freshwater and coastal marine habitation, in Uruguay, Mesosaurus gives fossil evidence that it also adapted to and inhabited a hypersaline aquatic environment. Mesosaur fossils also have turned up in the way of their coprolites (fossilized poop), which shows cracking, evidence of drying out by being on dry land. Their fossils are also important in correlating and verifying continental drift, in particular with the current continents of Africa and S. America’s drifting apart.
Mesosaurs were slow swimmers with the optimal swimming speed estimated at ranges from 0.15 to 0.86 m/sec, or 0.49 to 2.8 ft/sec under both normal salinity (5% salinity, ρ = 1020 kg/m3) and hypersaline conditions (39% salinity, ρ = 1278 kg/m3), considering λ values from 0.2 to 2.8. The interval of potential salinity conditions likely covers the range of values that plausibly occurred in the Uruguay hypersaline environment of Mesosaurus.
Millerosauria is the sister taxon to Procolophonmorpha and includes the families of Millerettidae (Mill-lur-et-tah-day) and Eunotosauridae (U-no-toe-sawr-ah-day). A few species of millerettids had a temporal fenestra, but unlike synapsids the millerettid fenestra was acquired independently as convergent evolution according to phylogenetic analyses. The presence of a quadrate emargination (notched margin at back of skull) suggests that millerettids had a tympanum (ear drum) capable of hearing high frequency airborne sounds. However, their stapes (the sound-conducting middle ear ossicle) is robust, suggesting that their auditory acuity was not as good as in more recent groups of anapsids such as turtles. Millerosaur fossils all come from South Africa. There are five genera in the family, Millerettidae. Eunotosauridae contains only one species in the genus holotype, Eunotosaurus. From 265-259 mya, Broomia (Broo-me-ah) is the oldest genus and has been analyzed as a direct descendent of romeriid captorhinomorphs.
|Artist: Andrey Atuchin Eunotosaurus foreground|
Eunotosaurus fossil remains come from the ‘Karoo Supergroup’ of South Africa during a period 265.8-259 mya of heat and extreme drying. It was 30cm/11.9in long and most likely was an insectivore.
Eunotosaurus, due to its flared ribs that were also broad enough to touch one another at the margins, was originally thought to be the ancestral lineage to turtles. Even though some may still think so, with extensive anatomical studies and phylogenetic analysis, Eunotosaurus is now firmly seated as a basal millerettid. There were nine pairs of widened ribs overlapping each other with the first 8 anterior ribs angling backwards while the one posterior rib pair angled forwards. The nine dorsal vertebrae, which were also similar to turtles, were connected to a pair of ribs. Cross-sections of the ribs indicate that they grew in three different phases as the individual animal developed. The first phase involves a primordium (first indication of development) rib ossifying into a rib bone. The second phase is the development of a shelf of bone above the main shaft of the rib to form a T-shaped cross-section. The third and final phase is the widening of the lower ridge into a teardrop-like shape that reinforced the rib.
There is only conjecture as to why Eunotosaurus evolved flared and broadened ribs. Two decent ideas is that there is evidence it was a burrowing animal and with the ribs making the back slightly concaved in shape, it would have afforded better accessibility in tunnel construction and tunneling mobility. Another thought is that it may have helped from being preyed upon, so most likely smaller predators wouldn’t be looking at it for dinner. However, a larger predator with a lesser bite force, might think twice before biting into nothing but hardened rib bone and a wide body girth not much worth risking choking on if swallowed whole.
|Credit: vida prehistórica Milleretta|
Milleretta (Mill-eh-ret-ah), one of the five millerettid genera had a temporal range of 252.5-251 mya going extinct during the Pr-T extinction. Found in the ‘Balfour Formation’ of the ‘Karoo Basin’ assemblage, the pedology (study of soils) of the strata shows it once was a forest floor where Milleretta lived that gradually gave way to much drier conditions that is probably what made it go extinct. With its range of sharp teeth it most likely scampered lively through the forest floor litter in search of insects and other invertebrates. Once the forests gave way to the drier climes, the insects it fed upon left as well, hastening its extinction.
At 60cm/23.6in long, it was double the size or more when comparing lengths with the other millerettids. Fossil anatomical remains allude to the fact that this millerettid was agile and quick. Two depressions at the base of the skull was where eardrums were housed which aided in an auditory way in locating insects and alluding predators. In contrast to Eunotosaurus with a T-cross section, Milleretta had plesiomorphic vertebrae making its ribs wider by growing its bone out the shaft to an airfoil-like section. Milleretta is Latin for: ‘little Miller’.
Now, that the brief intro into parareptiles is over with, let’s get back to the line of synapsid descendancy from pelycosaur to therapsids and finally to mammals.
Again, do not confuse Therapsida with the broader group, Theropsida that encompasses all synapsids. Therapsids have an Early Permian to Holocene temporal range from 275-0 mya and are those synapsids that are closely related to mammals in being mammals’ direct ancestor. Early therapsids themselves had evolved from pelycosaurs within the sphenacodontid clade. The specific therapsid group that gave rise to mammals are the cynodonts. Where dicynodonts made it just past the P-Tr extinction (Permian-Triassic extinction event) before going extinct, cynodonts were the only therapsid group to survive well beyond it and are still here with us in the form of mammals including you and me.
Therapsids had positioned their legs more upright underneath the body as opposed to the more sprawling stance of pelycosaurs. They achieved the more upright stance with the outside and inside toes becoming shorter than the middle toes giving the foot placement a more parallel axis to the body. Therapsid lateral temporal fenestrae had enlarged, improving on jaw musculature and lightening of the skull. Therapsid species begin showing up with more complex dentition in displaying teeth that are differentiated into front incisors for nipping, large lateral canines for piercing/tearing and posterior molars for mastication. It appears with the dentary changes, enhanced processing of food had become a nutritional advantage.
Through fossil evidence therapsids are the first fur/hair bearing group of animals that are known. We know that fur or hair had already come into use as some coprolites (fossilized poop) exhibiting the remains of therapsid bones also contained hair. But we know for a fact that the therapsid beaver/seal-like docodont, Castorocauda (Kass-tor-o-caw-duh) had hair as its fossils bear fur. Hair/fur also points to endothermy (warm-bloodedness) along with other therapsid features, such as highly vascularized bones absent of growth rings and the presence of nasal turbinates which is a long, narrow and curved shelf bone protruding into the nasal cavity where the conchae (porous sponge-like bone) warms inhaled air.
|Artist: Biarmosuchus Myosaurus Note hair/whiskers|
The origin of hair replacing epidermal scales changed the physiology and brains of synapsids. Fur gave insulation to adapt to colder environmental conditions and whiskers as sensory receptors gave the brain added information to analyze. Pits along with canals on the bone of the snout found in therapsid fossils, indicate where concentrations of nerves and blood vessels once were. In mammals, such structures allow specialized hairs (whiskers) to be used as sensory organs which most likely were the case for therapsids as well. The basal dicynodont, Myosaurus (My-o-sawr-us) already had hair and whisker traits 250 mya. In having these mammalian features, this is why therapsids originally were called ‘mammal-like reptiles’.
|Credit: WWW Proto-mammals|
With at least 510 genera and innumerable more species, if merely mentioned in this article, it would make it ten times lengthier than it already is. Since this is more of an evolvement article with therapsids leading to mammalians, this article is concentrating more on the line of evolvement of the therapsid group that led to mammals. So, if you want to know more of therapsids, below is a link to all the therapsid genera:
Along with some basal therapsid outliers, Therapsida is divided into three major clades which are: Eutherapsida (U-thuh-rap-sah-duh) as the crown clade further divided into, Neotherapsida (Nee-o-thuh-rap-sah-duh), which was even further divided into Theriodontia (Thear-e-o-don-chi-ah). These major divisions were further divided into the eutherapsid: Dinocephalia (Dye-no-seff-fah-lee-ah) that were herbivorous/carnivorous/omnivorous; the eutherapsid, neotherapsid: Anomodontia (Ah-nom-e-don-tee-ah) were strictly herbivorous while the eutherapsid, neotherapsid, theriodont: Eutheriodontia (U-thear-e-o-don-tee-ah) and Gorgonopsia (Gor-gon-op-see-ah) were carnivores except for the eutheriodontian, family, Bauriidae being herbivorous. The genera, Tetraceratops (Teh-tra-sair-ah-tops), Raranimus (Rare-an-e-muss) along with the suborder Biarmosuchia (Be-ire-mo-sue-key-ah) were primitive basal therapsids. Therapsid fossils have been found on every current continent.
|Credit: WWW Tetraceratops|
Tetraceratops is the most basal therapsid and only therapsid from the Early Permian living 275 mya in Texas and has affinities to the primitive biarmarosuchian therapsids. It has been one big headache for paleo anatomists and paleontologists in trying to figure Tetraceratops’ ranking. Originally classified as a sphenacodontid pelycosaur with strong consideration as an eothyrid pelycosaur; after further evaluation, they finally classified it as the basal most therapsid. The reason for the conundrum is that in having anatomical featured synapomorphies (characteristics present in an ancestral species shared exclusively by its evolutionary descendants) exactly like both groups, Tetraceratops truly is the bridge between derived sphenacodontids and primitive therapsids; the missing link so to speak.
Phylogenetic analyses show that Tetraceratops is the sister group of all other known therapsids. In the first analysis of reflecting anatomical interpretations, eight derived states are shared by Tetraceratops and therapsids.
The node linking Tetraceratops to other therapsids has a bootstrap frequency of 95%, the second largest value found by this analysis with a Bremer index of four. So, what does this mean?
In a statistical context, bootstrapping refers to using data at hand to infer the uncertainty of said data. This improves the cladistic statistical evidence by pulling on the current data’s ‘bootstraps’ to predict future end result knowledge. In practice, this is achieved by sampling, interpolating or permuting input data.
How bootstrapping became a term might be interesting to some, as it originated in describing a character which profusely liked to spill over a lot of lore. The expression, ‘bootstrap’ comes from tales originating from the mind and pen of the German writer, Rudolf Erich. Erich composed the fictional German personality, Baron von Münchhausen, whom found himself stuck in a deep hole. To get out, he grabbed his boots by the bootstraps and pulled himself upwards, until he was able to step out of the hole.
Bremer support and decay values are simply the difference in the number of steps between the score of the ‘most parsimonious trees’ (MPTs) and the score of the MPT that does not contain a particular clade, node or branch. MPTs require the fewest number of evolutionary events once summed over the entirety of events.
Cryptovenator was another synapsid that anatomically bridged affinities between sphenacodontids and primitive therapsids giving researchers hair-pulling fits as to exactly what it was to be classified as. Finally, bootstrap analyses and Bremer values helped give the final classification as a sphenacodontid. Below is a cladogram with (A) utilizing a bootstrap percentile and Bremer values, while (B) does not when taking into account Cryptovenator’s phylogenesis.
The seven derived features or synapomorphies that Tetraceratops and primitive therapsids share with derived sphenacodontids are:
1) loss of pre-canine dentition
2) loss of ectopterygoid dentition (thin dermal tooth bearing bone)
3) temporal fenestra has broadened forming a concave surface
4) reduced quadrate (upper element of jaw suspension)
5) pterygoid’s posteromedian flange moved behind interpterygoid vacuity
6) interpterygoid length is reduced
7) ventral plate of pterygoid is much reduced
Tetraceratops was no larger than 30.5cm/1ft at being around 28.4cm/11.2in. As being carnivorous and insectivorous, with its small size, it most likely fit a niche in capturing smaller invertebrate and vertebrate animals as the numerous larger pelycosaur predators during its time preyed on the larger herbivores. The generic name in meaning, ‘four-horned face’ is a misnomer, for it also had two smaller spine-like horns projecting outwards arising from its angular processes of the mandibles, making for three pairs of horns, or six horns. The other two pairs were facial projecting forwards as attached to the premaxilla and prefrontal bones.
Just to let one know, lately there is a push to re-reclassify Tetraceratops as a sphenacodontid again. Regardless, it only shows the affinitive bridge this synapsid has between pelycosaurs and therapsids.
Eutherapsida, other than the primitive most basal therapsid forms, contains all other therapsids in the subtaxa: Dinocephalia (Dye-no-suh-fall-e-ah), Neotherapsida (Knee-o-thair-ap-sye-dah), Theriodontia (Thear-e-o-don-tee-ah) and Eutheriodontia (U-thear-e-o-don-tee-ah). However, even though the paraphyletic biarmosuchians are primitive, they had enough similar features to eutherapsids, so are a sister clade to them. By the time dinosaurs rose in the Triassic, eutherapsids had already given rise to mammals. The eutherapsid lineage reached its greatest species richness and morphological divergence during the Permian, as they attained maximum dentition complexity in the Triassic therocephalian superfamily, Baurioidea (Bow-ree-oi-dee-ah).
Dinocephalians as the sister taxon to neotherapsids flourished 270 mya, but by 259 mya, had all become extinct leaving no descendants. With at least 12 families and innumerable genera, they were large bodied and were composed of carnivores, herbivores and omnivores. A distinct anatomical feature was having the hind limbs shorter than the forelimbs. The largest of dinocephalians was the herbivorous, Tapinocephalus (Tap-in-o-sef-a-luss) weighing up to 1996kg/4400lbs with a length of just over 3m/9.9ft. The omnivorous, Titanosuchus (Tye-tan-os-su-kuss) was slightly outmatched by Tapinocephalus’ in being around the same weight while only having a length of 2.5m/8.2ft. The two carnivorous dinocephalians in, Titanophoneus (Tye-tan- o-foe-knee-us) had a total length of 2.85m/9.2ft, while Anteosaurus (An-tay-o-sawr-us) had a length of 5m/16.4ft.
|Artist: John Sibbick Anteosaurus|
As the primitive pelycosaurs were waning, dinocephalians began radiating out replacing them in Permian niches, but only to the extent at the height of their diversity to have died out. Except for listing environmental causes such as disease, climatic factors or other stresses, no one knows why they suddenly became extinct to be replaced by the smaller and more derived therapsids.
Neotherapsids form a large clade of anomodonts and the most derived theriodont groups that lead to mammals. The temporal range for neotherapsids is from 270 mya to the present. During the Permian, neotherapsids had conquered much of the world with Permian timeframe fossils found in: Germany, India, Madagascar, Niger, Russia, South Africa, Tanzania, United Kingdom, the U. S. in Texas, and Zambia. Triassic neotherapsid fossils have been found in: Antarctica, Argentina, Brazil, China, Germany, Lesotho, Morocco, Poland, Russia, South Africa, Tanzania, and the U. S. in Arizona, Colorado, New Mexico, North Carolina, Texas, Utah, and Wyoming. Their radiation didn’t stop there as Jurassic neotherapsid fossils have been found in Lesotho, while more derived Cretaceous fossils have been found in Australia and Russia.
Groups of neotherapsids survived the ‘Great Dying’ P-Tr extinction 251.9 mya, the Tr-J extinction 201.3 mya and even the K-Pg extinction event 65 mya. The proto-mammal neotherapsid did not totally become extinct until 17.5 mya, leaving it up to mammals to carry on the neotherapsid lineage. Neotherapsids rose long before dinosaurs and survived long afterwards only for the proto-mammal neotherapsid to die out just before the advent of the family of man’s appearance in Hominidae showing up 17 mya.
The two main neotherapsid subgroups were in the sister suborders: Anomodontia (Ah-nom-e-don-tee-ah) from 270-201 mya and Theriodontia (Thear-e-o-don-tee-ah) from 265-0 mya.
The anomodonts are the most species-rich and abundant Permian/Triassic therapsids. During this period, this clade had a global distribution spanning a wide range of morphologies and ecologies. The beaked dicynodonts represent the largest anomodont subclade that possessed two maxillary tusks, while the rest of anomodont species were toothless, except for species in the basal anomodont clade, Venyukovioidea (Ven-nee-you-ko-vee-oid-dee-ah) that actually had more than a mouthful of teeth that were large in relation to the skull, so much so, the dentition was even too large for the closed mouth to fully cover.
|Artist: atrox1 Suminia|
Anomodonts arose 268 mya with some surviving the P-Tr extinction, but finally all dying out by 201 mya at the end of the Triassic during the Tr-J extinction. In ranging from lengths of 39.4cm/15.5in to 5.9m/19.5ft, anomodonts dominated the Late Permian when it came to terrestrial vertebrate fauna. They were the only herbivorous therapsid group to survive the P-Tr extinction event. Some anomodonts weren’t just terrestrial but were arboreal and fossorial as well.
Being wholly herbivores, anomodont success in surviving the extinction event might have been on how they assimilated their food. Jaw articulation allowed for anterior to posterior movements for slicing as well as masticating. Further, through a system of interlocking ridges and grooves formed from the palate and dentary the plant roughage was ground into a mush. This form of chewing food was advantageous during a time of a Late Permian drying climate in acquiescing for a method of ingesting nutrients from rougher and more fibrous plants that could tolerate the ambient hotter/drier conditions. There are at least 91 current known genera of anomodonts. Some anomodont species supported body hair and may have been endothermic as well.
|Artist: Christopher Chávez Eodicynodon|
|Artist: Corey Ford Kannemyeria|
With highly vascularized bones possessing Haversian canals eminently suggests that dicynodonts were endothermic. Juvenile dicynodont fossil bones even exhibit higher inner channel densities than the adults and most other therapsids as well. In addition, there are Late Permian carnivorous coprolites that contain hair along with dicynodont bones. In a new chemical study, it showed that both dicynodonts and cynodonts developed endothermic blood convergently (independently) before the Permian extinction in their more primitive forms. Besides the pair of maxillary tusks, dicynodonts had no teeth, but a keratinized beak for cropping plants.
The dicynodont, Lystrosaurus (Liss-tro-sawr-us), with a temporal range of 255-250 mya was one of the few theropsids as well as sauropsids that managed to survive the P-Tr extinction in sizable numbers. In Early Triassic strata, its fossils are the most common. Fossil beds formed from just after the extinction are 95% composed of Lystrosaurus remains. With very few predators and other competitive herbivores, they managed to radiate out into new lands spreading out from the current land of South Africa into what is now Antarctica, China, European Russia, India Mongolia and Poland.
|Artist: Michele Schirru Lystrosaurus attacked by Proterosuchus|
There are some 23 species currently known and depending on which species, ranged in length from 0.6m/2ft to 0.9m/3ft. Rather small, but about the only predators that survived the P-Tr extinction that was large enough to prey on Lystrosaurus was the 1.5m/4.9ft therocephalian therapsid, Moschorhinus (Moss-ko-rye-nuss) and the 3.5m/11ft. archosauriform, Proterosuchus (Pro-tur-row-sue-cuss). Where Moschorhinus was strictly terrestrial, Proterosuchus was also, but in addition was semiaquatic where it probably used the lie-in-wait ambush technique, due to its slow moving sprawling gait. Lystrosaurus most likely herded.
|Artist: Fabio Manucci Lisowicia|
Originating in what is now southern Poland, the dicynodont, holotype species Lisowicia (Lee-so-wiss-e-ah) lived 208.5-201.3 mya. Lisowicia is named after the Polish town of Licowice where the first fossil finds were found nearby. It was the largest heavily built dicynodont in weighing ~ 5.4 metric tons/6 U.S. tons and had a length of 4.5m/14.8ft. It was also one of the last or the very last of dicynodonts before going extinct taking along with it the dicynodont genepool.
The reasoning behind its ability to grow so large is that during its time period in the Late Triassic, the dinosaurian, sauropodomorphs were emerging and successfully out-competing other herbivores in access to plants. However, there is no evidence of sauropodomorphs ever inhabiting in what is now Poland, so with less ecological stresses and pressures, Lisowicia was able to trend towards gigantism becoming the apex herbivore in its region.
Unique among the other dicynodonts, Lisowicia as a quadruped held its body upright, with all four legs underneath the body just as mammals do. Other dicynodonts had both hind legs held directly underneath, while the forelimbs were a bit sprawled.
Theriodonts begin the direct path to mammals, where proto-mammal carnivore and herbivore therapsid groups become most mammal-like. Even the term, theriodont means: ‘the ones with beast teeth’ in reference to mammal-like dentition. Theriodont basal groups first appeared 270.6 mya, but most other theriodont groups contemporaneously appeared with their anomodont sister group 265 mya and are still here today in the form of mammals.
It is plausible that all theriodonts were endothermic, which includes the earliest primitive forms. The most basal theriodonts were carnivorous while some groups evolved later into herbivores during the Triassic. Theriodonts were most successful in speciation during the Late Permian through the P-Tr extinction event into the Early Triassic. By the Middle Triassic, even though theriodonts were losing ground in speciation, a successful line through eutheriodonts led to mammals.
Larger teeth evolved in the theriodonts, therefore to accommodate it, skulls also enlarged. The jaws also enlarged, in which mammals inherited giving both groups more capacity and efficiency in chewing. Also, several small jaw bones, which are still located in extant reptile jaws, migrated from the jaws into the inner middle ears aiding in hearing while allowing the jaw to open wider. This theriodont reduced quadrate trait was seated in the squamosal forming a joint with the articular allowing the anterior surfaces of the mandible dentition to contact and shear against the posterior surfaces of the maxillary dentition with closed jaws.
The quadrate bone as reduced in size was loosely attached to the skull by a fibrous cartilaginous pad and once separated from the jaw, really benefitted carnivores with large canines. The size reduction also allowed a fore/aft movement from its lower end. Again, as it is an important step to mammals, this theriodont reduced quadrate allowed the anterior surfaces of the mandible dentition to contact and shear against the posterior surfaces of the maxillary dentition with closed jaws. This created an effective cutting motion for carnivores in dismembering carcasses or slicing through and gouging out plant roughage for herbivores.
The two divisions of, Theriodontia are the sister clades of the suborder, Gorgonopsia (270.6-250 mya) and the suborder, Eutheriodontia (265-0).
Deriving from a Middle Permian therapsid, the first gorgonopsians were rather small, such as the most basal gorgonopsian, Nochnitsa (Nock-neet-zah) at ~ 0.76m/2.49ft but began trending in larger form reaching lengths of 4.3m/14ft with Inostrancevia (In-nos-stran-see-vee-ah). With the extinction of the dinocephalians in the Late Permian, gorgonopsians became the apex predator of their ecosystems only to become the one therapsid group that totally perished during the P-Tr extinction. Their fossils have been discovered in Africa and Russia.
Being carnivores, the distinguishing gorgonopsian feature was their saber-toothed like canines; large also were their incisors. However, the postcanines were small in size and number or totally absent as in the genus, Clelandina (Clee-lun-deen-ah). The dentition of gorgonopsians was intermediate between that of earlier pelycosaurs and that of mammals with strong heterodont features in the incisors and canines, while the postcanines were analogous to mammal premolars and molars. The dentition was intermediate between that of earlier pelycosaurs and that of mammals, but with polyphyodont dentition in that the teeth were replaced throughout the course of the animals’ lifetime.
|Artist: Vitor Silva Subfamily: Rubidgeinae|
The subgroup within Gorgonopsia is Rubidgeinae (Roo-bid-gee-ah-nay), which comprises several genera of large, robust gorgonopsians endemic to Africa. Rubidgeines’ jugal bone was broadly expanded, where in most gorgonopsians, the bone was narrow. The jugal is a skull bone in most vertebrates, but in addition for therapsids functioned as a facial structural support during biting. Rubidgeines used their head for capturing prey and had the presence of a reinforced skull consisting of a robust skull roof and supraorbital bosses in protecting the head.
|Artist: Vitor Silva Aelurognathus|
The gorgonopsian rubidgeine, Aelurognathus (Aye-loo-row-nu-thus) lived in what is now South Africa, 260.9-254 mya. It for sure was a predator, but its fossil remnants also point out that it was a scavenger as well. When compared to other gorgonopsians, it did possess decent sized canines but had rather small incisors. So, it couldn’t crush bone like other gorgonopsians could.
In the ‘Tropidostoma Assemblage Zone’ of South Africa there was a peculiar dicynodont fossil that had been really chewed on. Evidence of large incisor scrape marks on scattered backbone parts are evident from some unknown predator in which must’ve made the initial kill, but there were also innumerable small incisor scratch marks on the rear bones, suggesting other individuals fed on the hindquarters as well.
What this scenario tells us in its unfolding is that some large predator had made the initial kill and had commenced eating on the topside even biting into bone. Then, a pack of scavengers came along with enough members to scare away the large predator and went for the underside by removing the hind limbs then scraping the underbelly bones of most of the flesh. Along with the underside dicynodont bones, an added feature also was present in the fossil site of a broken tooth identified as belonging to Aelurognathus. The initial carnivore may have had its fill and simply abandoned its kill, but nonetheless, in either scenario, Aelurognathus scavenged.
In the family, Gorgonopsidae, Inostrancevia (In-nos-stran-see-ve-ah) as mentioned before was the largest gorgonopsian at 4.3m/14ft in total length. The skull itself was 60cm/2ft long that harbored a pair of canines 15cm/5.9in long. The canines had small serrations along the inside for slicing into flesh. It lived during Pangaea in an arid climate and ecosystem.
Fossils of Inostrancevia come from the ‘Sokolki Assemblage Zone’ in the ‘Kutuluk’ and ‘Salarevo Formations’ of northern Russia. The temporal range it roamed was during the Late Permian 260-254 mya. With the combination of its skull, canines and body size, Inostrancevia was a biting, walking, running and digging predator machine that no other animal during its time would want to stand up to it, except maybe another Inostrancevia. Once it sniffed out a burrowing animal, it could have easily dug it out. It likely pursued and killed the 3m/9.9ft pareiasaurid, Scutusaurus. This animal had plates of protective scutes underneath the skin that would’ve been of little hindrance to Inostrancevia’s canines.
|Photo: Ghadoghedo Inostrancevia canines|
There are three confirmed species of Inostrancevia in: I. aleandri (ul-lee-an-dree), I. latifrons (laah-tee-frons) and I. uralensis (yur-ul-en-sis). All three species had powerful leg muscles for speed and endurance and most likely could reach speeds up to 53kph/33mph. It’s known through fossil evidence that Inostrancevia did not possess scales, but whether it had naked skin, or hair is not known.
As the sister clade to gorgonopsians under theriodonts, Eutheriodontia is further divided into two sister clades of Therocephalia (There-oss-seph-ah-lee-ah) and Cynodontia (Sin-o-don-cha). Cynodonts in turn lead to mammals.
Thus far, it appears that eutheriodonts originally evolved from Permian Gondwana therapsids in the ‘Eodicynodon Assemblage Zone’ as a composite of the ‘Beaufort Group’ in present day South Africa. The two oldest eutheriodonts from 265 mya are, Glanosuchus (Glan-o-sue-cuss) and Ictidosaurus (Ick-tid-o-sawr-us). The temporal range of eutheriodonts through cynodonts to mammals is 265-0 mya.
However, by the Late Permian eutheriodonts had radiated out into Brazil, China, India, Kazakhstan, Madagascar, the African nations of Malawi, Niger, Tanzania and Zambia, Russia and the United States in Oklahoma and Texas. By 235 mya in the Triassic, another radiation wave further took them into what are currently Antarctica, China, Namibia, Russia, and Tanzania. It has been analyzed by the paleontological researchers, Adam K. Huttenlocker and Jennifer Botha-Brink that, “Increased environmental variability in the earliest Triassic was associated with rapid growth to a minimum body size requirement and consequently, shortened developmental times.”
Therocephalians and cynodonts began diverging early in the Permian, but the race of evolution was on, in which one would become the first mammalian where eventually cynodonts won out with the extinction of therocephalians 243 mya. Independently through convergent evolution, both eutheriodont groups evolved mammalian features such as a secondary palate and the loss of a postorbital bar. From a common ancestor, both groups inherited and retained the mammalian features of the loss of teeth on the palate, the expansion of the epipterygoid (slender bone situated above or upon the pterygoid; known as the alisphenoid in mammals) at the base of the skull and the narrowing of the skull roof to a narrow sagittal crest running between large temporal openings.
Therocephalians are a species-rich clade and an eco-morphologically varied group ranging from the middle Permian to the Middle Triassic, with their highest diversity radiating in the late Permian. Although most therocephalian species went extinct during the P-Tr extinction, those that survived were successful in radiating out into newer species. Therocephalians’ temporal range was from 265-243mya.
Older more primitive therocephalians before the P-Tr extinction like, Glanosuchus (Glan-o-sue-cuss) from 265-254 mya had already featured mammalian anatomical characteristics such as the reduction of phalanges (finger and toe bones) that became a mammal phalange trait and the primitive development of a middle ear including the malleus, incus and stapes bones with a very thin plate of bone that acted as an eardrum receiving sounds from the middle ear fluid and transferring those sound waves to a small air-filled cavity.
The more derived therocephalians as in the Early Triassic superfamily, Baurioidea (Bar-ree-oi-dee-ah) lacked the ossified postorbital bar separating the orbit from the temporal opening as witnessed also in being absent in all primitive mammals.
Therocephalians invaded all ecologies in most being terrestrial, while others were fossorial, arboreal and even semiaquatic as evidenced by sclerotic rings that stabilized the eye under water pressure along with strongly developed cranial joints supporting the skull when consuming large fish and aquatic invertebrates.
Therocephalians are further divided into three subdivisions separating Eutherocephalia (U-there-oss-seph-ah-lee-ah) from the basal families, Scylacosauridae (Sill-va-kah-sawr-e-day) from 265-249 mya as a sister group and Lycosuchidae (Lye-ko-sue-chi-day) from 265-260 mya as a cousin to the eutherocephalian clade. Eutherocephalians are the majority of therocephalians.
Eutherocephalians ranged from 260-242.5 mya. With a few sister groups of eutherocephalians during the Late Permian’s ‘Wuchiapingian Stage’ were effective in establishing southern Gondwana and Laurasia dispersal routes that persisted in Pangaea. Due to the long skulls and dentition, the term Eutherocephalia means ‘true beast head’.
|Artist: SmokeyBjb Purlovia|
Purlovia (Purr-lo-vee-yuh) from Late Permian Russia 259-252.3 mya is a holotype genus, but is classified under the eutherocephalin family, Nanictidopidae (Nah-nic-tuh-dop-ah-day) along with its closest relative, Nanictidops (Nah-nic-tuh-dops) from South Africa. Its fossil was collected from the ‘Vyazniki’ tetrapod assemblage in a composite of former terrestrial intermittent floodplains.
Purlovia possessed long canines in both the lower and upper jaws with the rest of the buccal (cheek) dentition being much smaller. The jaw muscles were very thick and robust with the mandible (lower jaw) curving slightly upwards. Aside from these mouth features, Purlovia was an herbivore. Rather small at 0.6m/2ft, its triangular long head was nearly a third the size of the body at 20cm/7.9in long. The fossil skull is triangular shaped when observed from above with the postorbital, the region behind the orbitals (eye sockets), nearly half its length. Purlovia had long phalanges toe/finger bones for its body size. Unlike other more advanced therocephalians, Purlovia lacked a secondary palate.
|Artist:Matt Celesky G. masutinae after Suminia|
As a sister taxon to eutherocephalians, the primitive therocephalian, Gorynychus (Gor-e-nye-cuss) lived in the mid-late Permian 265-252.3 mya. The genus harbors two species in, G. masyutinae (mazz-e-ut-e-nee) and G. sundyrensis (sun-dire-in-sis). The genus fossils come from Russia in the claystone/sandstone ‘Urpalov Formation’ of the ‘Vanyushonki Member’s lower red beds. The genus is ancient Greek with the name referring to ‘bloody claw’. It was named for the Russian dragon, ‘Zmey Gorynych’.
Gorynychus was definitely a transitional therocephalian with a mixture of primitive lycosuchid and scylacosaurid traits (which are the earliest known therocephalians) and derived eutherocephalian characteristics. The distinguishing trait of Gorynychus separating the genus from all other therocephalian species is its autapomorphic dental morphology possessing coarse denticulated incisors and postcanines.
|Artist: DiBgd Gorynychus spp.|
With large incisors and G. sundyrensis even having double maxillary incisors on both sides, Gorynychus was definitely the apex predator during its time in the Permian ‘Kotelnich’ ecosystem. At around 1.6m/5.25ft in length, it was the largest predator and could subdue any terrestrial, arboreal or semiaquatic prey that was represented in the ‘Kotelnich’ fauna. The only competition it would’ve had would have been the very basal gorgonopsian, Nochnitsa (Nock-neet-zah) that was one of the smallest and most primitive gorgonopsians at ~ 0.76m/2.49ft and was nocturnal while Gorynychus was diurnal.
The Triassic began 251.9 mya with an impoverished biosphere due to the P-Tr extinction where some 83% of all genera became extinct and is known as the largest mass extinction of insects. This is why this extinction event is known as the ‘Great Dying’. Regardless that the Triassic is the shortest period of the Mesozoic Era, the therapsids and archosaurs that survived the extinction event, had given rise to mammals, dinosaurs and pterosaurs by the time of the Middle Triassic. The supercontinent Pangaea’s hot and dry climate during the Early Triassic had begun to give way to a more humid climate as the massive continent of Pangaea began to break up into the smaller continents of Laurasia to the north and Gondwana to the south by the Late Triassic 215 mya.
|Artist:Julius Csotonyi Jurassic Period|
During the Triassic, vascular plants like lycophytes, ginkgophytes, cycadophytes, ferns, horsetails and glossopterids had survived the extinction event, but were being outclassed by the emerging spermatophytes (seed plants) such as, conifers and bennettitales. The seed fern, Glossopteris dominated Pangaea’s southern hemisphere. For marine life during the Triassic, secondary endosymbiotic algae were the most abundant plankton. Modern types of corals first appear in the Triassic where ammonite species were hit hard, but soon recovered and rebounded during the Triassic. Very few fish families survived the extinction event, but remained uniform in what did survive while many aquatic reptile species arose during the Triassic such as the nothosaurs, plesiosaurs and ichthyosaurs. Alas however, as so it began with an extinction event, the Triassic also ended with one 201.3 mya going into the Jurassic that greatly reduced the therapsid species numbers, particularly herbivorous kannemeyeriid dicynodonts and the traversodont cynodonts.
Cynodontia (Sin-o-don-tee-ah) is Latin in reference to ‘dog teeth’. It is a major clade that led to basal cynodonts that is a sister taxon to epicynodonts and another main sub clade, eucynodonts (true dog teeth) that contained the sub clades cynognathians and probainognathians that finally led to ancestral mammals. As the major clade, cynodonts are still with us in mammalians.
|Artist: Nobu Tamura Procynosuchus|
The most primitive of cynodonts was the 260 mya, Procynosuchus (Pro-sin-no-sue-cuss) with the Greek name meaning, ‘before dog crocodile’. At 60cm/2ft, it not only appeared like an otter, anatomically, it alludes to the fact that it was built for a semi-aquatic lifestyle, for its wide zygapophyses (vertebra articular processes) allowed for anguilliform locomotion (water propulsion) and a high degree of lateral flexibility. The tail was also long for a cynodont which would have been useful for undulating through water. One primitive feature was its splayed legs. Procynosuchus originated from what is now South Africa, but had radiated out into Pangaea’s Permian Zambia and Germany.
|Artist: WillemSvdMerwevez Charassognathus|
Basal and primitive cynodonts like, Procynosuchus and Charassognathus (Cha-rass-o-nay-thus) from 259.1 mya did not lead to mammals but instead, all went extinct by 247 mya. The eucynodont, cynognathians went extinct during the Late Jurassic 150 mya, but showing up during the Early Triassic are the longest living cynodontians. If man doesn’t cause the extinction of mammals, including himself, perhaps they will outlast all cynognathians. Aside from mammals, all other probainognathian cynodonts went extinct by the Early Cretaceous 125.3 mya.
When compared to other Permian/Triassic fauna, cynodonts were small to mid-sized with an average length of 0.76m/2.5ft. The largest cynodonts were in Cynognathus (Sin-no-nay-thus, or, Sigh-no-nay-thus) at 1.2m/3.9ft, the more derived Trucidocynodon (Tru-see-doe-sin-o-don) at 1.3m/4.2ft, the traversodontid herbivore Exaeretodon (Ex-sair-ret-o-don) at 1.8m/5.9ft and Diademodon (Dee-ah-dem-o-don) at 2m/6.6ft in total length.
Most basal cynodonts at least, were hairy/furred and warm blooded, but still a tetrapod that laid eggs. However the more derived had developed epipubic bones. Originally for cynodonts epipubic bones developed to support abdominal and hind limb muscles for a more erect gait and greater ease of locomotion. However, the epipubic bone came to function later as an abutment for the female pouch by coming up from the pelvis along the ventral surface and that is why it is sometimes referred to as the marsupial pouch bone. So, like today’s opossum, the cynodont epipubic bone’s original service to gait and stride was at the expense of prolonged pregnancies where highly altricial births became the result as in extant marsupials. Marsupials do have a placenta, but due to the epipubic bones, it does not last very long, thus ending in premature births. In having longer full term pregnancies with the placenta in place, only full-term placental tetrapods reversed evolution in the devolution of the bone.
|Placode cells on fetus|
The scales, feathers and hair story all share the exact same beta-keratin origins, so it is not too difficult to imagine that one evolved replacing the other. They are all wholly composed of proteinaceous and structurally fibrous keratin. In the embryonic state all three are derived and developed from a collection of cells known as anatomical placodes. Placode cells express the same developmental genes in whether expressing reptilian scales, bird feathers or mammal fur/hair. This tells scientists that there was once a common ancestral origin for all three. Placodes first produce scales then leave the embryo by dying off, but when genetically left turned on, rather than producing scales, will later produce hair or feathers. Just to let ya know here, fur is the exact same thing as hair. We only call it hair when discussing humans; on all other mammals we express it as fur.
All primitive or derived cynodont dentition (teeth) were fully heterodont (differentiated) and supported a braincase that bulged outwards at the back of the head. Both these features are characteristics of mammals.
|Dvinia skull and model|
All basal cynodonts ancestrally were carnivorous, but later more derived cynodonts in the Late Permian such as the 50cm/1.6ft, Dvinia (D-vee-nee-ah) were omnivorous, while Middle Triassic cynodont traversodontids and Late Triassic cynodont tritylodontids evolved into herbivores from earlier carnivorous cynodonts.
|Artist: Viergacht Progaleasaurus|
The cynodont, Progalesaurus (Pro-gal-e-sawr-us) occurred 252.7-251.2 mya appearing just before the P-Tr extinction only to disappear during it. The name implies ‘before galesaurus’, where Galesaurus (Gay-luh-sawr-us) is Greek meaning, ‘weasel lizard’. This is in relationship to the more basal Progalesaurus in regards to the more derived Galesaurus where both belong to the same family of galesaurids as does Cynosaurus (Sin-o-sawr-us).
Coming from the ‘New Lootsberg Pass’ in the ‘Karoo Basin’ of South Africa with only one fossil discovered, Progalesaurus is a holotype. It was a small carnivore at ~ 60.4cm/2ft long. The skull was only 9.35cm/3.68in in total length and had remarkably large nares (nostrils) for early cynodonts.
Progalesaurus’ discovery increases the number of valid Early Triassic cynodonts to four, shedding light on the tempo of early cynodont diversification after the end-Permian mass extinction. Therefore, has been placed at the base of Epicynodontia outside of the more evolved eucynodont members.
Cynodontia is further divided into the stem based infraorders Epicynodontia (Epp-e-sin-o-don-tee-ah) and Eucynodontia (U-sin-o-don-tee-ah). Eucynodonts are further divided into the two major subdivision clades, Cynognathia (sin-o-nay-the-ah) and Probainognathia (Pro-bain-o-nay-the-ah).
|Artist: DK Thrinaxodon|
The epicynodont, Thrinaxodon (Thry-nax-o-don) had a temporal range of 251-247 mya during the Early Triassic. Its fossils come from South Africa and Antarctica when Africa and Antarctica were conjoined during Pangaea. Just after the P-Tr extinction it was one of the few carnivores that had survived and perhaps its fossorial burrowing habits was a main attribute in its survival. Although one of the largest carnivores during its time due to the extinction event in killing off most large carnivores, its length was only ~ 89cm/35in.
With short limbs and a more basal sprawling gait better suited for tunneling than running, Thrinaxodon was an intermediate between more basal therapsids and mammals in displaying primitive features while adopting more derived ones. In addition to the slightly primitive splayed limbs, it also exhibited canine dentition replacement if one was lost or broken, while juveniles possessed a greater number of teeth than adults. Juvenile fossils also displayed parasphenoidal dentition which is unusual for therapsids in general, but has been exhibited in gorgonopsians as well. Parasphenoid teeth define that anterior vomerine teeth are aligned transversely, while teeth on the posterior vomers, as supported by the parasphenoid bone, are arranged in longitudinal rows and increase in number posteriorly. Additionally, teeth are added laterally and shed medially.
The many derived features this 251 million-year-old epicynodont brought to the forefront displays numerous mammalian characteristic traits. The snout near the nostrils and maxilla bones were pitted insinuating whiskers and containing a secondary palate which separates the nasal passages from the rest in the mouth of the upper jaw, would have given Thrinaxodon the ability to breathe uninterrupted, even with food in its mouth.
Mastication (chewing) was an apparent greater development of the temporalis muscle [muscle running from the side of skull to the back of the mandible (lower jaw) involved in closing the mouth and chewing] relative to the masseter muscles (muscles running through the rear part of cheek from the temporal bone to the lower jaw on each side closing the jaw in chewing), indicating a strong posterodorsal movement of the mandible. Also, Thrinaxodon juvenile fossils show one of the first occurrences of permanent adult replacement teeth that will erupt under baby teeth in cynodonts.
A main mammalian character trait shown by Thrinaxodon burrows that had caved in burying the occupants is parental care. In these fossilized burrows the occupants were of differing ages from younger to older juveniles and adults; in other words, these were families. Being in groups like this with various ages shows familial nits.
Ribs were also absent in Thrinaxodon’s abdominal region, making it more flexible living in burrows to turn the thoracic half of the body around in tight fittings. The lack of abdominal ribs also hints to endothermy (warm-bloodedness) that will be explained further next under Cynognathus. Also, for a fossorial lifestyle, the 55.5cm/35in body was much longer than wider, while the tail was shortened.
Even though Thrinaxodon had a slightly sprawling posture, its distal medial femoral condyle (the knee ball joint end of femur located on outside perimeter) articulates with the acetabulum in a way that permits the hind limb to present itself at a 45-degree angle to the rest of the body. This makes a large difference in comparison to the distal medial femoral condyle of pelycosaurs that only allowed the femur to be parallel with the ground. This forced pelycosaurs to assume a sprawling-like posture, where Thrinaxodon could utilize a more upright posture.
|Thrinaxodon and Broomistega fossil|
One really unusual Thrinaxodon fossil is what was found inside a preserved burrow in South Africa’s ‘Karoo Basin’ which was composed of floodplain sandstone. It is of a Thrinaxodon individual huddled together with a juvenile temnospondyl amphibian, Broomistega (Brew-me-steg-ah). At first it was a mystery how this odd couple could have been together in death, but with further study on a finished synchrotron radiation scanning, a scenario came out.
Since it was a filled-in burrow, it was the Thrinaxodon individual’s burrow and the semi-aquatic Broomistega was the guest. Whether the amphibian was invited in as some form of symbiotic relationship, or merely barged in and could not be evicted by the epicynodont, the story began to unfold. The two rested together side-by-side with the epicynodont on its back and the amphibian also belly-up. Both were forced and pressed against the burrow’s sides but there was no stiffening of the bones to suggest rigor mortis.
This pressing together against the burrow’s walls gives away clues that some force entered the burrow and with disarticulated sediment filling the tunnel that force was floodwaters. However, the amphibian was not swept into the burrow; it had already entered, on his own accord but had been injured beforehand with broken ribs that were in the process of healing. It also had two bite marks on the skull roof, but did not match the epicynodont’s dentition. This was not a predator prey relationship. During this period in the region, the Triassic climate was very hot and the Thrinaxodon was most likely aestivating in his burrow when the Broomistega entered to seek refuge from the heat and recuperate from its wounds.
|Credit: Gfycat Thrinaxodon/Broomistega|
Eucynodont species saw the extinction of basal cynodonts, including the rise and extinction of cynognathians and proto-mammals to the arrival of true mammals. As previously stated, Eucynodontia as an infraorder has two major subdivision clades as the earlier, Cynognathia and the more derived, Probainognathia.
Cynognathians are the first of two major subgroups under Eucynodontia. A common cynognathian synapomorphy was a very deep zygomatic arch that extended above the middle of the orbit. They lived from the Early Triassic into the Early Jurassic. Cynognathian speciation exploded and radiated out just after the P-Tr extinction in evolving new adaptations to climatic and environmental conditions then quickly filling the niches left by species that didn’t survive the extinction. However, during the Middle Triassic cynognathian species began going extinct, but a few like, Scalenodontoides (Skale-in-o-daunt-oi-deez) managed to hang on until the very Early Jurassic before going extinct 201.6 mya. Although lately, I’ve heard a few paleontolgists’ mumblings that Scalenodontoides was not a cynognathian, but instead remained phylogenetically as a therocephalian.
Cynognathians are divided under the stem clade Gomphodontia (Gom-foe-don- tee-ah) as a sister taxon to Cynognathus which gomphodontians are further divided under the families, Diademodontidae (Dee-ah-dem-o-don-tuh-day) from 247.2 to 202 mya in the Early to Late Triassic, Trirachodontidae (Tri-rik-ah-don-tuh-day) from 250-237 mya during the Early to Middle Triassic and Traversodontidae (Truh-vers-uh-don-tuh-day) from 242-201.6 mya during the Middle Triassic to Early Triassic.
|Artist: Julius Csotonyi Cynognathus|
The basal cynognathian, Cynognathus (Sin-no-nay-thus), as stated earlier was a larger cynodont from snout-to-vent at 1.2m/3.9ft and was anatomically built like a robust stout dog. The skull made up 30% of the total length. Originating from South Africa in the Early Triassic 247 mya it had spread out into Tanzania, Zambia and throughout Pangaea’s southern hemisphere with fossils found in what is now, Antarctica, Argentina and China. As Pangaea began drifting apart, its southern portion became hot and dry killing off plants and herbivores. With less prey, this predator by the Middle Triassic 237 mya had gone extinct.
As a carnivore, Cynognathus had a mouthful of teeth with wide jaws. Incisors were for nipping, canines were for tearing and cheek teeth for chewing. Variation in the teeth and jaw muscle arrangements showed that it was capable of chewing junks of flesh before swallowing. It also possessed sectorial postcanine teeth with two serrated cusps distal to a recurved apex. For these cusps, just picture a middle long slightly curved cone with two much smaller cones on each side. The hind legs were directly underneath the body, while the forelimbs were slightly sprawled.
|Serrated cusps distal to apex|
The lack of belly ribs, in the stomach region, alludes to the presence of an efficient diaphragm, an important muscle for mammalian breathing. This is just another element that showed an evolving trend toward mammalian characteristics in cynodonts. Cynognathus ribs didn’t extend into the abdomen, which allowed room for a diaphragm creating greater respiration abilities and also would’ve made the lower body more flexible.
Trirachodontid cynognathians all come from the Triassic southern African continent and China, but just after the P-Tr extinction event quickly spread out over Early Triassic geographical regions. Possessing wide skulls and short narrow snouts, the synapsid temporal fenestrae were large and wide running along the back of the skull.
The Early and Middle Triassic climate was semi-arid with seasonal rainfall where many trirachodontids were fossorial burrowers. They were carnivorous and/or insectivores. Trirachodontididae is divided into two subfamilies which are, Trirachodontinae (Tri-rik-ah-don-tuh-nay) that is composed of three genera and Sinognathinae (Sign-o-nath-uh-nay) composed of two genera.
|Artist: SmokeyBjb Trirachodon|
The trirachodontid trirachodontine, Trirachodon (Tri-rik-o-don) in Greek refers to ‘three ridge tooth’ and for good reason. Wow…a lot of ‘trirach’s’ just listed there, huh… Anyway, back to the topic at hand. Trirachodon dentition possessed gomphodont canines that were neomorphic (an altered gene mutation possessing a novel pattern of gene expression), while the cusps of the postcanines were not homologous with the cusps of the sectorial teeth. Sectorial teeth, sometimes referred to as carnassial teeth, are the upper and lower molars modified to allow enlarged and self-sharpening edges to pass by each other. With these teeth it dieted as an omnivore on large insects, small vertebrates and certain vegetation.
Trirachodon’s ~ 249-244 mya fossils were found in the African nations of Namibia in the ‘Omingonde Formation’ and South Africa in the ‘Beaufort Group’. At 50cm/1.6ft, it was rather small. From histology studies of the fossil bones, all the bone elements consists of a moderately vascularized, periodically interrupted and fibro-lamellar bone tissue suggesting that the overall growth of Trirachodon was rapid during favorable seasons, but decreased or ceased during the unfavorable ones. As the environment during these Triassic times of semi-aridity with seasonal rainfall, Trirachodon was sensitive to such environmental fluctuations. As in Thrinaxodon’s case, Trirachodon most likely aestivated in burrows.
Also as Thrinaxodon was living in seasonal floodplains, so too, did Trirachodon and in being communal, it gave and preserved a slew of fossils as twenty individual fossils were found at one dig site that were originally drowned then buried by flood sediment debris. Trirachodon not only lived in burrows, it built them in complex multiple layers much like community living with the main tunnel leading to separate chambers. Scratch marks throughout the tunnels have even been preserved. This was a very social animal. Trirachodon may have constructed more elaborate burrows than Thrinaxodon, but of course they had a few million more years of practice to do so.
|Artist: FinwalSMD Beishanodon|
The trirachodontid sinognathine, Beishanodon (Bye-shan-o-don) had a temporal range of 251.3-247.2 mya with the fossil found in the Gansu Province of China’s Early Triassic ‘Hongyanjing Formation’ concerning the portion formed during the Triassic’s ‘Olenekian Epoch’. Discovered in 2010, Beishanodon is the second sinognathine genus to be found in China with the first being Sinognathus (Sign-o-nay-thus). Both were assigned as sinognathines based on their dentition characters. The placement of Beishanodon in the subfamily taxon of Sinognathinae was based on the ovoid-elliptical outline of the upper postcanines and a long axis of the postcanine tooth row directed toward the center of the subtemporal fenestra. As China’s sinognathine trirachodontids, what the Beishanodon and Sinognathus fossils have in common with Africa’s trirachodontids is their large skulls evolved convergently as compared to their small upper postcanine widths. Beishanodon most likely was an herbivore feeding on leaves, roots and stems.
Traversodontidae, as a sister taxon to trirachodontids is a large family clade of herbivorous gomphodontian cynognathians. They ranged from the Middle Triassic to the Early Jurassic 242-201.6 mya in what are now Africa, N. America, S. America, Europe, India and Russia. The family has eight monotypic genera and one polytypic genus in having two species and an additional two subfamilies in Massetognathinae (Mah-say-toe-nay-thu-nay) and Gomphodontosuchinae (Gom-foe-don-to-sue-she-nee). There are at least three genera assigned to Massetognathinae, while Gomphodontosuchinae has at least five genera.
Traversodontids, as gomphodont members, had short skulls ending in a snout that was much narrower than the back of the skull. The snout tip flared out making it wider than the middle portion of the snout. Traversodontids were herbivores with teeth attuned for masticating vegetation. Set behind the large canines, the wide postcanines were closely spaced, so much so, the crowns touched one another. The maxilla (upper jaw) tooth rows are inset while the maxillae and zygomatic arches extend outward, which is highly suggestive of traversodontids in having cheeks. This family name comes from the buccolingually (affecting the cheek and tongue) expanded postcanine teeth, which are linked to an herbivorous or omnivorous diet.
Andescynodon (An-deez-sin-o-don) as the most basal traversodontid gomphodont had a temporal range of 242.8-240 mya from the Middle Triassic of Argentina. Discovered in the ‘Cerro de Las Cabras Formation’. As a tectonically altered region, the formation also exposed megaflora fossils, such as: two families of extinct seed ferns known as, Corystospermaceae and Peltaspermales, extinct members of the horsetail order, Equisetales and among other various floras, the extinct simple leaf genus Gontriglossa. As Andescynodon’s wide teeth suggests an herbivory diet, these may have been a few of the plants it dined on.
Andescynodon was ~ 97.3cm/38.3in in length while the femur was 4.3cm/1.7in, the humerus was 1.4in and the skull was 27.2cm10.7in. The reason I put in hind limb, forelimb bones and the skull length is because one can give a good estimate of the fossil animals’ total body length through allometric (allometry: change in organisms in relation to proportional changes in body size) equations if the fossil is disarticulated and missing anatomical components.
So, in examining skull length variables in relation to total skull lengths versus limb variables, one can use the allometric equation:
log y = log bₒ + b1 log x + log e
The equation utilizes base 10 logarithmic calculations once deriving the power growth equation » y = log bₒ xb1e. With this, bₒ is the y-intercept, b1 is the line’s slope and e represents a probable multiplicative error. For some species with no total length data, I had to utilize this with Andescynodon being one of them. Although it is interpolation, at least it is not guesstimation. For those that enjoy allometric equations, here ya are; for those that don’t, just skip it.
The postcranial (all parts of the skeleton except for the skull) bones of Andescynodon are similar in their structure to Diademodon’s body anatomy. A trait dissimilar to traversodontids is that Andescynodon had four incisors on each side of the upper jaw (maxilla). Also distinctive from other traversodontids was the diameter thickness in its femoral walls which was 29% of the cross-sectional diameter. The femur as well had a number of radially orientated vascular canals within the thickened bone walls. This is analogous to the thickened femur walls of Thrinaxodon and Trirachodon that were burrowers. Thick femoral walls are also indicative of extant burrowers like the naked mole rat, porcupines and digging lizards such as the Gila monster. However, other than that there are no other anatomical features that would suggest it was a burrower.
The extinct subfamily, Massetognathinae under Traversodontidae contains three genera living during the mid to latter Triassic, where four species of them share the genus name, Massetognathus (Mass-eh-tog-nay-thus), while the two other holotype genera are Dadadon (Da-da-don) and Santacruzodon (San-tah-cruz-o-don). Massetognathines are a sister taxon to gomphodontosuchines.
The four species of Massetognathus are M. terrugi (tuh-rue-ğee), M. ochagaviae (o-cha-gah-vee-ah), M. major (may-jor) and the best known, M. pasculi (pas-quall-lee). All species come from the ‘Chañares Formation’ in Argentina and the ‘Santa Maria Formation’ in Brazil. Massetognathus species lived 235 mya and averaged in length ~ 46cm/18.1in. With the genus name meaning, ‘chewing muscle jaw’ they had incisors, fang-like canines and flat-topped molars covered by low ridges, as herbivores, an adaptation for grinding tough plant stems, tubers and roots.
The differences in the species is that M. terrugi was the most common Chañares terrestrial tetrapod with a rather large skull possessing a more defined sagittal crest. M. ochagaviae, as the most common Santa Maria species: had a higher skull that lifted up as well as consequently lifting the mandible; had a slight dorsally pointed dentary ventral border under the coronoid process and was a cynodont species during its time with the least amount of postcanines. However, the base lateral to the labial margin of upper postcanines extended outward forming an isosceles triangle in an occlusal view. M. major had the largest skull of the four species with a narrow snout possessing fewer teeth than the other three Massetognathus species. Finally, M. pasculi is considered the only valid species for Chañares gomphodonts. Its rotation of the dorsal plate relative to the trochlea exhibited a progressively greater rotation that’s closely related to mammals.
There is direct fossil evidence that Massetognathus species laid eggs; evidence of detailed bone structures indicating that they were endothermic (warm blooded); had dog-like tails, claws and a body covered with hair.
As the other traversodontid subfamily, Gomphodontosuchinae, it was devised to include the distinct dentition relations between six traversodontid genera. The six genera are: Gomphodontosuchus (Gom-foe-don-to-sue-cuss), Menadon (Men-ah-don), Exaeretodon (Ex-ear-ret-o-don), Scalenodontoides (Skale-in-o-daunt-oi-deez), Protuberum (Pro-to-beh-rum) and Ruberodon (Rue-burr-o-don). These six genera had an external crest on the surface of the tooth crowns that functioned by shearing plant food items, while the internal mouth basin facilitated crushing. This dentition configuration of traversodontid gomphodontosuchines represents a vast evolutionary step towards improvement in assimilating plant food processing. Gomphodontosuchine fossil remains come from 235-205.6 mya in what are now India, Brazil, Argentina and Madagascar.
|Artist: Voltaire Paes Neto Menadon in front|
The gomphodontosuchine species, Menadon besairiei (buh-sair-e-eye) had a temporal range of ~ 232-228 mya with fossils found in Brazil and Madagascar. This is not an unusual circumstance to have the same species found in different parts of the world thousands of miles away from each other. At the time Madagascar was attached to the eastern portion of Africa and Brazil along with eastern South America attached to the western portion of Africa during the existence of Gondwana. So, even though they have not been found yet, Menadon fossils should turn up somewhere in Africa.
Menadon possesses a deep snout that held four upper incisors with the first and second incisors lying horizontally with the three lower incisors also lying horizontally. Unique among traversodontids is that Menadon’s had hypsodont (high crowned) postcanines. In counteracting high wear from masticating abrasive plant roughage, the hypsodont teeth grew continuously much like sloths. At ~ 96cm/37.8in in length, the body was squat with short limbs holding it up. Anatomically in life Menadon kind of looked as if it had the face of a wombat attached to the body of a hairy snapping turtle minus the shell.
Probainognathians belong to the other major clade of cynognathians that had a temporal range of 247.2-0 mya during the ending of the Middle Triassic to the Holocene’s present. The tritheledontids and tritylodontids are the two most derived probainognathian groups that survived the Tr-J extinction event and it is within these two members that eventually lead to mammals. These Late Triassic-Early Jurassic probainognathians bridge the gap between the most derived theriodonts with the most primitive mammalians.
By the time probainognathians had evolved, the external and internal morphology of endothermy, hair/fur and whiskers (otherwise known as vibrissae in scientific circles) were well established in eucynodonts. To further express, from the most basal, it appears through the fossil record that all cynodonts had evolved endothermy (warm bloodedness) and had fur and whiskers, so please allow me to elaborate a bit on the evolvement of whiskers, which are actually hair seated in stimulus nerve endings. The pits on cynodont and earlier therapsid snouts are good indicators whiskers had evolved, not all at once but in stages.
Deposited sediment material from 300-180 mya that is layered and stratified in South Africa’s ‘Karoo Basin’ is rich in synapsid fossils. Due to the treasure trove of therapsid remains, there is a chronological timetable from older to younger species groups. From well preserved fossil skulls possessing whisker pits and maxillary (upper jaw) nerve canals supplying sensorial stimulation for whiskers are evidence of an evolvement taking place from rudimentary whisker sensing to a more pronounced sensing.
By scanning with X-ray computed tomography (CT scanning), Dr. Julien Benoit along with Professors Paul Manger and Bruce Rubidge developed 3-D images of various therapsid cynodontian snouts. After the 3-D modelling, what they found is that already in place in these extinct animal skulls, was a maxillary canal that once housed the trigeminal nerve responsible for facial sensation and motor movement, but also functions as a sensory tool for mammals with whiskers.
As viewed in the illustration above, the maxillary canal and trigeminal nerve were already functioning assets for stem (basal) therapsids to the most derived prozostrodontians and mammals. Prozostrodontia (Pro-zoh-stroh-don-tee-ah) is a cynodont probainognathian clade including mammals and their closest non-mammaliform relatives. In the illustration, please note how the trigeminal nerve (in green) bulks down becoming more streamlined and reduced until reaching its current position in prozostrodontians and extant mammals. MSX2, the same gene in eucynodonts that promoted a large cerebellum and the loss of the reptilian parietal foramen by totally ossifying the skull roof, also is responsible in giving rise to eucynodont hair, whiskers and mammary glands.
There are around twelve basal probainognathian genera in: Lumkuia (Lume-ku-e-ah), Ecteninion (Eck-ten-nee-nee-on), Aleodon (Al-e-o-don), Chiniquodon (Chin-nuh-quo-don), Gaumia (Guh-me-ah), Kunminia (Kun-min-nee-ah), Probelesodon (Pro-be-ell-so-don), Probainognathus (Pro-bain-o-nay-thus), Trucidocynodon (Tru-sid-doe-sin-o-den), Diegocanis (Day-go kay-niss), Therioherpeton (Thar-e-ah-her-puh-ton) and Prozostrodon (Pro-zoh-stroh-don). There are three families of probainognathians in Tritheledontidae (Tri-thell-uh-don-tie-dee), Brasilodontidae (Bra-sil-o-don-tie-dee) and with the final family Tritylodontidae, (Tri-till-o-don-tee-day). Lastly, the subclass under Probainognathia is the clade Mammaliaformes (Mam-mal-e-ah-forms), which are the Mesozoic forerunners directly linked to true mammals.
It is trending to classify the genera: Aleodon , Chiniquodon, Gaumia, Kunminia and Probelesodon into the family, Chiniquodontidae (Chin-nuh-quo-don-tee-day) due to their close anatomical relationships.
The seven genera tritheledontids are in the following cladogram:
The two genera brasilodontids and four genera of tritylodontids are in the cladogram below, but keep in mind that there are five known brasilodontid genera and seventeen tritylodontids.
The current most basal and oldest probainognathian is Lumkuia, which during the Middle Triassic had a temporal range of 247.2-242 mya. Its fossils have been found in the ‘Cynognathus Assemblage Zone’ of the ‘Beaufort Group’ in the South African, ‘Karoo Basin’. At ~ 21.5cm/8.5in, it was small with the skull measuring only 6cm/2.4in. With the small size it was a carnivore/insectivore dieting on small vertebrates and large invertebrates, like insects.
|Artist: SmokeyBjb Lumkuia skull / head|
Living during the time of Cynognathus, Lumkuia was much more derived with teeth crowns that were high and narrow curving inwardly from the top and it lacked a pineal foramen (a skull roof opening for the pineal eye and nerve). Also absent were costal plates covering the ribs.
|Artist: Gabriel Ugueto Trucidocynodon|
Another basal probainognathian was, Trucidocynodon that lived during the Late Triassic 220 mya. Its fossils were discovered in Brazil’s ‘Santa Maria Formation’ showing the length to be 1.3m/4.2ft. In anatomical features, it was much like another basal probainognathian in Ecteninion that yet another new family, Ecteniniidae (Eck-ten-nee-nee-e-day) is trending, including these two genera along with the recent 2013 discovery of, Diegocanis.
This carnivorous weasel-looking, Trucidocynodon was rather large for cynodonts and it most likely was due to environmental pressures in competing with the divergence of larger contemporary South American rauisuchians and archosaurs. The limbs of Trucidocynodon not only stood erect underneath the body, but the forelimbs were digitigrade (walk on toes) and it is the first synapsid to exhibit cursorial limbs (anatomically built for sprint running). This would have allowed it to run down larger prey and in keeping up with other large predators.
As more derived than the basal probainognathians, tritheledontids (formerly known as ictidosaurs) were highly specialized cynodonts and were exceedingly more ‘mammal-like’ than their predecessors. Tritheledontids had a temporal range of 221.5-189.6 mya showing up in the Late Triassic and dying off in the middle portion of the Early Jurassic. Living only on Gondwana, their fossil remains only come from South Africa and South America.
Tritheledontids were small reaching lengths of no more than 20cm/7.9in and were insectivores/carnivores. As being a transitional bridge from earlier synapsids to mammalians in their jaw bones; in particular in the tritheledontid, Diarthrognathus, the jaw anatomy was primitive when viewing the joint between the quadrate and articular bones, but showed advanced mammalian features in the joint between the squamosal and dentary bones. The articular and quadrate bones were evolving to become middle ear bones and in the transition gave a stronger bite force.
As carnivorous cynodonts, tritheledontids are the divisional line between cynodont and mammaliaform evolution. With the jaw anatomy of the squamosal/dentary jaw joint and quadrate/articular joint as both already functional along with the masseter muscle (muscle running through the rear part of the cheek from the temporal bone to the lower jaw on each side closing the jaw in chewing) in conjunction with opposing muscles holding the jaw in a sling-type fashion, these are true mammalian features. If that isn’t enough to convince skeptics, in addition, tritheledontids had already evolved other mammalian traits such as a prismatic tooth enamel covering and broad cheek teeth. Of course, hair/fur/whiskers, warm-bloodedness and mammal respiration morphology was already in place.
Brasilodontidae family members had a temporal range of 252.3-205.6 mya throughout most of the Triassic. Brasilodontids are transitional basal cousins to tritheledontids and mammaliaforms. Besides the two genera, Brasilitherium and Brasilodon, there are three other genera taxa in, Minicynodon (Min-e-sin-o-don), Pachetocynodon (Puh-nash-toe-sin-odon) and Protheriodon (Pro-thear-e-o-don). All fossils come from South America except for Pachetocynodon fossil remains coming from India.
The tritheledontid, Riograndia had a temporal range of 225.42 mya with its fossils coming from the ‘Caturrita Formation’ of Brazil. The formation is composed mainly of clayey fluvio-lacustrine deposits yielding to more sandy facies with occasional gravel from a permanent year round braided river system. Riograndia had large olfactory bulb casts with a wider cerebral hemispheres region and cerebellum than found in other non-mammaliaform cynodonts. Considered the most basal tritheledontid, the total length of Riograndia was ~ 15cm/5.9 in and was a carnivore/insectivore.
|Artist:Jorge Blanco Lft:Brasilodon Rt:Riograndia|
The brasilodontid, Brasilodon lived contemporaneously with the tritheledontid, Riograndia in the same faunal ecology of Brazil 227-225 mya during the Norian epoch of the Late Triassic. It was an insectivore with a length only of 12cm/4.7in, while weighing ~ 20g/0.42oz. There were two species in: B. tetragonus (tet-rah-go-nuss) and B. quadranagularis (quad-drang-goo-lair-is).
As seen in other more basal probainognathians, the prefrontal bone, postorbital bone, and postorbital bar are absent in the two Basilodon species. The anterodorsally projected iliac blade (ilium extension) with a reduced postacetabular process, reduction of the pubis anterior and the medially located lesser trochanter is indicative of a mammalian pattern of pelvic musculature, by being functional enough to swing the femur in a nearly parasagittal plane (relating to a situated plane adjacent or parallel to the plane which divides the body into right and left halves). The stapedius (very small muscle stabilizing the smallest bone known as the stapes) is well developed as is the presence of several foramina indicating that the middle ear allowed Brasilodon to hear well.
Although highly derived cynodonts, tritylodontids were the last family of known ‘mammal-like’ synapsids, lasting up until the Early Cretaceous. Showing up in the initial portion of the Late Triassic, they had a temporal range of 251-113 mya. They appear to have quickly radiated out beyond their South Africa origins by the end of the Triassic with tritylodontid fossil remains being found in South Africa, the South and North Americas, Eurasia and Antarctica. There have been two new recent tritylodontid genera discoveries in: Shartegodon (Shar-teg-o-don) and Nuurtherium (Nu-ur-thear-e-um). Both fossils come from SW Mongolia’s Late Jurassic , 250.8-145.5 mya. Tritylodontids are noted at times as mammaliaforms.
Most likely descending from the Cynognathus lineage, tritylodontids had erect limbs, endothermy morphologies, and a mammalian skeletal anatomy, such as a high sagittal crest on the skull. Also in other derived mammalian traits, on the back of the skull existed huge zygomatic arches for the attachment of large jaw muscles and the secondary palate was well developed. The primary reason tritylodontids aren’t considered mammalian or even mammaliaform to some is that they retained vestiges of the reptilian joint between the quadrate bone of the skull and the articular bone of the lower jaw.
|Tritylodontid Kayentatherium skull|
Tritylodontid dentition differs from most cynodonts in that they did not possess canines and the front pair of incisors was enlarged much like extant rodents. There also existed a large gap between the incisors called a diastema separating the incisors from the square shaped cheek teeth. The upper jaw cheek teeth consisted of three rows of cusps running along the length of each tooth with grooves in between. The lower teeth with two rows of cusps fitted into the grooves of the upper teeth. This is how tritylodontids would grind food much like today’s rat, although unlike rats, tritylodontids had a palinal jaw stroke (front-to-back), instead of a propalinal one (back-to-front). Most tritylodontids were herbivores with some being omnivores. The name, Tritylodontidae refers to the dentition shape in meaning ‘three knob teeth’.
If you might recall the explanation of the pubic bone under Cynodontia, like mammaliaforms, tritylodontids possessed epipubic bones, which infers that they either laid eggs as monotremes do, or produced undeveloped fetus births like marsupials do. As well, tritylodontids possessed evidence of diphyodonty (an animal with two sets of teeth as in temporary baby teeth replaced by adult permanent teeth), a trait associated with suckling and therefore the production of milk. There are at least 17 genera of tritylodontids with their sizes ranging from 30-100cm/0.98-3.3ft. We’ll only discuss three out of the seventeen tritylodontids.
|Artist: Michael Long Oligokyphus|
As a tritylodontid, Oligokyphus (Ol-e-koe-phi-fuss) lived at the very end of the Late Triassic 205.6 mya, survived the Tr-J extinction, while the last species went extinct 283 mya during the Early Jurassic. There were five species in: O. biserialis (bi-ser-e-al-is) from 205.6-201.6 mya, O. triserialis (tri-ser-e-al-is) also from 205.6-101.6 mya, O. lufengensis (loo-fin-gen-sis) from 201.6-196.5 mya, O. major (may-jor) from 189.6-183 mya and O. minor (my-nor) also from 189.6-183 mya.
Each Oligokyphus species differed from other tritylodontids in distinctive dentition with minor differences in tooth cusps and skeletal anatomy from one another. All other known tritylodontids had two principle cusps on the lower postcanines, where in contrast Oligokyphus spp. possessed two longitudinal rows of three principal cusps and a posterolingual (relating to base of tongue) accessory cusp on the lower postcanines. O. lufengensis fossils found in China, England, Germany and Arizona, USA differed a bit from those South Africa Oligokyphus spp. in lacking the anterior cingulum on the lower postcanines. Oligokyphus was a terrestrial animal and its success in radiating out is a tribute to no terrestrial barriers like an ocean yet in existence separating the land regions as they do today.
There were no canine teeth as they were predisposed by the use of very large incisors. The upper and lower postcanine cusps fitted well together serving as an excellent means in masticating plant material and the teeth did not occlude (obstruct). Oligokyphus had a lateral extension of the maxilla replacing the absence of the premaxilla. As far as the body goes, total length was only 50cm/20in, but the body and neck were very flexible due to having an atlas (cervical vertebra supporting the whole head), axis (2nd cervical vertebra serving as a pivoting neck and head support) and a double occipital condyle (occipital bone protuberances functioning in articulation with the atlas vertebra superior facets). In fact, the family name means, ‘small curved animal’.
|Artist: Gabriel N.U. Dinnebitodon|
Dinnebitodon (Den-nah-bite-o-don) at ~ 28.7cm/11.3in was a small tritylodontid with its fossil remains being discovered in Arizona’s ‘Kayenta Formation’ from 189 mya. Other than dentition differences, it resembles its close contemporaneous tritylodontid relative, Kayentatherium (Kye-en-taa-thear-e-um). The Dinnebitodon fossil skull had three incisors on each side of the upper jaw, with the second incisor being large and well developed at 9mm/0.35in by 7mm/0.28in.
Along with the incisors, a young adult Dinnebitodon would’ve had five postcanine teeth with a sixth postcanine erupting in later life. The postcanine teeth shape resembled rounded-off squares with three rows of cusps on each tooth’s occlusal [teeth fitting relations between the maxillary (upper) and mandibular (lower) teeth] surface. With this type of dentition resembling extant animals that diet mainly on seeds, Dinnebitodon may have been more of a granivore rather than strictly a plant eating herbivore.
|Artist: S. Yamamato/H. Matsuoka Montirictus|
The final tritylodontid we’ll discuss here is Montirictus (Mon-tee-rik-tues) that in being the last known surviving tritylodontid living well into the Cretaceous, it had a temporal range of 140.2-136.4 mya. Montirictus’ fossil remains come from the ‘Kuwajima Formation’ of current day, central Japan. The total length of Montirictus was 40cm/15.8in. It also was a bit sturdier and heavier than most tritylodontids weighing around 2.2kg/4.9lb. In Montirictus’ timeframe it coexisted with derived dinosaurs, along with now extinct turtles, lizards and the arrival of the first marsupials.
Its fossil finding disproves thoughts that once mammals appeared they out-competed tritylodontids creating their extinction by the end of the Jurassic. Most likely in still laying eggs, it is an attribute that a tritylodontid could reach a life span down into the Cretaceous. The upper cheek teeth were composed of sub-equal cusps and buccal/lingual cusps that were crescentic shaped. As well, both buccal and lingual anterior ridges along with V-shaped buccolingual cross-sections of two anteroposterior grooves between the three cusp rows were present.
The Jurassic Period spanned 56 million years from 201.3-145 mya. At the beginning of the Jurassic, the supercontinent Pangaea had already begun rifting into two landmasses with Laurasia to the north and Gondwana to the south. Both these landforms became separated by the newly formed Tethys Sea. The rifting created more coastlines and shifted the continental climate from dry to humid, replacing many of the Triassic arid deserts with lush rainforests.
|Artist: Evgeny Dvoretckiy Jurassic Period|
During the Jurassic with rainy summers and dry winters, terrestrial animals became adapted to a seasonal climate while abundant water could be found in intermittent streams, permanent rivers, ponds and lakes. With a warm humid climate, lush jungles developed, especially in the higher latitudes. As the most diverse floral group, conifer species dominated Jurassic landscapes such as: araucarians (a genus of conifers) and trees from the Pinaceae and Taxaceae families. Shrubby cycadeoids dominated the lower latitude vegetation, while gingkoes and tree ferns made up most of the common forest canopies.
Terrestrial fauna became dominated by ornithiscian, saurischian and theropod dinosaurs. Evolving from theropods, the first avialan birds also appeared during the Jurassic. The appearance of the earliest lizards and the evolution of therian mammals, including primitive placental mammals also arose during the Jurassic. Numerous now extinct turtles inhabited rivers, streams and lakes. Crocodilian archosaurs made the transition from a terrestrial to a permanent semi-aquatic mode of life. The ocean coasts became inhabited by evolved marine reptiles such as ichthyosaurs, pliosaurs and plesiosaurs while aerially, pterosaurs were the dominant flying vertebrates.
Several massive batholiths emplaced the long ranging American Cordillera consisting of an almost uninterrupted sequence of mountain ranges forming the backbone orogeny of western North America, Central America and South America. During the Early Jurassic, the USA western interior was covered in the largest erg deposits (vast sea of sand) so far ever recorded geologically for Earth. Today, parts of this vast sand sea can be seen at Zion National Park.
The Jurassic began as the Triassic ended in the Tr-J Extinction collapsing coral reef communities and wiping out all conodont families in the oceans. On land, all less derived therapsids and the larger temnospondyl amphibians went extinct. Except for crocodilians and dinosaurs, all archosauromorphs died out. ~ 76% of all terrestrial and aquatic animal life met its demise. 60% of diverse pollen assemblages disappear. However, with the extinctions it allowed dinosaurs to reign supreme in filling ecological niches voided by the extinctions while aquatic numerous reptiles were able to swim the shallow seas.
In what was the main cause is still open for debate, but it appears as if the world’s largest volcanic event was the culprit, or at least had a strong supporting role in the extinction. With continental rifting going on, the ‘Central Atlantic Magmatic Province’ (CAMP) was being formed by pulsation of huge volcanism 201 mya. We know this because of its current location in the central portion of the North Atlantic Ocean. It created a massive upwelling of volcanic igneous rock covering an area of 11 million km2. The volcanic flow of ~2–3 × 106 km3 lasted for 600,000 years. This also is not only Earth’s largest region of volcanism; it is also Earth’s largest volume of volcanic flow. With the CAMP output of massive CO2 the climate heated up and the oceans acidified with the oceanic waters acting as a sink for the noxious gas converting carbon dioxide into carbonic acid.
The Path to Mammals:
Recent discoveries have shown that stem mammaliamorphs held a much wider distribution in time and geographical range from the Triassic/Jurassic transition to the Jurassic/Cretaceous transition than once believed. No matter the pronounced sauropsid dominance of size and strength during the splitting of Pangaea resulting in the formations of Gondwana and Laurasia, these little rats that could were very successful in speciation that ultimately led to today’s extant lot of mammals.
As mammaliamorphs, tritylodontids are really the kickstart culmination in the transitioning to mammalian traits. So, we’re going to look for a few minutes at 3-D model imaging of dentition microwear and mesowear of tritylodontid fossil teeth.
What these two wear studies’ references infer: the type of plant material herbivorous animals ate, how they masticated their diet and exactly how as herbivores they ingested food as either browsers or grazers. In addition to analyzing tooth type, where microwear detects fine wear damage in scratches and pits of the tooth surface, mesowear images the overall tooth wear from the original shape of the tooth. These microwear and mesowear dentition studies aids the paleontologist in what the ecology of the environment was during the extinct animal’s time and further sets the stage to clade relationships.
A 07/25/2019 published study on the microwear and mesowear by the researchers, Daniela C. Kalthoff, Ellen Schulz-Kornas, Ian Corfe, Thomas Martin, Stephen McLoughlin and Julia A. Schultz showed that tritylodontids were generalist feeders with none ever being a dietary specialist. In the microwear analysis, fine wear was predominant with numerous pits and to a lesser degree evident fine scratches. There was also coarser microwear evidence such as crude scratches, larger puncture pits and gouges alluding to episodic feeding on harsh food items or was due to exogenous effects (contaminant foods with soil grit and dust particles), or the wear could have been due to both episodic and exogenous conditions. Food items for this type of wear would have been softer vegetative components with low intrinsic abrasiveness, plant reproductive structures such as seeds and even insects; so possibly tritylodontids were not wholly herbivorous, but to a degree were also insectivorous. Behaviors that were ruled out were rooting for food, digging and caching behaviors.
Below is the scatter and pit plot of the researchers tritylodontid species findings. In the plot, the parameter symbols are: (SP) number of small pits; (LP) number of large pits; (FS) number of fine scratches; (CS) number of coarse scratches. Convex hulls embrace areas taken by taxa with N > 2; symbols represent individual means.
PC1 of the plot is the first principal component and explains ~ 74.5% of variances. PC1 features a high positive loading for the number of small pits. Kayentatherium wellesi and Dinnebitodon amarali plot furthest right on the scatter plot with the highest small pit values, followed by Oligokyphus major with somewhat lower values. Oligokyphus sp. from the USA and the two Russian species of Stereognathus have the lowest small pit values and plot at the left end of the scatter plot. PC3 shows a high negative loading above the cut-off for the number of fine scratches. With the highest values, D. amarali, Kayentatherium wellesi and the single tooth of Tritylodon longaevus tend towards the lower end of the scatter plot. Oligokyphus sp. (US) and O. major can be found at the upper end of the plot. PC2 is not shown, because character loadings were below the cut-off of 0.7, so its numbers did not show up on this graph as it was only negligibly influenced by the number of large pits.
Mammaliamorpha (Mam-mail-e-mor-fah) is a phylogenetic classification in ordering a broad clade consisting of the last common ancestor to tritylodontids and the crown group mammals (true mammals).
|Artist: Masato Hattori Adelobasileus|
Before we dive into mammals, I would like to give an identity to a rather small mammaliamorph known as Adelobasileus (Eh-dale-o-bah-sill-e-us) that existed 225 mya during the Triassic. This predates tritylodontids by a good 10 million years. With its fossil remains recovered from the ‘Tecovas Formation’ in West Texas, the find only consists of a partial skull. However, the distinct cranial features of the fossil skull, in particular of the cochlea (part of the inner ear shaped as a spiral cavity in the bony labyrinth, thusly intensifying hearing) highly suggests that Adelobasileus was a transitional form in the evolving process of cynodonts into mammals. Many paleontologists feel that Adelobasileus is the common ancestor, or at least a very close relative to the common ancestor of true mammals. But, currently with only scant fossil remains, it may be considered a mammal but outside the crown group of true mammals.
First off, mammals do not have the market cornered when it comes to viviparity, for live births have been around in the animal world for a long time in both extinct and extant species. Extinct and extant reptiles have been viviparous. Certain amphibian caecilians and fish species are viviparous.
So, how did mammalians transition from oviparity to viviparity; or put more simply…go from an eggshell (hatchling) to a placenta (live birth)? To begin with, there really isn’t that much difference between an embryo housed in an eggshell or a placenta and thusly, the egg contents of a human female is much like the egg contents of a chicken. The amniotic fluid functions pretty much the same in both, as a gravid eggshell or placenta protects the embryo and fetus from cushioning blows and facilitates the access of nutrients, water and biochemical processes to the embryo and latter fetus. Initially, the amniotic fluid is water derived from the mother, but eventually is replaced by the growing fetus’ urine. Yes, a throwback from our fish origins, we start out in a watery-liquid environment. In fact, the human fetus possesses gills for its first nine weeks before being spoliated and replaced permanently by developing lungs.
No matter the female amniote, vitellogenin (VTG) is a protein present in the amniote female’s blood, from which the substance of egg yolk is derived. Belonging to a particular group of several lipid transport proteins, VTG is classified as a glycolipoprotein in having the properties of a sugar, fat and protein. VTG precursors provide the major egg yolk proteins that are the source of nutrients during early embryonic development.
At first, marine vertebrates started off with a single copy of the vitellogenin gene, while the amphibian, reptilian, bird and mammalian lineages each experienced duplications that gave rise to more modern genes. A pseudogene is a section of a chromosome that is an imperfect copy of a functional gene creating a mutant copy. With the exception of monotremes, mammals have all their vitellogenin genes turned into pseudogenes that are a section of a chromosome that is an imperfect copy of a functional gene.
Evidence shows the placenta of mammals, including humans, evolved from much simpler tissues that attached to the inside of eggshells and enabled the embryos of non-mammalian and our distant ancestors, such as birds, reptilians and pelycosaurs, to have access to oxygen as the placenta does to mammalian embryos.
The allantois is the fetal membrane lying below the chorion in most vertebrates and is formed as an outgrowth of the embryo’s gut. So, it naturally functions as storing waste (chiefly uric acid). In egg-laying birds and reptiles it grows to surround the embryo, while in placental mammals it forms part of the placenta. Once the allantois has been established as an extension of the embryonic gut, the umbilical cord then partially develops from the allantois containing remnants of the yolk sac. Sufficiently formed by the fifth week of development, the umbilical cord replaces the yolk sac as the source of nutrients for the embryo and growing fetus.
That first membrane layering the inside wall of an egg enveloped by a shell is the chorion. The placenta primarily evolved from the chorion to act much like the eggshell in protectively enveloping the fertilized embryo and fetus while mediating the flow of gases and nutrients. The placenta in the beginnings of the first semester of pregnancy utilizes ancient eggshell cell growth genes, then later switches to its species-specific genes.
The placental membrane separates maternal blood from fetal blood with the fetal portion of the chorion. The maternal placenta component, which evolved much later with placental animals, is known as the decidua basalis. In a first phase of embryo growth, placental cells primarily activate genes that mammals have in common with birds and reptiles, suggesting the placenta initially evolved through repurposing genes the earlier mammals inherited from their immediate ancestors when they arose more than 120 million years ago. In a second phase, cells of the mammalian placenta switch to a new wave of pseudogenes that are species-specific. So, shrews activate newly evolved shrew genes from previous gene mutations where humans do the same in activating species-specific newly evolved human genes. That is what aids in making us, well…us.
The placenta is essentially an organ that fuses to the uterine wall to securely hold in place a pregnancy. But yet, it is the only temporary organ produced in mammals as it exits the female’s body upon giving birth, only to develop once again with a new pregnancy.
In tying all this together into a neat knot, globins are heme-containing globular proteins involved in binding and/or transporting oxygen, where heme is an iron-containing compound of the porphyrin class forming the non-protein part of hemoglobin. Today, globins are found virtually in all organism kingdoms. An ancestral hemoglobin gene most likely was present before the divergence of plants and animals 1,500 mya. One-celled life in the beginning was anaerobic, so oxygen was detrimental to it. It seems the initial function of oxygen-binding hemoproteins was to protect cells from oxygen once it had sufficiently appeared in the biosphere as a product of photosynthesis.
The function of embryonic hemoglobin, which appears early during mammalian development, very likely is to protect against free radicals derived from oxygen and nitric oxide, such as peroxynitrite. Hemoglobin has a high affinity for oxygen appearing well before there is an effective system of placental oxygen transfer. Therefore, globin evolution functions, such as the transport of oxygen to blood and intracellular metabolic activity, were acquired much later. The groundwork for this was laid down by whole genome duplications in the vertebrate lineage.
Lactation appears to not only be a eutherian (true mammal) trait as basal mammaliamorphs were already practicing infant nurturing via lactation millions of years before true mammals even appeared. Evidence of tritylodontids in having no teeth to have baby teeth erupt then be replaced by permanent adult teeth strongly suggests lactation. The more derived mammaliaform morganucodont fossils also display this form of tooth replacement. With this evidence, female milking is a mammalian ancestral trait.
Basal and more derived synapsid eggs had shells that were parchment-like, which made the eggs impervious to desiccation. However, the outside of the shells required moisture. Synapsid moms were already nurturing in that they would moisturize their laid eggs with a glandular skin secretion. Mammary glands evolved from these apocrine-like glands that combined multiple modes of secretion while developing in association with hair follicles. Through comparative analysis studies, the research showed the evolutionary origin of milk constituents is a result in which these secretions evolved into a nutrient-rich liquid (milk) long before mammals arose.
Along with the milk secretory constituents, a variety of antimicrobial and protein immunity components were co-opted into the milk. Calcium-binding phosphor-proteins originally had a role in calcium delivery to eggs and in addition, by evolving into large, complex casein micelles, they took on an important role in the transport of amino acids, calcium and phosphorus. As well, an ancestral c-lysozyme lost its lytic functions in favor of a role as α-lactalbumin that modifies a galactosyltransferase to recognize glucose as an acceptor. This led to the synthesis of novel milk sugars, of which free oligosaccharides may have predated free lactose.
Early day mammaliaforms were already endothermic. This required fluid to replace incubatory water losses of eggs, which in turn depended upon very small sized eggs making large eggs an impossible environment for the embryo to grow in. As a result this had a rapid growth effect and limited tooth replacement indicating the delayed onset of feeding and reliance on milk. Thus, milk had already supplanted egg yolk as the primary nutrient source, and by the Jurassic Period vitellogenin genes were being lost. Nipples as the milk delivery system are simply derived sweat glands.
All primary milk constituents and requirements evolved before the appearance of mammals, with some paleontologists insisting that the milk constituents had origins predating the split of the synapsids from sauropsids. So, as, Ph.D. Olav Oftedal retorts, “Thus, the modern dairy industry is built upon a very old foundation, the cornerstones of which were laid even before dinosaurs ruled the earth in the Jurassic and Cretaceous Periods.”
|Artist: Jorge Gonzalez Pseudotherium|
A basal mammaliamorph is Pseudotherium (Sue-doe-thear-e-um) with its holotype fossil coming from the early half of the Late Triassic ‘Ischigualasto Formation’ in Argentina 231 mya. It was 25cm/9.8in long with a very long and flat snout bearing very long fangs near the tip of the snout. The fangs most likely were used to seize invertebrate prey such as insects and other invertebrates. Through high resolution CT scans, the inner ear was pretty much developed and along with other mammalian features, the post-orbital bar was absent, although vestigial postorbital bones are present and there is the presence of turbinals (3 thin curved shelves of bone in the nasal cavity’s sides) that heat inhaled air in endothermic vertebrates.
Mammaliaforms have a temporal range of 225-0 mya with the clade composed of the mammalian crown group and its ancestors going all the way back to the most recent common ancestor. ‘Crown group’, sometimes referred to as, ‘crown assemblage’ is a collective grouping of all its living representatives along with their extinct ancestors as descended down to the most recent common ancestor of them all. However, extinct side branches of the crown group, like Tasmania’s thylacine that went extinct in 1933 is still part of the marsupial crown group. For mammaliaforms, the origin of crown group mammals extends all the way back to the Jurassic, although there are holotype mammals extending into the Triassic.
Early-day mammaliaforms were small and most stem mammaliaforms possessed the epipubic bone used by the animal in strengthening the torso while supporting the abdominal and hind limb musculature. But the bone obstructed the abdomen in expanding for full term live births. This was in conjunction with tritylodontids as derived clade traits in being synapomorphic. So, all stem mammaliaforms most likely laid eggs, or gave birth to premature young as marsupials do today. Also in the early mammaliforms, the stance was intermediate in slightly being primitively sprawled to being derivatively upright underneath the body.
By the time mammaliaforms had arrived, mammalian teeth and their physical, structural and mineral arrangements were already set into their sockets so to speak. The mammalian tooth is diphyodont meaning two sets of teeth are produced that includes the set of baby teeth and the second set of permanent or adult teeth. Basically, mammal dentition is either heterodont (varieties in shape) or thecodont (set in jawbone sockets) arranged. Primarily composed of four constituents, the enamel is a chemically derived crystalline calcium phosphate called hydroxyapatite. The other three are the dentin (calcified tissue covered by the enamel), cementum (a specialized calcified compound that cements the root to the alveolar bone by anchoring the periodontal ligament) and the dental pulp that is the only living section of the tooth located in the center. Incisors and canines tear and rip food while molars and premolars grind food through jaw action for more adequate digestion. Originally the cynodont molar crowns had cusps with three points in a straight line called triconodont.
The more derived mammaliaforms later produced cheek teeth crowns with offsetting cusp points called trituberculate that fitted the upper jaw’s cheek teeth to the lower jaw’s cheek teeth. From crystallite discontinuities prismatic enamel evolved from mammaliaforms to compensate for worn or loss teeth not being replaced by spreading out the force of a bite or chewing aiding the tooth from injury or wear.
Meckel’s cartilage is comprised of three distinct regions. The anterior and posterior regions undergo endochondral ossification and contribute to the mandibular development, forming the malleus and incus (ear bones) respectively. The middle region degenerates and gives rise to the sphenomandibular ligament (internal lateral ligament affixed to the lingula of the mandibular foramen). Morphogenetic studies have shown that the developmental potential for ossification of Meckel’s cartilage is conserved in extant mammals. Mammaliaform fossils revealed that this pattern evolved first in the mammaliaform phylogeny. Further, these findings allude to the disconnection of the ear from the mandible had occurred independently in multituberculate mammalians, in monotremes and in therians. The inner ear of mammaliaforms is derived in having a single petrosal bone enclosing the entire inner ear and a promontorium (projecting part) for an elongate cochlear canal.
The evolving separation of the jaw and the ear bones of mammaliaforms allowed the skulls of later mammals to expand sideways and backward enabling bigger brains to develop. Also, in the Meckel’s cave, the trigeminal ganglion that is the first part of the pathway from the whiskers to the brain, occupies that cavity. Later on in primate evolution, from its convex border the trigeminal ganglion splits into three large nerves vis-à-vis the ophthalmic (V1), maxillary (V2), and mandibular (V3) nerves.
|Artist: Jonathan Hughes Sinoconodon|
Thus far, the most basal mammaliaform is Sinoconodon (Sign-o-con-o-don). Although it was a very advanced conodont and closely related to the more basal derived mammaliamorph, Morganucodon (Mor-gan-ew-co-don), Sinoconodon still featured polyphyodont dentition in tooth replacement throughout its life and continuously slowly grew throughout life. But, it possessed one of the traits commonly used to define mammals in an evolved jaw joint between the dentary and the squamosal bones, replacing the primitive tetrapod assemblage between the articular and quadrate bones. These combined primitive conodont and derived mammalian features make it a wholly transitional species with no other extinct or extant animal like it.
Sinoconodon appeared in China’s Early Jurassic, ‘Lufeng formation’ with an age of 193 mya. The jaw articulation and braincase of Sinoconodon compares more to the cynodont therapsids, Probainognathus and Thrinaxodon than to mammals. The reasons researchers could tell that Sinoconodon grew continuously throughout life is that originally the fossils found were first thought to merely be different species of Sinoconodon, but later determined that they were representative of one species at differing growth stages. This demonstrates the transition from therapsid to mammal, where the medial surface of the groove in the squamosal housing the quadrate was lost. Appearing much like a miniature weasel, Sinoconodon was only 15cm/5.9in in total length.
|Credit: Encyclopedia Britannica Morganucodon|
Just mentioned in the above second paragraph, Morganucodon was a more derived basal mammaliamorph and although closely related to mammals, it does lie outside the common ancestry of extant mammals. However, it and its family members of Morganucodontidae (Mor-gan-ew-co-don-tee-day) with dentary-squamosal and articular-quadrate jaw joints are superb examples of mammalian transitional fossils. Tiny at, 9.5cm/3.75in, it had the body form of a shrew. With a temporal range on the Triassic-Jurassic boundary it lived ~ 205 mya in what is now Wales. The Wales fossil was the first to be found, but later fossils have been found in other parts of Europe, North America and the Yunnan Province of China. There are currently five fossil species that have been found of the gnus, Morganucodon.
It was an insectivore with teeth designed to easily chew on insects with elytra (singular: elytron; chitinous hardened forewings) and most likely preyed on other small invertebrates. The fossil eye orbits are large indicating Morganucodon was nocturnal, while most likely staying in burrows during the day. It still laid leathery eggs, however like monotremes, with the combination of deciduous teeth being replaced by the onset of permanent teeth, along with a toothless stage during infancy, is a strong indication the mothers fed their young through lactation. There is also strong evidence that it possessed specialized skin glands producing nonpolar chemicals such as lipids and oils to keep fur groomed and water resistant; much like most mammals have today. Due to plant material like the conifer, Hirmeriella being found in the same strata near the Morganucodon fossil sites, it likely lived in forested environments.
Docodonta (Doe-co-don-tah) is an extinct large order of mammaliaforms that lived from the ending of the Early Jurassic to the Early Cretaceous 175.6-122.46 mya. The superfamily, Docodontoidea (Doe-co-don-toid-dee-ah) within the order, forms a clade of at least 16 genera and 28 species. Docodonts did not lead to therian mammals, but through convergent evolution did possess similar specific traits such as tooth shape. Docodonts had a series of tall molar cusps along two rows with an anteriorly placed pseudo-talonid (the flat heel of a molar crown used to crush food) to fittingly receive the pseudo-protocone (a corner cusp) of the upper molar. The reason pseudo is added is that they do serve the same purposes in mammalian mastification, therefore the pseudo-protocone is analogous to the protocone, but the anteriorly placed pseudo-talonid is opposite to the posterior talonid basin of true tribosphenic (three peaked molar cusps) mammals. Indeed though, docodonts do show similar physical traits as portrayed in some of today’s mammals.
Primarily, docodonts were small terrestrial and fossorial animals. However, there have been docodont fossils found that show evidence of certain species also being semiaquatic and arboreal. Docodont fossil are found in what once composed the subcontinent, Laurasia in what is now Asia, Europe and North America.
|Artist: Mark Klinger Castorocauda|
The docodontid, Castorocauda (Kass-tor-o-caw-dah) had a temporal range of 164 mya during the Middle Jurassic radiation of mammaliaforms. The holotype fossil was found in the Daohugou lakebed sediment of the ‘Tiaojishan Formation’ in what is now the Inner Mongolia region of China.
Castorocauda, looking very similar to an extant beaver; in fact, the genus name is Latin for: ‘beaver tail’. It was semiaquatic spending most of its life in freshwater chasing after and dining on small fish with teeth similar to those of Eocene whales and extant seals, while the vertebrae is similar to extant otters. With adaptations highly specialized for an aquatic environment, it evolved these adaptations convergently and is not a direct descendant of beavers, otters or platypuses.
Its fossil had preserved hair that included a very advanced dense pelage, guard hairs, whiskers and underfur. The fossil also reveals the tiny auditory ossicles of the middle ear and a broad tail with scales interspersed with hairs that grew less frequent toward the tip. Although Castorocauda had docodont dentition, in addition it possessed unlike docodont teeth that would have been specialized for capturing slippery prey like fish and aquatic invertebrates, making it a piscivore.
Its legs and feet are anatomically very similar to a platypus’ limbs, which are webbed used for swimming and digging with the forelimbs. Although still considered small at 42.5cm/17in in length and 650g/1.4lb in weight, it has thus far been the largest mammaliaform found.
A Lagerstätte, or more specific, Konservat-Lagerstätten are sedimentary deposits that exhibit exceptional body form articulated fossils and preserved soft tissue due to quick burial in an anoxic environment. The Castorocauda fossil came from just such an environment. Known as the ‘Yanliao Biota’ of Inner Mongolia/China, it is an assemblage of volcanic tuff and aquatic sediment stratum spanning from 190-146 mya. The ‘Yanliao Biota’ strata were formed during the time of the destruction of the North China Craton and the beginning of heightened subduction activity driving the Paleo-Pacific plate underneath the Asian continent. This resulted in a series of tectonic, paleo-geographic and paleo-environmental changes due to the ongoing and frequent volcanic activities.
|Yanlio Biota fossils Top Castorocauda|
The ‘Yanliao Biota’ region preserved Jurassic fossils from amphibians to mammals and body parts such as dinosaur feathers, mammalian fur and soft tissue, like the skin of reptiles and pterosaurs. With an average temperature of 15 °C/59 °F, the climate had seasonal intermittent cold temperatures and by credent evidence, may be where feathering, pycnofibers and fur for insulation first arose.
One last docodont to be mentioned here out of its ~ 24 species representatives is the Middle Jurassic 160 mya, Docofossor (Dah-koe-foss-sur). Found in the province of Hebei in a 160 mya sedimentary layer of China’s ‘Tiaojishan Formation’, the body length from snout to vent was only 9cm/3.5in. Anatomically built like a mole, it most likely acted like a mole in being fossorial.
Having a stubbed tail instead of a long tail that would be a hindrance in tunneling, it was also equipped with strong shoulders and forelimbs ending in shovel-like fingers with flattened claws, while the hind limbs were short and sprawling. With all this being indicative of a subterranean lifestyle, in addition, the phalanges formula on the hands were 2-2-2-2-2 instead of the 2-3-3-3-3 ancestral trait. African golden moles have almost the exact characteristics for its digging and tunneling lifestyle. The golden mole’s anatomy is influenced by the genes BMP and GDF-5, which fuses the phalange (finger) joints together, thus decreasing the size and making them stouter. This perhaps first occurred with Docofossor, as it is the first known fossorial mammaliaform.
The snout was blunt with an overhang from the top down and was used as a spade shovel, while the massive olecranon (convexly curved bony prominence of the ulna), along with a projecting parafibula (third bone of lower leg restricted to the knee area of early mammaliaforms) were also an adaptation for digging forcing the rear knee joints into a bent position.
Haramiyida (Huh-ram-e-yee-dah), a sister group to Mammalia is an order clade of long-lived mammaliaforms in having a temporal range of 216.5-65 mya extending from the Late Triassic all the way to the end of the Late Cretaceous. Haramiyidans are a more primitive group than the multituberculates, in which we will discuss later under Mammalia, but they have so much similarity with multituberculates in anatomy and morphology that many paleontologists feel they are direct ancestors to the multituberculate group; although this trending thought is not conclusive. Some studies however, have even grouped haramiyidans with multituberculates in a clade called, Allotheria (Al-lo-thear-e-ah), although it is now being rendered obsolete as euharamiyidans are now being viewed as basal mammaliaforms. So, currently as it stands haramiyidans rank no further than mammaliaforms. This just proves how transitioning animals are so close in appearance that at times it is very difficult to say they are more primitive or more evolved in characteristics.
Now with the above said, let’s look at a China haramiyidan subgroup known as, Euharamiyida (U-huh-ram-yee-dah) that typifies the difficult taxonomy. Euharamiyids are a clade of small haramiyidans, averaging no more than 9.7cm/3.8in with a temporal range of 164.7 to 125.45 mya that stretched from the Late Triassic to the Early Cretaceous.
Haramiyidans have been known since 1846, but that discovery along with future others were only of the teeth. As you might well be aware of, an ancient animal no larger than the average mouse will not preserve and weather well in fossilizations. But in China there have been some incredible current fossil finds of these animals.
|Lft: Shenshou Rt: Xianshou spp. fossils|
|Lft 2: A. allinhopsoni Rt 2: A. jenkinsi fossils|
The recently discovered genera of euharamiyids are: Shenshou (Sheng-sue) and Xianshou (She-in-shoo). The one Shenshou species is S. lui (loo-ee). Two species were found in the genus, Xianshou which are: X. songae (sawn-gay) and X. linglong (ling-long). Along with these three species are two other 2013 species discoveries in the genus, Arboroharamiya (R-bor-o-huh-ram-ee-yah) in: A. jenkinsi (jen-kin-see) and A. allinhopsoni (al-lin-hop-so-nee). Shenshou and the two Xianshou species lived 160 mya while the two species of Arboroharamiya lived 159 mya, with all the species coming from the ‘Tiaojishan Formation’.
|Artist: Chuang Zhao euharamiyids|
For the above illustration, identification is from left: first is X. linglong; 2nd from left in background is S. lui; in the middle is X. songae; the glider is A. allinhopsoni and the last one to the right is A. jenkinsi.
All of these species were arboreal (tree dwellers), with A. allinhopsoni possessing a patagia that could glide it from branch to branch. None of the five species were adept at being terrestrial as their pes (feet) were adapted for holding onto tree limbs and branches and were not anatomically equipped for running. With the unique dentition having numerous cusps on the crown, they all most likely were insectivores/granivores (dieting on seeds and nuts).
Overall, the morphology in these five species was mammalian. Although some of the ear bones weren’t evident in the fossils due to the tiny sizes, incredibly most were. Among all the mammalian traits, such as hair, legs beneath the body and warm bloodedness in addition, these five euharamiyid species had a mammal middle ear with the mandible jaw bones migrating in forming the malleus, incus and stapes ear bones turning air vibrations into ripples within the ear fluids.
In considering euharamiyids as mammals it has caused a stir, for it accepts the Allotheria clade as a group within Mammalia putting haramiyidans beyond mammaliaforms as true mammals. This would throw back in time true mammalian lineages’ first appearance during the Late Triassic between 235-201 mya as opposed to the Middle Jurassic 176-161 mya. However, an intensive recent study on an early mammaliform contradicts these findings.
The earliest known mammaliaform recently found in eastern Greenland existed somewhere around 210 mya. It had teeth similar to a mammal, but a jaw bone and ear like a reptile. However, this haramiyidan known as, Haramiyavia (Huh-ram-me-ah-vee-ah) did possess a dentary condyle of the mammalian jaw hinge and the postdentary trough for mandibular attachment of the middle ear; a transitional condition of the predecessors to crown group mammals, but it negates being related to multituberculates or other mammaliaform groups leading to crown group mammals.
|Artist: April Neander Haramiyavia|
Research studies give quantified detail in its skull evolution and dental function in the forerunners of mammals. Haramiyavia had a unique way of chewing in a back and forth sawing action. Its teeth of multiple cusp-rows were adapted to omnivory or most likely herbivory and are distinctive from the teeth of other early mammal relatives that are presumed to be insectivorous. This makes it as the earliest known herbivorous mammaliform species and haramiyidans as less derived than mammals keeping them in the mammaliform group, but as the sister group to mammalians.
The research findings place Haramiyavia and the haramiyidan lineage members on a more ancestral position in the mammalian evolutionary tree; thus, on a separate branch from mammals. This reaffirms original arguments that the explosion of modern mammal diversification did not occur in the Triassic Period, but millions of years later during the Middle Jurassic.
|Artist: Jorge Gonzalez Cifelliodon|
Found in rock strata of the ‘Yellow Cat Member’ in Utah, USA formed 139-124 mya, is one of the latest and last mammaliforms in, Cifelliodon (Seff-fell-lee-o-don). Its fossil was found among iguanodon, dromaeosaur and ornithopod dinosaur remains in strata forged ~ 130 mya. Cifelliodon belongs to the family, Hahnodontidae ‘Han-no-don-tuh-day’ and is classified under the same order as Haramiyavia, in the order, Haramiyida (Huh-ram-me-eed-ah). Haramiyidans are a group of derived mammaliforms, but are more basal than multituberculates.
Cifelliodon was ~ 50.8cm/20in in body length, which was large for mammals of that time frame. Its mammalian body shape and dentition puts it as a transitional between more basal mammaliaforms and more derived mammals and is one of the closest relatives to the common ancestor of today’s true mammals. CT scans of the skull reveal that it possessed large olfactory bulbs and with tiny eye orbits was more likely in processing smells rather than relying on vision to locate prey items such as, small vertebrates and large insects.
|Artist: April Neander Agilodocodon on limb|
As one can see from some of the examples above, mid Jurassic mammaliforms had already brought the mammalian lineage into new environmental folds by adapting and evolving into not just terrestrial, but also into arboreal (tree living), fossorial (subterranean living) and semiaquatic habitats. Besides the arboreal mammaliforms mentioned above, the earliest known arboreal mammaliaform that lived ~ 165 mya is Agilodocodon (Adj-il-lah-dee-koe-don) in what is now China. It had claws for climbing trees and although primarily omnivorous, it possessed teeth adapted for piercing and ripping woody plants to get at the sap to lap up and ingest.
So, what defines a mammal is in possessing the cumulative trait phylogenies that all their predecessor therapsids had evolved. This includes fur, being endothermic, three middle ear bones, erect limbs beneath the body supporting an upright posture, a jaw joint composed of the articular bone (located at back of lower jawbone) and quadrate bone (located at back of upper jawbone), diphyodonty tooth replacement with prismatic enamel, ability to breathe while chewing, a secondary bony palate and giving live births, except for monotremes, who lay leathery reptilian eggs. Two mammalian characteristics we haven’t yet discussed are the neotcortex and suckling. All of these traits will be possessed by the animals further discussed. In reference to mammary glands, Mammalia is ‘mamma’, which is Latin for breasts.
The neocortex is derived embryonically in mammals from the brain’s telencephalon, as in anatomical terms it’s the rostral portion of the forebrain. The neocortex (neo for new) is a set of six layers found only in the mammalian cerebral cortex that has increased the abilities in brain functions like higher order sensory perception, motor skills, cognition, analysis to cause and effect, spatial reasoning, self-awareness and communicative ingenuity whether in body movement/facial expressive interpretation or vocalization. The involvement in the neotcortex affected the increase in the size of the brain, in turn affecting a larger skull size. The neocortex gave rise to inhibitory control of aggressive behaviors in response to social pressures, thus creating increased gatherings and social harmony.
Only therian mammals (prototherians, metatherians and eutherians) have the ability to suckle. Without the ability to suckle, of course totally dependent placental infants would die of starvation. Suckling requires a multitude of synchronous activities involving the tensor veli palatine (slender triangular muscle situated in the pterygoid fossa of skull), palatoglossus (mucus covered small fleshy fasciculus muscle that’s narrower in the middle than at either end), mylohyoideus (paired muscle running from the mandible to the hyoid bone forming the floor of the oral cavity of the mouth) and intrinsic tongue muscles that close the fauces (arched opening at the back of the mouth leading to the pharynx). These small mouth muscles work instinctively in conjunction to draw the dorsal surface of the tongue against a tensed soft palate forming a seal between the oral cavity and oropharynx. Depressing the tongue in front of the seal induces negative pressure, thus drawing milk into the mouth. All these functions embryonically evolved physically and physiologically into placental instinct, including humans as well.
|Credit: brtc.tamu.edu Suckling Capability|
As far as the evolution of mammalian suckling goes, through CT scans, a team of scientists studied the mouth anatomy of the cynodont, Brasilitherium, mammaliaform fossils and extant pouched young marsupials. What they found was the origin of a medial slip of the reptilian posterior pterygoideus (muscle attached to mandible and skull to open and close the jaw) had migrated to the lateral surface of the pterygopalatine boss that supported the lateral edge of a non-muscular soft palate. Equipped with this, suckling arose in mammals after fibers of this slip migrated into the soft palate to form the tensor veli palatine, and the palatoglossus separated from the pharyngeal constrictors.
All of this added stress onto the pterygopalatine boss that led to the resultant formation of a hamulus (small hooked projection). In losing the transverse process of the pterygoid, the lateral posterior pterygoideus differentiated to form the medial pterygoid. Only the most basal extant mammals in monotremes cannot suckle, which is due to their ancestors modifying the fauces region of the mouth to break down invertebrates between keratinized pads on the posterior end of the tongue and under the ventral surface of a long palatine. In doing so, they lost the tensor veli palatine and palatoglossus, thus negating the ability to suckle.
To expound a bit more on placental births, which were initially discoursed under Mammaliamorpha, we’ll give a little more detail on globins. If ya might recall, oxygen was detrimental to Earth’s early one-celled anaerobic life. These microbes had developed oxygen-binding hemoproteins as a shield against free oxygen radicals. Although globin proteins are still a defense against free oxygen radicals in amniote embryos, globin functions, due to its affinity to oxygen, also eventually evolved into transporting oxygen to blood.
Four alpha globin genes are found in all mammal species in having multiple copies of these genes, or pseudogenes. But, a functionally active globin gene in one species may be a pseudogene in another. In lower more primitive vertebrates there is a single beta-globin gene, but a number of duplication events in the eutherian lineage have yielded a reel of five genes. As illustrated in the figure below, beta-globin genes evolved by successive duplication of an ancestral gene. In the eutherian lineage, thus gave rise to HBE, HBG, HBD, and HBB (ϵ-, γ-, δ-, and β-globin) genes.
|Credit: Opazoet et al. Globin Genes|
Globin evolution functions, such as the transport of oxygen to blood and intracellular metabolic activity, were acquired much later. The groundwork for this was laid down by whole genome duplications in the vertebrate lineage.
From the Late Triassic, throughout the Jurassic and into the latter half of the Late Cretaceous, early mammals remained small and secretive while filling terrestrial, fossorial, semiaquatic and arboreal niches. After the Cretaceous/Paleogene extinction event 66 mya, it emptied ecological space once filled by dinosaurs, other reptile groups and even other mammalian groups. Surviving mammals quickly filled the niche voids, even becoming arboreal with bats taking to the air 52.5 mya. Since Cenozoic time, therian mammals have obtained and held dominance in the medium-to-large sized ecological niches.
Even though confined to small eco-spaces, mammals early on radiated out and diversified into new environs. In so doing they developed a broad spectrum range of limb specializations coupled with different locomotor strategies dependent upon their habitat niche. Mammal anatomical locomotion reflects diverse heterogeneity in different patterns of development and growth. This affected the timing of epiphyseal growth plate closure in the long bones of the limbs such as the tibia, fibula, femur, metatarsals and phalanges in the hind limbs and the humerus, radius, ulna, metacarpals, and phalanges in the forelimbs. This genetic shift in timing of the development of anatomical tissue or bone is known as heterochrony.
Examples of radiative heterochrony are Zhangheotherium (Zane-ghee-o-thear-e-um) from 125.2 mya and the 106 mya, Kryoryctes (Cry-or-riss-tees), a fossorial monotreme. Kryoryctes was a subterranean digger; therefore his limbs were short and stout ending in portly claws due to the overall anterior limb mobility of the forelimbs being sacrificed in favor of maximizing the fossorial abduction movement. In contrast, the symmetrodont (Mesozoic mammals characterized by the triangular aspect of the molars), Zhangheotherium had long sprawling limb bones and a large plantar (sole of foot) due to its scansorial habits, which was a form of locomotion and mobility geared for ease of movement through tree branches and on tree trunks. One thing both animals had in common was a lack of agility in terrestrial movement.
An early mammalian clade order was Eutriconodonta (U-tri-con-o-don-tah) that lived from the Early Jurassic 190 mya to the Late Cretaceous 70 mya. Formerly known as Triconodonta, eutriconodonts were highly successful in competing with dinosaurs for environmental niches. As a group, they have presented some of the best preserved and most complete Mesozoic mammal fossils found to date including bones, soft tissue, fur and even internal organs. There is clear evidence of a diaphragm from a Spinolestes (Spin-o-less-tees) Spain fossil that is just like the diaphragm of mammals today. The fossil also gives detail of its fur down to the cellular structure of compound hair follicles composed of primary and secondary hair endowed with unusually stiff spine-like guard hairs.
|Artist: Oscar Sanisidro Spinolestes and fossil|
As early small mammals, with the largest being no more the size of a badger developed a diverse variety of body shapes for terrestrial sprints, fossorial niches, semiaquatic streamlined swimming, arboreal climbing and aerial gliding; they were filling the world’s biological niches. They were insectivores, carnivores, herbivores, piscivores and omnivores while roaming in what are now Africa, Asia, Europe, North America and South America.
In the eutriconodont family, Gobiconodontidae (Go-bi-con-o-don-tuh-day) was the genus, Repenomamaus from the Early Cretaceous of 125-123.2 mya. The two species were R. robustus (ro-bus-tuss) and R. giganticus (gye-gan-tuh-cuss). They both were the largest of eutriconodonts, but at 1m/3.3ft, R. giganticus is thus far, the largest known Mesozoic mammal. In fact, R. giganticus was larger than most of the microraptorine dromaeosaurids such as, Graciliraptor. Both species’ fossils come from China’s ‘Yixian Formation’.
|Artist: Roman Yevseyev Repenomamaus eating a lizard|
|Artist: Julius Csotonyi Repenomamaus eating a dino|
The bodies of Repenomamaus were proportionately longer than the short legs, while its gait was in a sprawling manner. Repenomamaus possessed a pubic bone dictating that the genus was either egg layers or delivered underdeveloped young. They were carnivorous and/or scavengers as one fossil find of R. robustus showed the skeletal remains of a juvenile Psittacosaurus within the stomach area.
|Artist: Mark A. Klinger Juramaia|
Thus far, the earliest true mammal is Juramaia (Jur-ah-my-ah); in fact, the name infers ‘Jurassic Mother’, due to being the most basal eutherian yet, in which eutherians are the most derived placental mammals. It lived during the Late Jurassic, 160.89-160.25 mya in the Jurassic’s Oxfordian Stage strata of what is now the western Liaoning Province of China. Juramaia lived in a temperate forest and with forelimb bones adapted for climbing, it likely was more arboreal than terrestrial. By judging from the shape of its mammalian teeth it was an insectivore.
Eutherians are the group that evolved to include all placental mammals providing nourishment to unborn young via the placenta. With a mean average length of 85mm/3.4in, the diminutive Juramaia as a eutherian, pushed placental mammal evolution back 35 million year from where it last stood in the split between marsupial and placental lineages. Juramaia’s discovery fills in fossil record gaps by aiding in the updated calibrate of dating mammal evolution with DNA-based methods.
As we can attest to from this lengthy article, the mammalian evolution has been a long and arduous process of stages from the first appearance of synapsids to the true mammal. Mammalian classification has been very difficult while changing significantly over time. As new mammal fossils are constantly being discovered, especially nicely preserved articulated China finds, phylogenetic reviews require new arrangements. To keep it simple, I’m going to use the older theriform classification. Under Crown Group Mammals, I will expound a bit on the newer mammal clades.
There has been and is going to be a lot of terms containing ‘theria’ within their terminology, so let’s make an attempt at explaining each one, then conclude it all with a cladogram.
First off is Theriiformes (220-0 mya), which is a major mammal subclass primarily referring to mammals most closely related to therians.
Yenotheria (167-0), in replacing Prototheria is a sister sub group of theriiforms and has just recently been proposed to be represented as a basal subclass for extant monotremes and their extinct ancestors.
Holotheria (220-0 mya) is an infraclass that includes the first basal holothere, Kuehneotherium (Que-knee-o-thear-e-um) that is a transitional mammalian from mammaliaform dentition to extant crown therian molars along with the therian descendants of Kuehneotherium’s last common ancestor, such as kuehneotheres and trechnotheres.
As the sister group to holotheres, Allotheria (153-17.5 mya), meaning ‘other beasts’ includes all other mammals besides therians, such as multituberculates and the questionable gondawanatheres in whether they are true mammals, or are euharamiyidans.
Trechnotheria (216.5-0 mya) is all therians arising from Jurassic and Cretaceous extinct mammals that were endemic to Africa and Asia.
Cladotheria (165-0 mya) with the sister genus, Zhangheotherium is a legion group including all living therians plus their ancestors.
With the superorder, Dryolestoidea of Mesozoic mammals related to therians, Zatheria (165-0 mya) includes the common ancestors of the order, Peramura, and its sister clade, the infralegion group, Tribosphenida.
Lastly, is the subclass, Theria (160.57-0), which includes all placental mammals that give live births in Metatheria (marsupials) and Eutheria (placentals lacking the epipubic bone).
|Extinct mammaliaforms & archaic/extant therians|
Much like the triconodonts, one of the most successful mammal groups during the Mesozoic were the clades of multituberculates. Named for the multiple tubercles (cusps/bumps) on their back teeth, these rodent-like allotheres competed auspiciously with the smaller dinosaurs for ecological niches at least for the last 20 million years dinosaurs existed. Including gondawanatheres, there were ~ 262 species that had radiated out into the entire northern hemisphere of Laurasia and also into certain regions of the southern hemisphere of Gondwana.
Multituberculates, although now extinct, were a highly successful mammal group reaching their diversified peak during the Early Paleocene, then gradually began declining in species numbers during the latter half of the Paleocene and the whole of the Eocene, becoming extinct in the Miocene. Existing for ~ 152.5 million years they had a temporal range of 170-17.5 mya from the Late Jurassic to the Miocene. The oldest known basal multituberculate is Ferugliotherium (Fair-uh-glee-o-thear-e-um) from 70 mya of the ‘Campanian Los Alamitos Formation’ in Late Cretaceous Argentina. What makes it differ from other multituberculates is that it possessed prominent transverse ridges and furrows on the molars.
|Credit: Past Time Paleo Rugodoson|
Another basal multituberculate is, Rugosodon (Rue-go-saw-don) from 161.2 mya in what is now China. The dentition of Rugosodon indicates it was omnivorous. Even though many multituberculate groups, along with the dinosaurs were wiped out during the K-T extinction event, others survived to live on another 48.5 million years with the genus, Patagonia from what is now Argentina. Going extinct by 17.5 mya, Patagonia has been considered an allothere multituberculate, but the classification recently has been disputed grouping it in with the gondawanatheres that are closely related to multituberculates.
Fruitafossor (Fruh-tah-foss-sore) with long thick claws attached to short stocky forelimbs was a digging mammal existing ~ 150 mya in the USA, Colorado’s Late Jurassic ‘Morrison formation’. This mammal is an early offshoot of mammals related to the therian lineage, even though it wasn’t a therian itself. With a number of prototherian (which includes monotremes) and therian features, anatomical traits include a platypus-like girdle (shoulder) and a vertebral column very similar to therian extant anteaters, armadillos and sloths.
|Artist: Gabriel Ugueto Fruitofossor|
Fruitafossor’s claws were used as an aid in leading a fossorial lifestyle, but don’t let the name fool you; it didn’t eat fruit, in fact, fruiting plants weren’t yet even around. Much like today’s anteaters and aardvarks, it used the claws for digging up ant beds and termite mounds. The genus name gets its namesake from the Colorado town of Fruita where its fossil was found nearby. With open-rooted replaceable peg-like teeth, it wouldn’t chew its diet of termites and ants, but instead simply swallow. The teeth possessed no enamel for protection. Its teeth are very much like the colonial insect eating mammals of today which is known as myrmecophagy (mire-me-coff-foe-gee).
At 27.9cm/11in, Fruitafossor was about as large as the eastern chipmunk. With its powerful short and stout forelimbs, it has been given the nickname of the cartoon character, Popeye.
An archaic and basal eutherian-like mammal was Eomaia (Oh-my-ah). Living during the Early Cretaceous 125 mya, it gave birth and produced milk, but even though it had features like full-term placentals, its pubic area was too small to give full term births, so is listed as a sister group to therians. However there is still a strong push in considering it a therian.
|Eomaia fossil (Note: fur)|
Eomaia was only10cm/3.9in in total length and was an insectivore and while being scansorial and arboreal, scampered up and down trees after insects. To climb trees and bushes, it had recurved strong claws attached to tenacious flexor (grasping) muscles. The tail was twice as long as the torso length.
|Artist: S. Fernandez Eomaia|
The exceptionally well preserved fossil coming from the ‘Yixian Formation’ of China, details the full skeletal anatomy and fur. Even though it didn’t give full term births, it did give birth to underdeveloped young. Its epipubic bones projected forward much like marsupials. Other metatherian traits included an enlarged malleolus (little hammer) at the bottom of the tibia, a joint between the first metatarsal bone and the entocuneiform bone in the foot, which is offset further back than the joint between the second metatarsal and mesocuneiform bones. These offsetting foot bone joints in the tarsus, aren’t analogous to metatherians as their bone joints are level with one another. Eomaia also had the same metatherian incisor dentition with five upper and four lower incisors.
Although during the Cretaceous, reptiles in dinosaurs, pterosaurs and crocodilians were the ruling class of life, mammalian clades had taken a foothold into ecological niches that would eventually lead to the Crown Group Mammals. The Cretaceous, named after the Latin word, ‘creta’ for chalk, while always abbreviated with the capitol letter ‘K’ after the German word, ‘kreide’ for chalk, lasted from 145-66 mya. There is a push to roll its beginning up 5 million years to 140 mya, making it still the longest Earth period of 74 million years instead of 79 million years. But, there is no formal agreement yet by the ‘International Union of Geological Sciences’ to do so. Its beginning is based on stratification while its abrupt ending is based on an iridium boundary layer due to a bolide (large meteorite), or maybe a comet or asteroid impact.
|From lft. to rt. Early-Mid-Late|
The Cretaceous was a warming period forming shallow inland seas. Although substantially different in location during the Cretaceous, the current landform continents were formed as the complete tectonic breakup of Pangea had occurred. In its beginning, Gondwana was still intact, but broke up as South America, Australia and Antarctica rifted away from Africa during the period. In the middle of the Cretaceous, deep sea currents became stagnant creating anoxic conditions as the detritus material was not decomposed. The stagnation of the currents was caused by very gentle temperature gradients arising from the equator to the poles weakening global winds that drive ocean currents resulting in less upwelling of the deep ocean waters promoting stagnation as the resultant. This, later formed dark anoxic shales that laid down petroleum preserves in regions we now call the Persian Gulf and the Gulf of Mexico.
Leafed trees and grasses first appear in the Cretaceous although conifer forests still dominated. Angiosperms (flowering plants), first appearing 125 mya begin spreading until becoming predominant 83-72 mya. Of course social insects like bees arise from wasps as well during the same time as angiosperms. Gymnosperm (plants that produce unprotected seeds) taxa begin dwindling with others like plants from the order Bennettitales (Ben-nuh-tee-tales) becoming extinct before the Cretaceous ends.
In the seas, siliceous diatoms, ammonite genera and reef-building rudist clams become common along with modern sharks, rays and teleost (ray-finned) fish. Marine reptiles during the first half of the Cretaceous were ichthyosaurs only to go extinct 90 mya, while plesiosaurs thrived throughout the period with mosasaurs showing up 101 mya. Hesperornithids, ancestral to modern birds first appear in swimming coastal marine shorelines as well as in inland freshwater bodies in search of fish.
|Artist: Chuang Zhao Cretaceous with symmetrodont, Origolestes sleeping among dinos|
The first true bird had flown during the Cretaceous with more to evolve. Snakes first appear evolving from monitor lizards still retaining vestigial limbs to slither across terrain and swim coastal shores. Land animals were dominated by the apex archosaurian herbivores and predators like the dinosaurs, but there was a competent level of smaller animals like non-avian dromaeosaurid Microraptor species and of course the mammalian multituberculates that according to fossil records outnumbered dinosaurs in certain ecologies. Towards the end of the Cretaceous derived monotremes existed, true marsupials had evolved from metatherians and eutherians had begun to diversify.
|Credit:Ext Stocktrek Images|
The Cretaceous ends abruptly with a large celestial body slamming into what is now the Gulf of Mexico off the coastline of the Yucatan Peninsula creating the 180km/112mi Chicxulub Crater. The Cretaceous-Paleogene Extinction abbreviated as the K-Pg (formerly: K-T for Tertiary) erupted suddenly in wiping out three quarters of all plant and animal life ~ 66 mya. The mass extinction was responsible for 75% of all species going extinct. Except for some of the exothermic crocodilians and sea turtles, any tetrapod over 25kg/55lbs did not survive. The extinction in the seas killed off all current sea-faring reptiles as well as destroying ammonites. Mosasaurs soon went also as ammonites were their primary food source. All dinosaurs, except for birds died off due to the mass extinction event. No pterosaur ever flew the skies again. Mammal lines as well were wiped out with 90% of all mammal species vanishing, although the survivors recovered quickly.
|Artist: Karsten Schneider|
What saved all mammal groups from extinction was small size and burrowing habits. Crocodilian survivors that would lead to modern crocodiles, alligators and gavials were semi-aquatic and could withstand semi-torpidity. As well, the survival groups were essentially insectivores, omnivores and scavengers. Predominant herbivores and carnivores didn’t fare well with most of these species going extinct no matter what animal group they were in.
As the bolide (as big as Mars’ moon, Phobos) slammed into Earth, the immediate impact created a tremendous amount of thermal heat and a supersonic resonance into the atmosphere that immediately radiated outwards in all directions snuffing out all life in its wake. Within seconds gypsum from the sea floor was instantly vaporized from the seismic shockwave and along with molten material was dispersed as aerosol spreading out into the atmosphere. In a matter of minutes after the impact granite bedrock chunks were heated, ejected and flung all over the surrounding terrain and water.
Along with the concurrently outpouring of lava from the Deccan Traps of what is now India in belching out carbon dioxide (CO2), the impact also released prodigious amounts of the gas encircling the whole Earth’s atmosphere. At first, this created a cooling effect with the dust and debris circling the atmosphere by blocking out the sun. As a result sea levels dropped and the Earth was put into darkness for two years. But once gravity finally took hold, the dust cleared out settling down back to Earth. But with all the excess CO2 remaining in the atmosphere, a global greenhouse warming effect occurred resulting in sea level increases as verified from Early Cenozoic Era biostratigraphic zonation of calcareous microplankton/nannaoplankton.
This CO2 is telling, for as the more primitive mammals that survived the K-Pg extinction were small, they began to speciate into larger sizes filling in niches dinosaurs once occupied, but only up until about 10 million years after the K-Pg extinction. In the Paleogene during the Cenozoic Era when the Paleocene-Eocene Thermal Maximum occurred, this maximum created an intense global climatic warming and acidification effect also on the oceans brought about by all the en masse carbon released into the atmosphere where the oceans acted as a carbon sink just after the K-Pg extinction. Although it was short-lived the maximum brought about the extinction on average of ~ 40% of all benthic foraminifera species. It also halted the size increases in mammals.
On islands, the term insular dwarfism is used to describe a decrease in species size. Also, competition with highly successful competitors competing for niches will make for smaller species as in mammals competing with already established contemporaneous dinosaurs. However, it appears that climate is a factor as well. During the Paleocene-Eocene Thermal Maximum, according to stratified thermal footprints, the Earth’s temperature raised 5-8 °C/41-46.4 °F for a good 10,000 years while staying elevated for 100,000 years. Known as a ‘hyperthermal event’ the climate changed in a relative and rapid elevated temperature increase for 170,000 years. In that time frame, mammals belonging to the same species decreased in size. For instance, the horse size decreased 30%. The reason…the larger the animal is, the more heat is retained; the smaller the less as more heat is given up. It's all about surface area.
Crown Group Mammals:
The extinction changed mammalian fortunes in no longer having to contend with the ruling dinosaurs. The variety of mammalian jaw shapes stayed the same through and just after the extinction event. But gradually, different mammals rose in niche habitats offering new diets while the older species that survived the extinction died off. The extinction eventually took its hold on the more archaic mammals allowing today’s distant relatives and ancestors to fill niches that weren’t previously available, like grazing and eventually evolved into today’s mammals. So through the fossil record between the Cretaceous and Paleocene, an extinction and turnover of mammals occurred with the more primitive groups decreasing while the more derived mammals began increasing. However, this took some time.
Within 100, 000 years after the extinction event, more derived and larger mammals began showing up. By that time, mammal fossils could be found to be as large as 71.1cm/28in. After 300,000 years had passed into the Paleocene from the extinction, the family, Arctocyonidae (Arc-tuh-sigh-on-uh-day) arose with nine species in the genera Chriacus (Kree-a-cuss). Evolving after the extinction, Chriacus with a body length at 1m/3.3ft and tail length of 40cm/15.8in is verification in mammal enlargement. Although it isn’t a direct ancestor, it lies within the lineage range of the common ancestor and early ancestors of the derived hoofed and carnivorous mammals.
Chriacus was omnivorous dieting on fruits, insects, small reptiles and mammals and with ankle jointing that allowed its hind feet to reverse 180 degrees, like squirrels in climbing down tree trunks; it most likely raided the contents of bird nests. Chriacus still possessed more basal and primitive conditions and went extinct during the Eocene. As such its encephalization quotient was no greater than 0.41. The Encephalization Quotient: (EQ = Ei/Ec) helps determine the ratio between species brain sizes designated as (Ei), compared to species with the same body mass designated as (Ec). This process is carried out through scanning fossil endocranial images then comparing them to extant mammals. The EQ of today’s mammal with the same body mass as Chriacus is 1.0. Chriacus possessed less than half the intellect and is most likely a main reason why the family group went extinct by the end of the Early Eocene in that competing against more derived mammals became too much of a challenge.
A mammal order that survived the extinction was composed of odd genera in, Leptictida (Lep-tic-tee-dah). Leptictids are presently thought to have been the first mammal group to have branched off from basal eutherians. Representative of the order is the genus, Leptictidium (Lep-tic-tee-dee-um). Bipedalism within animal groups is convergent evolution as found in certain extinct archosaurs, man and Leptictidium. It could walk and run upright in kangaroo-like fashion, while its forelimbs ended in hands that were dexterous enough to grasp hold of items as most all other leptictids could, so leptictids are the first known mammals to utilize bipedalism and dexterous fingers.
With a temporal range of 70-33 mya, stretching from the Late Cretaceous to the Oligocene, the leptictid clade survived the extinction going on to live a total of 37 million years. The order, Leptictida had a total of seven families, one subfamily and consists of twenty known genera.
With a total of eight species, the genus, Leptictidium had a temporal range of 50-35 mya. Genus species first appeared in the Early Eocene during the time of a warmed climate and high humidity. Adapted to living in forests, once the Oligocene forests gave way to open plains, they went extinct and as one of the last of leptictids to go, they left no descendants.
Leptictidium could possibly hop as well as run, although its tibia and fibula weren’t fused together fully and the shock of repetitive jumps could have been damaging even though the feet were built to handle jumping locomotion, but other leptictid species were well adapted for both running and jumping. The tips of the phalanges (fingers) were long and tapered while the long tail served as a counter balance during locomotion. Total lengths were from 60-90cm/2-3ft with the tail making up almost half the lengths.
Stomach contents from fossil evidence shows that Leptictidium species were omnivorous with the stomach area showing the remains of leaves, small lizards, small mammals and particularly insects. Upper molar teeth varied in Leptictidium varied from other leptictid species in being more transverse.
Leptictids were part of the Late Cretaceous-Paleocene evolutionary mammal radiation and after ranging throughout Europe, they then dispersed into Asia and North America. Leptictids as placental mammals are considered a paraphyletic clade that led to the infraclass, Placentalia (Plah-sin-tal-e-ah) that is composed of the eutherian extant mammal group that carries fetuses to full term births.
With an elephant shrew-like snout and kangaroo-like body, Leptictidium looked like something out of a ‘Star Wars’ movie as evidenced in the museum exhibition photograph below.
|Photo: Ghedoghedo Lepticidium fossil|
Crown Group Mammals as a clade is a collection of all extant (currently living) representatives including their ancestral descendant lineages leading all the way back to their most recent common ancestor. There are around 6,400 extant species of mammals and unfortunately, we cannot include all of them into this treatise as it is far too long as it is. There are three living orders of mammals in Monotremata (Mahn-uh-treem-et-ah), Marsupialia (Mar-sue-pee-ail-e-ah) and Placentalia (Pleh-sin-tail-e-ah) which are those mammals that give full term live births.
|Crown Group (extinct/extant)|
For sure the above is a very simple diagram, but for understanding purposes with group-clade names changing all the time, below here is a much, much simpler cladogram. Now we’re giving these simplistic diagrams because immediately following will be cladograms that will require a tad more concentration.
The stem and crown group concepts, show relationships to plesions and scions (cf. C. RASKE & J. EFFERIES 1989). Plesion is Greek for, ‘close’ and generally means: although close in similarities, a branch is phylogenetically listed outside the crown group due to splitting off earlier than crown group members. Scion is simply the descendants of a common ancestor, so is looked upon as a monophyletic group that’s an extension of the crown group downwards into the stem group. Other new terminology introduced here: adelphotaxon – (PL: adelphotaxa) a sister taxon; zygotaxon – the monophyletic clade consisting of two adelphotaxa; basal node – the node at the base of a total group (stem and crown); crown node – the node at the base of a crown group.
|Crown Group terminology|
So, with the info provided above, can you decipher the diagram below of crown and stem groups:
All extant mammals, except three species have seven cervical vertebrae (neck bones). Two exceptions are the two-toed sloth and the manatee in possessing only six neck vertebrae, while the third exception in the three-toed sloth has nine neck vertebrae. Mammal lungs behave as a sponge in shrinking and expanding and are honeycombed with alveolar sacs. The ribcage along with the diaphragm muscle expand and contract forcing air in and out of the lungs that work much like a blacksmith’s bellows. The heart is four chambered and due to endothermy, mammals have higher metabolic rates. The integumentary organ (skin) is composed of three layers in the outermost being the epidermis, the middle layer as the dermis and the innermost layer is known as the hypodermis.
In eutherians (true mammals), all females have two ovaries and a uterus while virtually all males have testicles that have descended out of the body cavity into a scrotum. In metatherians, marsupial females have two ovaries, but also two uteri while male marsupials possess undifferentiated testicles located in a scrotum lying in front of the penis on the underbelly as anatomically opposed to eutherian male mammals. Marsupial pouches (known as a marsupium) are not universal in all extant Australian and South American marsupial species, such as the pouchless short-tailed opossum and the shrew opossum that utilizes skin folds. Of course, prototherians lay eggs, but contrary to beliefs, once hatched the premature young do lap milk from the mother’s belly, but are very capable of sucking pooled milk from a bowl.
In speaking of extant mammals, you may see monotremes listed as the order Monotremata, or as the paraphyletic subclass, Prototheria, which is now being pushed aside for, Yenotheria, or even under the proposed infraclass, Australosphenida (Auss-tra-los-fin-e-dah).
For the placentals: Marsupialia, as an infraclass refers to all pouched mammals, but so does the clade, Metatheria but with the addition of all mammals more closely related to marsupials than to placentals. Placentalia, due to the confusion in not including marsupials, which are also placentals, has essentially been supplanted by the clade, Eutheria.
Now, to really add to the confusion, newly proposed eutherian placental mammal groupings based not solely on fossil anatomy and extant mammal morphologies, but as well geographical ecologies, molecular genetics, nuclear mitochondrial datasets, physiologies and inherited common phylogenies have been incorporated into the mix. So, with this new data, new terms in classifications have been incorporated and they are:
|New Eutherian Groupings|
Atlantogenata (At-lan-toe-gee-nah-tuh) is one of two magnorders (refers to the Latin word, magnus for large and/or of ranking importance) of major placental clades ranging from the Paleocene to Recent. Atlantogenata is further divided into two superorders which are: Xenarthra (Zee-nar-thra) from 59-0 mya and Afrotheria (Aff-roe-thear-e-ah) from 65-0 mya. Xeanarthran examples are glyptodonts, sloths, anteaters and armadillos, while afrotherian examples are aardvarks, elephant shrews, golden moles, tenrecs, hyrax, sirenians (dugongs/manatees) and elephants.
Boreoeutheria (Bor-e-o-u-thear-e-ah) from the Paleocene to the Recent is the other magnorder of placental mammal clades. In having a common ancestor ~90 mya, it is also further divided into two superorders of: Euarchontoglires (U-arch-on-toe-lears), which is synonymous to Supraprimates (Sue-prah-pri-mates) in meaning ‘beyond the limits of primates’ has a temporal range of 65-0 mya and Laurasiatheria (Lar-is-she-ah-thear-e-ah) with a temporal range of 75-0 mya. Euarchontoglires examples are rodents, primates (including Homo sapiens), lagomorphs and colugos. Laurasiatherian examples are: moles, shrews, bats, perissodactyl/artiodactyl ungulates, cetaceans, pangolins and carnivores.
In both superorders making up Atlantogenata, Xenarthra and Afrotheria are basal sister clades listed as a monophyly group through genetic evidence. As a sister taxa, both these superorders are at the base of mammal radiation which advances the thought that they have their origins as an ancient Gondwanan placental clade. Xenarthrans are endemic to the Americas (North, Central and South) originally evolving and diversifying in South America during the continents long Cenozoic isolation. Their ancestral lineage was fossorial diggers. Afrotherian relationships are based on DNA sequences and comparative anatomy and their fossils are not restricted to Africa, although they appear to have evolved there when it was an isolated continent, then radiated out through land bridges.
In both making up Boreroeutheria, Euarchontoglires and Laurasiatheria are sister groups and most likely split from each other ~ 90 mya. Retrotransposon is a genetic component that copies and pastes them into varying genomic locations by converting RNA into DNA. This retrotransposon transcription process is what Euarchontoglires species are based on in their relationships. Laurasiatherians have pretty much been grouped together by similar gene sequences, there are no real anatomical features to give support. However it has been proven that they evolved on the supercontinent of Laurasia after it had split off from Gondwana.
The above proposed arrangements can even become more difficult, as there is general support in combining Boreoeutheria and Xenarthra to form the clade, Exafroplacentalia, in which is now a push to rename that clade, Notolegia and no, we are not going any further in discussing that.
Yes, mammalian ancestry is a bit of a quagmire, but there are reasons for it that will eventually be ironed out. It has been very difficult in structuring mammalian taxa as the fossils have been very tiny, therefore very scant in remaining articulated due to the ravages of time, erosion, earth movements and demineralization. Interordinal is kind of like the word interstitial in meaning between, so in phylogeny, interordinal relationships refer to taxa in between node and stem taxonomic orders. Unfortunately, the mammal fossil record does not address the issues of interordinal relationships far older than the KT boundary, nor phylogenetic inferences of early placental biogeography, or the actual pattern and progress of morphological evolution viewed on an accurate phylogeny with an authentic time scale.
With these fossil gaps, for future molecular analyses there is a lack of sequenced phylogenies in the missing fossil data. Therefore, there can be no portrait of genomic sharing with previous studies in speciation processes such as lineage sorting, introgression from species hybridization, or simply hybrid speciation. The above concerns can obscure phylogenetic analysis in making some parts of the taxon lineage difficult to resolve; even with genome data, but with every new find puzzle pieces are falling into their respective place. Anyhow for the time being, we are going to keep it simple in utilizing Prototheria (proto: original), Metatheria (meta: transcending) and Eutheria (eu: true). Withal, if you want to look up terms that will be the new future reference then by all means…go for it!
Below is a simple phylogeny, a listing and a very simple cladogram of the extant mammal groups. Please do note that some of these classification terms may be falling out of favor for newer terms as future fossil finds and further molecular genetics prove these terms’ relationships as invalid.
|Simple Mammalian Clade|
Now, before we get to the three extant mammal groups comprising the bulk Crown Group Mammals, I’d like to add the cladogram below that in its simplicity, kind of diffuses any confusion that might have arisen with the reader trudging through this.
|Crown Group Cladograms|
Trechnotheria with a temporal range of 216.5-0 mya is a subclass group clade of mammals extinct and extant related to therians and therefore the crown group of all extant prototherians, metatherians and eutherians. Trechnotherians are noted for their dentition shearing mechanism along with a hypertrophied vallum (ridge) surrounding a fossa (slight depression) of the molars. This dental arrangement eventually led to tribosphenic dentition noted in all derived mammal groups. Each evolutionary radiation of trechnotherians involved successive transformation stages of the molar form through convergent evolution among the various groups. This eventually resulted in the tribosphenic pattern found in the immediate ancestors of prototherians, metatherians and eutherians.
While the term, Monotremata primarily refers to extant mammals, Prototheria has been relegated to ancestral extinct lineages to extant monotremes. However, it is now being replaced with the infraclass, Australosphenida. Being the limb of the tree to first branch off from the common ancestor to modern extant mammal lineage, monotremes are the most basal and primitive mammals living today. Diverging from the mammalian main line ~ 166 mya, monotremes still retain ancestral primitive theropod characteristics along with modern mammal features. Some primitive traits are: lay eggs, possess a complex pectoral girdle, shortened limbs are held lateral to body due to orientation to the humerus and femur, lack vibrissae (whiskers) and have a cloaca which serves as a single opening for digestive fecal excrement, reproductive, and urinary tracts. Certain modern mammalian features are lactation, hatchlings cannot suckle but can suck, possess a four chambered heart and although it was through convergent evolution, they also possess three middle ear bones as eutherians do.
Prototherian fossils as the common ancestor or even as a close relative to monotremes have been difficult in finding. Plus, for the fossil finds that were discovered and analyzed, due to the scarcity of the remains it has been up and down as to whether they were prototherians or basal eutherians. Herein lies how that up and down rollercoaster ride has gone.
The prototherian fossil record extends back to the early Cretaceous; however origins of the group lie much farther back in time. The monotreme shoulder girdle developed before the shoulder girdles of other extinct and extant mammalian groups. Anatomic evidence, such as the shoulder girdle suggests an origin as long ago as the middle of the Jurassic Period as mentioned earlier in being approximately 166 mya. Primitive features in monotremes reflect an ancient origin, therefore determining the relationships of monotremes should take into account these primitive retentions. But this attempt is problematic because the archaic features reflect an origin far removed from that of living placental and marsupial mammals. Perhaps with future fossil finds, a mechanism may be found to explain how these features could be possessed by prototherians that are closely related to mammals without the primal traits.
As a reminder, the Prototheria subclass and the proposed infraclass, Australosphenida are synonymous. Prototheria originally contained the extinct mammalian orders: Docodonta, Morganucodonta, Multituberculata and Triconodonta along with the extant order, Monotremata. There were common anatomical characteristics among these orders in that the side wall of the braincase was formed by a bone called the anterior lamina, while in therians the wall is formed by the alisphenoid bone. Also, all groups including therians have tribosphenic molar dentition in containing three cusps on each molar.
Recently, the four extinct orders were dropped from Prototheria leaving only the monotremes. This made Prototheria redundant. A bit later, once Australosphenida came into classification, it made Prototheria obsolete, but, Australosphenida has its own dilemmas. Having a temporal range of 210-0 mya, Australosphenida includes all extinct and extant monotremes along with the extinct family members of Ausktribosphenidae (Ausk-tri-bus-fin-ah-day), Henosferidae (Hen-us-fear-ah-dee) and the genus, Kollicodon (Kol-lie-koe-done).
Prototheria is a paraphyletic subclass that includes: docodonts, eutriconodonts, morganucodonts, multituberculates and monotremes. Paraphyletic refers to any group of organisms descended from a common evolutionary ancestor or ancestral group, but doesn’t include all the descendant groups. So monotremes are simply a branch of prototherians, but are not specifically related to the other branched groups. I do like the term, Prototheria because as included with, Metatheria and Eutheria, in meaning: ‘first beasts’, ‘changed beasts’ and ‘true beasts’ respectively, this arrangement rather nicely defines the three living crown groups of mammals of monotremes, marsupials and full-term placentals. Besides, since early ancestral mammal fossil finds are scant for the three living mammal groups, if more fossil finds come to light, it may bear out a justification in retaining these three terms.
With a temporal range of 167 mya during the Middle Jurassic, Ambondro’s fossil was found in the ‘Isalo III Formation’ of Madagascar and is the first known mammal to possess tribosphenic molars. Monotremes, when matured lose their teeth, but juveniles have very similar tribosphenic dentition as Ambondro had. The jaw assemblage along with teeth patterns and wear, had paleontologists group Ambondro as a prototherian ancestral to monotremes. This tiny mammal was no more than 8cm/3in long and most likely dined only on small invertebrates.
Interestingly enough, the fossil bed Ambondro’s remains were found in, also contained the fossils of shoreline invertebrates, crocodilians, plesiosaurs and sauropod dinosaurs. The Ambondro jaw and teeth fossil gives verification that the tribosphenic dentition of interlocking teeth through cusped molars coevolved more than once proving that the monotreme tribosphenic type had evolved separately from the latter therian groups’ tribosphenic dentition as Ambondro’s anatomical jaw features differed from the earliest, but later therian jaw features.
Ambondro, belongs to the basal clade related to prototherians (or the proposed basal subclass, Yinotheria if you prefer) in the family of Henosferidae (Hen-nuss-fear-ah-day). Two other henosferid family members coming from what is now South America are, Henoferous (Heno-fur-us) with Argentina fossil remains and Asfaltomylos (As-fall-toe-my-los) with its fossil remains coming from Patagonia. Henosferous was from ~ 180.1-168 mya while Asfaltomylos was from ~ 178-168mya. Both prototherians lived at a time when Australia was linked to Patagonia, Argentina/Chile allowing for dispersal from one landform to another. Australia is the terrain that monotreme mammals first evolved from prototherians.
Before we start into the extant crown group members, there are innumerable extinct mammal groups that weren’t treated here in this article; maybe some of your favorites were passed up. So if ya would like, below is a listing of most of the extinct mammalian lineages:
Through DNA sequencing, molecular genetics and fossil discoveries, the monotreme line is estimated to have diverged 220 mya from other mammalian lineages. Monotremes diverged from the mammal mainline even before triconodonts, multituberculates and kuehneotheres had and is the only living group of prototherians. Being the first extant mammals to diverge from the main mammalian crown line, monotremes still possess some primitive synapsid and reptilian anatomical characteristics. They retained the egg-laying gene vitellogenin, which produces a protein necessary in eggshell formation that is not found in the metatherians and eutherians. However, in retaining the presence of vitellogenin suggests that a form of symplesiomorphy occurred where the common ancestor of monotremes, marsupials, and full-term placental mammals was oviparous, but was lost in all other extant mammal groups. Symplesiomorphy, meaning together is a form of plesiomorphy (near form) where ancestral character states are shared by two or more taxa.
Through electroreception, monotremes can perceive natural electrical stimuli produced by an organism’s movements. ‘Mono’ and ‘treme’ in monotreme means ‘single opening’ in only have one opening called the cloaca as is the case in reptiles, birds and amphibians. Therians have three for reproduction in the vagina/penis, urination in the urethra and the anus for fecal excrement. The male monotreme uses its penis only for sperm transferal. Monotremes also possessed a gait much like tetrapod reptiles with the legs splayed outwards from the body.
All modern day monotremes have spurs on their rear legs although in echidnas it is vestigial and only in the male platypus does the spur inject venom. Many non-monotreme extinct mammal groups also had spurs. One more convergent evolution aspect to monotremes is in their suspended ear bones (that were originally jaw bones) evolved independently from therians. Nonetheless, in prototherians the ear openings did not migrate as the openings did in therians, but instead the ear openings remained near the base of the jaw.
Monotreme infants are called puggles and with the mother possessing no nipples will lap or slurp the milk filled with antibiotics secreted by specialized sweat glands onto the mother’s belly or her pouches.
The mammalian line that would eventually lead to prototherians and monotremes, through genetic analyses suggests that the lineage began some 220 mya. However, the oldest prototherian line in the fossil record found thus far is in the 165 mya Inner Mongolia Jurassic lake beds of the mammalian, Pseudotribos (Sue-doe-tri-bos). At only 12cm/4.7 in total length, Pseudotribos was an insectivore dieting on invertebrates such as insects and worms.
|Artist: Mark Klinger Pseudotribos|
As one can detect from reading, tribosphenic teeth are important in identifying mammalian species from one another and other mammal groups. This is due to the importance in how mammal chewing evolved in allowing them to facilitate greater and quicker ingestion of nutrients to complement their higher thermoregulation rates. In the word, tribosphenic, ‘tribo’ is Greek meaning: grinding/pounding, where ‘sphen’ is also Greek in meaning: cutter/grinder. In true tribosphenic teeth of marsupials and placentals along with their ancestors, the cutter is in front of the grinder. This combined shearing and grinding dentition arrangement with the cutter in front and grinder in back allowed for more versatile feeding functions. It held great importance for early mammalian diversification and is also why you can enjoy chewing your meal while still breathing. Why in going over this is that the generic name Pseudotribos is Greek meaning: ‘false chewing’. The reason being is that Pseudotribos indeed had tribosphenic molar dentition, but the cutter and grinding tooth elements were arranged in reverse with the grinder in front of the cutter. This form of tribosphenic dentition is more in line with prototherians than to therians and along with its short stout splayed limbs, lends more credence to Pseudotribos in being more ancestral to monotremes than to therians. Regardless, there is an infringing push to link Pseudotribos to therians.
The prototherian family, Ausktribosphenidae (Ausk-tri-bose-fuh-nigh-day) has a temporal range of 122.7-112.6 mya and consists of two genera in, Ausktribosphenos (Ausk tri-boss-fee-nose) and Bishops (Bish-ops). The family genera lived during the Early Cretaceous ranging from 125-113 mya. Both were about the size of a house mouse at 9.7cm/3.8in in length. Their fossil remains have only been found in one certain region of Australia known as the Flat Rocks site in Victoria and are related to monotremes, although they left no direct descendants.
Ausktribosphenids possessed tribosphenic teeth in the prototherian dentition arrangement. A post-dentary trough was present in Ausktribosphenos with six lower pre-molariform teeth alongside the last two being strongly molariform. Bishops also possessed a trough with at least six pre-molars, which is almost non-existent in placental mammals. Bishops is somewhat more advanced in that unlike Ausktribosphenos, it lacked a bone in the mandible (lower jaw). This little bone is important because it is one the bones that formed the ear bones assemblage in more advanced mammals creating keen hearing.
Teinolophos (Ty-noll-la-foss) from 123 mya was tiny at only 10cm/3.9in in total length, but is closely related to monotremes in being one of the earliest relatives to the platypus. It might have even been a basal platypus minus the beak. Teinolophos definitely was semiaquatic in possessing semi-webbed feet. Its fossil finds were discovered at the ‘Wonthaggi Formation’ at Flat Rocks, Victoria, Australia.
|Credit: paleozoografica Teinolophos|
Devised molecular clocks suggest that echidnas diverged from the platypus somewhere between 17-80 mya. However, research studies by the scientists: Timothy Rowe, Thomas H. Rich, Patricia Vickers-Rich, Mark Springer and Michael O. Woodburne firmly established Teinolophos in lying within the ancestral monotreme crown pushing the divergence of echidnas from platypuses back to ~ 119-120 mya.
This Early Cretaceous date is confirmed by High Resolution X-ray Computed Tomography (HRCT) scans on Tienolophos fossil remains, in which revealed the presence of a hypertrophied mandibular canal coursing along the entire length of the mandible in a position lateral to the molariform (having molar tooth form) roots, which the ramus (movable hinge joint bone) exits medially beneath a large medial tubercle (bone protuberance). As far as mammals go, the platypus is the only other mammal besides Tienolophos that has a hypertrophied mandibular canal along the mandible’s entire length.
With a temporal range of 61.7 mya, Monotrematum (Mon-o-tray-mah-tum) is the first monotreme and is classified under the platypus family of, Ornithorhynchidae (Or-nith-or-rink-ah-dye). It is also the only monotreme found thus far outside of Australia and its surrounding oceanic islands. It had an average size of 85cm/33.5in which is around 30.5cm/1ft larger than the extant platypus.
Unlike the extant platypus, Monotrematum retained its teeth into adulthood and as a carnivore, its diet had a larger variety of smaller animals than what the current day platypus diets on.
Due to land connecting Australia to South America via Antarctica, monotremes apparently had dispersals northward through Antarctica to the northern hemisphere as attested to Monotrematum’s fossil remains found in the ‘Salamanca Formation’ of Patagonia, Argentina. At the time, South America, Antarctica and Australia were a part of southern Gondwana. Of course due to Antarctica’s current location in the lower colder latitude, there haven’t been any monotreme fossil finds. But unfortunately when the snows and ice have melted to expose land due to man-induced global climate change, perhaps fossils will be revealed.
The first true platypus was in the genus, Obdurodon (Obb-dur-o-don) that had a temporal range of 26-5 mya. There were three species in: O. dicksoni (dik-so-nee), O. insignis (in-sig-niss) and O. tharalkoochild (there-all-coo-child). The odd species name of ‘tharalkoochild’ is an aboriginal wording coming from the creation story of the platypus. It tells of Tharalkoo who was a stubborn young female duck. Her parents warned her not to swim down river because Bigoon the water-rat would have his wicked way with her. Scoffing, she disobeyed and went anyway and sure enough was ravished by Bigoon. By the time Tharalkoo escaped and returned to her family, the other girl ducks were laying eggs, so she did the same. But instead of a fluffy little duckling emerging from her egg, her child was an amazing chimera that had the bill, webbed hind feet, and egg-laying habit of a duck, but with the fur and front feet of a rodent, in which was the first platypus.
All three Obdurodon species were discovered in Australia but hail from different regions with O. dicksoni fossil remains coming from 23-2.6 mya at Riversleigh of N.W. Queensland, O. insignis from 15.9-11.6 mya in the Tirari Desert of central Australia and O. tharalkoochild from 15-5 mya at ‘Two Tree Site’ fossil beds of Riversleigh in Queensland, Australia. All three, like Monotrematum retained their teeth into adulthood. Also, all three were larger than the modern day platypus with O. tharalkoochild on platypus steroids reaching a length of 1.3m/4.3ft.
|Artist: Peter Schouten O. tharalkoochild (inset: molars)|
Unlike the current living platypus, the Obdurodon spp. did not bottom feed on water bottoms, but instead more on the surface, on the shorelines of lakes and rivers and terrestrial as well venturing into the forests back then near shores and banks. All three species dieted on small aquatic and land animals, such as fish including lungfish, crustaceans, and frogs. The wear patterns on O. tharalkoochild’s teeth are suggestive of crushing, instead of shearing, which would have been caused by consuming hard-shelled animals such as turtles.
|Artist: Dr. Anne Musser Steropodon|
With a temporal range of 105.5-93.3 mya Steropodon (Stir-rop-o-done) and Kollikodon (Koll-lee-o-done) from 99-96 mya are both considered stem monotremes. Stem groups are paraphyletic composed of a total group minus the extant crown group members. Therefore these two species are not ancestral to any living species. If you can recall, under Mammalia we mentioned that the 106 mya Kryoryctes (Cry-or-riss-tees), with short and stout limbs was a subterranean digger. Kryoryctes is also tentatively considered a stem monotreme, but there is some ongoing debate on whether it is ancestral to the echidna lineage as the humerus (long bone in the arm running from the shoulder to elbow) appears similar in morphology to extant echidnas. Of these three, only Steropodon seems to have been semi-aquatic.
|Kryocetes fossil bones|
As far as length goes, at ~ 1m/3.3ft, Kollikodon was one of the largest of Mesozoic mammals, where Steropodon was no Mesozoic mammal runt either at ~ 45cm/1.5 ft, while Kryoryctes’ length was also 45cm/1.5ft. All three of the fossil remains come from Australia with both Kollikodon and Steropodon found in New South Wales, Lightning Ridge ‘Griman Creek Formation’ and Kryoryctes remains found in Slippery Rock’s ‘Dinosaur Cove’.
|Artist: Nixdrawsstuff Kollikodon|
Today, there are only five extant species of monotremes in the platypus and four echidnas. Extant adult monotremes of today lack teeth. But where juveniles have teeth, monotreme lower molars lack a talonid, and consequently there is no basin with facets produced by the wearing action of a protocone (the center of the three cusps of a primitive upper molar), while also absent is the cristid obliqua (tooth crest) connecting the talonid (crushing region of a lower molar) to the trigonid (first three cusps of a molar).
Ornithorhynchidae (Or-nith-or-rink-ah-dye) is the family of platypuses with a temporal range of 61-0 mya. It includes three genera with five species. The ones already mentioned are extinct in, Monotrematum and the three species of Obdurodon. The only extant species of platypus (mainly referred to as the duck-billed platypus) is Ornithorhynchus anatinus (Or-nith-or-rink-us = a-nah-tuh-nus).
Platypus venom is made in the crural venom glands located on the inner thighs that are connected to hollow spurs on their hind legs’ ankles via a duct. Females have spurs but produce no venom. Even for the males, the venom is normally produced during mating season only, initiated through hormone levels. The venom is a complex blend of nineteen peptides composed of D-amino acids and superadded with a non-nitrogenous composition. However, the venom is not fatal to humans.
The natural range of the platypus is along the eastern coastline river systems of Australia and on the island Tasmania that once linked up to Australia. There is also an isolated population introduced by man on the western side of Kangaroo Island just off the coast of southern Australia. Inhabiting small streams and rivers, the platypus is semiaquatic. It lives over an extensive range from the cold highlands of Tasmania and the Australian Alps to the tropical rainforests of coastal Queensland and as far north as the base of the Cape York Peninsula. Along the banks it digs burrows for resting and nesting.
Unlike its extinct relatives, today’s platypus loses its teeth as adults and only feeds submerged looking for prey on the river bottoms. When diving, it closes its eyes, nostrils and ears relying on electroreception to locate its prey composed of insects and insect larvae, freshwater shrimp, crawdads (crayfish), small shellfish and worms along the river bottom. How it eats is once capturing prey along the river bottom gravel, it surfaces and begins crushing the caught food with the bits of gravel to ingest, then spits out the gravel.
Lower than therian mammals, the platypus maintains an endothermic temperature of ~ 32 °C/90 °F and most likely inherited this lower body temperature from its ancestors that also came from Australia, but during a time when Australia’s climate was subpolar due to its tectonic plate location. More nocturnal and crepuscular, the platypus may still exhibit active diurnal periods.
Below is a short video of a platypus playing with and feeding from its handler:
Echidnas, also known as spiny anteaters make-up the family, Tachyglossidae (Tac-e-gloss-e-dee) encompassing three genera in, Tachyglossus (Tac-e-gloss-us) with one extant species, Zaglossus, (Zah-gloss-us) with three extinct species and two extant species and Megalibgwilia (May-gah-lib-gwah-lee-ah) with two extinct species. Megalibgwilia’s temporal range was 89-50 kya (k = 1,000).
|Photo: Pavel German Tachyglossus|
|Two extant long-beaked a) Z. bartoni b) Z. bruijni|
The species are the Australian extant: T. aculeatus short-beaked echidna; three extant New Guinea long-beaked echidnas in, Z. bartoni (eastern long-beaked), Z. bruijnii (western long-beaked) and Z. attenboroughi (Attenborough’s long-beaked) along with the two extinct in, Z. hacketti and Z. robustus and the two extinct Australian echidnas, M. ramsayi (ram-say-i) and M. robusta (ro-bus-tuh)
Tachyglossids are terrestrial within forested and wooded zones. For digging they have large shovel-like claws utilized in burrowing straight down and clawing for prey. For more efficient burrowing, the hind legs point outwards from the body with the claws curling backwards to better shovel dirt backwards. The rostrum is beak-like in being long and narrow and is where up to 400 in Tachyglossus and 2,000 in the long-beaked species electroreceptors are located. Adults have no teeth, so use their long tongue coated with sticky saliva to apprehend prey, where Tachyglossus primarily feeds on ants and termites, Zaglossus in addition to a sticky salivated tongue, the tongue also has small fine spines to snare and feed on various insects, insect larvae and worms. Once food is in the mouth, echidnas break it down with hard pads located on the roof of the mouth and back of the tongue.
The body, after ~ 8 weeks of growth from hatching is covered in coarse fur and keratinized quills (modified hairs). Eight weeks is normally how long the quills harden and mature at which time the mother weans her puggle. Females lay one reptilian-like leathery egg that’s kept in a skin pouch until hatching 7-10 days later.
|Echidna keratinous spines|
Much like their relative, the platypus, echidnas maintain a low body temperature of ~ 31 °C/87.8 °F, which is one of the lowest maintained body temperatures in mammals. The body temperature can fluctuate by up to ~ 7 degrees up or down. However, unlike the platypus, echidnas have a low metabolism rate and live in slow-mode fashion, but it appears to aid them in being long-lived for up to 17 years in the wild and up to 50 years in captivity.
|Large short-beaked echidna|
|Large Z. bartoni echidna|
Echidnas are much larger than we perceive them to be. Maybe sometimes due to the resemblance to the hedgehog, we mistakenly perceive and infer echidnas as the same proportions. Anyway, the extant short-beaked echidna is perhaps the smallest at 38.1-43.72cm/15-18in with a mass of 3.5kg/7.7lbs in adults. Adult long-beaked extant echidnas range anywhere in length from 45.7-99.1cm /18-39in, with a mass of 5-16.5kg/11-36.3lbs.
|Credit: megafauna.com.au Z. hacketti|
The extinct species were even larger with Z. robusta reaching 65cm/25.6in and Z. hacketti reaching a length of 1m/3.3ft, with a height of 0.6m/2ft, while weighing upwards to 30kg/66lbs. This Pleistocene Western Australia echidna in Z. hacketti is one of the largest known monotremes; even as one of the largest of all prototherians. Its legs were much straighter than other monotremes making it more adept in maneuverability through forests. Z. hacketti fossil remains were found in Mammoth Cave with evidence of incisions and burn marks. Most likely the remains were brought into the cave after being hunted by aborigines, prepared, cooked and eaten.
|Credit: WWW Megalibgwilia|
Also the fossil remains of the two Megalibgwilia are from Australia, but across the whole continent, while also found in Tasmania. However, M. robusta fossils have only been found in New South Wales, Australia, inferring that it was more endemic. Although Megalibgwilia spp. had longer and straighter legs than the extant species, they were only a little larger than the contemporary western long-beaked echidna. As an insectivore, they ate a variety of smaller insects. Living during the Miocene, M. robusta is the oldest known tachyglossid, while M. ramsayi lived during the Late Pleistocene. Surviving in warmer and wetter climates, all Megalibgwilia spp. became extinct by 50,000 years ago once the climate in Australia became much more arid.
|Albino short-beaked echidna|
Echidna body coloration can range from browns to grays to black, even within their own species. Albinism is the congenital absence of pigment in the skin and hair making them white while giving pinkish eyes. Being albino affects virtually any animal; even echidnas. If ya want to click on the link below to watch the 37 second video, you can watch the short-beaked albino in action foraging.
What is intriguing to me is that some folk say that monotremes are living fossils because they are the only extant mammals that lay eggs along with other primitive anatomical features. Yet, just like any other modern day mammal group, they experience rapid eye movement (REMS) while dreaming. Therefore unlike other animal groups, all mammals, even echidnas and the little mouse dream along with having the occasional nightmare.
All therians extinct and extant give birth to their young instead of hatching via a shelled egg. This includes all metatherians that give altricial births, such as marsupials and all eutherians, in which give full-term births, such as all extant and extinct true placental mammals that are, or were indigenous to Africa, the Americas, Asia (including India) and Europe.
In studying the earliest mammals drifting towards modern day eutherian mammals, dentition fossils are very important since the first of mammals were very small. Most were no larger than a mouse, while the few largest were no bigger than a kitten. Jaws and teeth fossils are about the only remains of these creatures left to study. We’re evaluating the Mesozoic here, which is dubbed ‘The Age of Reptiles’ and for good reason. Dinosaurs during this age of time were very successful and competitive, but every niche wasn’t dominated by reptiles and those ecologies are the micro ones that the earliest mammals took up residence in.
In vertebrate animals, neonates (newborns or hatchlings) are either precocial or altricial. Altricial is defined as in newly born or hatched, the young are undeveloped and helpless requiring parental care, whereas precocial neonates just born or hatched are in a developed enough state to feed and fend for themselves. For placental mammals the neonates (newborns) are either precocial ~ capable of its own mobility, or altricial ~ incapable of its own mobility.
According to an eutherian study conducted by mammologist, Theodore I. Grand, neonates of altricial placentals are categorized into 4 categories and they are ~ I: small brain, weak musculature; II: small brain, strong musculature; III: large brain, weak musculature and IV: large brain, strong musculature. Each mammal species exhibits a distinct mother/infant strategy from the altricial red panda cub (condition I) and the golden lion tamarin (condition III) to the precocial wildebeest calf (condition IV). An example of condition II (small brain, strong musculature) has not been found. This suggests that muscle does not grow in advance of the brain and that the brain acts as a pacemaker of growth. Grand’s model proposes that early growth rates of brain and muscle correlate with nutrition, maternal effort during gestation, lactation and parental care, whereas postnatal muscular growth correlates directly with adult body size and locomotor repertoire.
As far as precocial mammals go, examples are most ungulates, guinea pigs and species of hares. Hares serve an important example when it comes to evolutionary processes, as being closely related to rabbits that are highly altricial, exemplifies the fact that precociality is not a conservative feature within species.
Perhaps viviparity convergently evolved loosely here and there among synapsids rather than being genetically descended down from one ancestral line. This is highly possible as most folks feel that snakes are oviparous (lay eggs) only. While some folks more educated of snake reproduction say no, as snakes are also ovoviviparous in that snakes do give live birth even though the neonates are hatched from eggs inside the mother. However, there are a few snakes, like the more primitive boas that indeed do give live births from a sac or placenta carried inside the mother.
What we do know of placental evolution is that it has evolved through inner familial gene expression as well as convergently. The placenta's wide range of functions are supported by novel genes that have evolved following gene duplication events, while acquisition of gene expression by the trophoblast is required in other mammal placentas. In placentas, high-affinity fetal hemoglobins play a key role in placental gas exchange even if they’re not expressed in the placentas. One of these gene expressions termed the: ERVW-1 gene, expresses the manufacture of the protein, syncytin-1in female placentals that functions within the placenta allowing various fetal life-supporting exchanges between the mother and its offspring. Syncytin-1 proteins, otherwise known as enverins are produced by the retrovirus W, a virus member group that actually makes up 1% of the human genome. So, we always fret about viruses like the H1N1 and coronavirus, but without viruses, there’d be no placental mammals, which of course includes us.
These fetal hemoglobins evolved following duplications within the beta-globin gene family with convergent evolution occurring in ruminants and primates. In primates there was also a rearrangement of a case of genes in relation to an upstream locus control region. By the expression of sugar and amino acid transporters at the trophoblast microvillus and basal membranes, substrate transfer from mother to fetus is maintained. In contrast, placental peptide hormones have arisen largely by gene duplication, yielding chorionic gonadotropins (group of hormones stimulating gonad activity) from the luteinizing hormone gene and placental lactogens. This process is transpired from the growth hormone and prolactin genes. There has been a stark degree of convergent evolution with placental lactogens emerging separately in families and time frames within ruminant, rodent, and primate lineages, with chorionic gonadotropins evolving separately in equids and higher primates.
Dryolestoidea (Dry-o-les-toid-e-ah) was a superorder containing two orders in Dryolestida (Dry-o-les-tee-dah) from 164.75-17.5 mya and Amphitheriida (Am-fee-thear-id-ah) from 176-161 mya. Dryolestids are a big order containing 37 genera within 9 families, while in the family of, Amphitheriidae (Am-phi-thear-id-day) there were two genera of amphitheriids.
Considered the mother of all full-term Cenozoic Era extinct and extant full-term placental mammals, this mammal itself did not have its fossils discovered and classified, or its molecular genetics analyzed, nor even its phylogeny traced…it was made-up by a team of scientists utilizing all the physical features of known fossil and extant placental mammals to feed into a new computer program that detailed even the tiniest of details such as the inner ear bones to come up with the very first Crown Group full-term placental mammal that unmistakably, according to the final result, arose approximately 300,000 years after the 66.043 mya extinction K-Pg boundary.
What the researchers utilized was a computer program called the TNT-NN algorithm where non-negative least-squares (NNLS) in the spatial domain are used to solve inverse problems. There are other techniques that are extremely faster, like the Fourier technique, but these techniques produce solutions that violate the non-negativity of moment constraints. Inversions in the spatial domain of TNT results are a fast iterative method that doesn’t violate non-negativity constraints. Iterations are repetitive mathematical computational procedures applied to the result of a previous application usually implemented as a means of obtaining successively closer approximations to the solution of a problem.
|Credit: Smithsonian Inst. bronze Protungulatum|
Before the skeptics out there start scoffing, guess what was later found near that extinction boundary in the oldest portion of 65 million year old Paleogene strata from Saskatchewan Canada’s ‘Ravenscrag Formation’. It was a rat-sized and rat looking fossil named, Protungulatum donnae (Pro-tun-gue-luh-tum = don-nee). The genus name means ‘first ungulate’ as it is the forbearer of all ungulates, but it is also the oldest known full-term placenta that lived between 65-64 mya. Exactly as the researchers predicted on their six year TNT testing and results, enlisting over 4,500 anatomical traits and phenomic characters from 4,541 fossil and extant mammals.
So, as many are pointing out…if ya don’t like the thought of humans and apes evolving from a common primate ancestor, then surely in knowing all modern day mammals (including us) came from a rat-look-alike creature, it has to sting a bit in shortening a few religiously lit fuses.
What sparked the research was a fossil find coming from Mongolia’s Gobi Desert in the ‘Ukhaa Tolgod’ locality of a shrew-like mammal with the given genera name, Maelestes (Male-s-tees). With a temporal range of 75-71 mya, it had an epipubic bone evidencing that it was not a true full-term placental but a true therian giving birth much like metatherians to undeveloped young as modern marsupials do; although there is no evidence of a pouch.
The Maelestes fossil find is important in understanding the radiating of Cretaceous eutherians and the radiation of full-term placentals post the Cretaceous in the absence of dinosaurian groups. Above is an illustration of a Maelestes skull compared to other Late Cretaceous archaic mammal skulls.
The 150 mya, Fuitafossor, as was introduced under, Mammalia and although not a therian itself, it was an encroaching relative to therians. Its shoulder girdle was characteristic to prototherian monotremes, but it possessed more therian anatomical traits suggesting that it is one of the earliest known mammalians leading to the evolutionary line of modern day mammals. Placental therians are divided into the two major clades, Metatheria and Eutheria.
To start, where all eutherians whether extinct or extant are full-term placental eutherians, not all metatherians are marsupials. However, all non-marsupial metatherians are more closely related to marsupials than to full-term placental mammals. Let’s get this straight, as many folks confuse and interject the term ‘marsupial’ when it should be ‘metatherian’.
All metatherians have anatomical features that are similar, but the difference between marsupials and other metatherians is in dentition. Marsupials have tribosphenic teeth where all other metatherians possess varying tooth forms. There are a few other differences, like tooth replacement in more primitive metatherians, but, all more derived metatherians have the same dental formula including five upper and four lower incisors, canine pairs, three premolars, and four molars. There have been some excellent basal metatherian fossil finds from China and Mongolia that has allowed scientists to study skulls in fine detail. The main characteristic in likeness is how the cranial vascularization functions similarly in basal metatherians and advanced marsupials in the skull and neck.
The central nervous system (CNS) in mammals is vascularized through angiogenic (angiogenesis: formation of new blood vessels) vessels growing from a perineural (located around a nerve or bundled nerve fibers) vascular plexus. Growing evidence points to a central role of the brain vasculature in neurogenesis. What is determinately attempting to be said here is that the vascular system (blood/lymph) of the brain, besides carrying oxygen and nutrients also, through contact regulation, interacts with neural stem cells for their propagation and performance.
Metatherians were the most dominant mammal clade during the Cretaceous, in particular in North America. Today’s American opossum is viewed as dimwitted, in particular for the reason of ‘playing possum’ as it apparently plays dead when threatened. This reaction is not intentional, but is due to a primitive nervous system that short circuits during duress while taking a while to re-circuit then recover. Originating during the Early Miocene, the genus, Didelphis (Di-dell-fiss) consists of six American opossum species. It has been very successful in varying ecologies, with the Didelphis species still radiating out into new environments. From 23 mya being offered up as saber-tooth cat and dire wolf meals to current road kills, Didelphis opossums are still nonetheless successful in perpetuating their reign.
It was once thought that metatherians diverged from eutherians ~ 148 mya, but in evaluating eutherian 160 mya fossil evidence, through a comprehensive molecular phylogeny analysis, metatherians and eutherians bifurcate the mammalian line in diverging from each other during a mean average of ~ 180 mya. Originating in Late Jurassic Asia, metatherian faunas forming clades radiated outwards becoming widespread over Laurasia in successfully colonizing Asia, Africa, Europe and the Americas by the Early Cretaceous. However, for a time metatherians evolved side by side with eutherians, but around 15-20 mya, metatherians were unable to compete with the more progressive latter placental forms and died out except in South America, Australia and its surrounding islands where they were isolated sheltering them from eutherian competition.
Their success in these two continents was most likely due to water barriers from rising seas isolating them in keeping evolving placental forms out. But, while there were still land connections, metatherians began migrations from S. America down into Australia via Antarctica about 50 mya. Around 5 mya, when a land bridge formed between South America and Panama, metatherian marsupialiforms as descended from herpetotheriids (a sister family to all living marsupials) were able to cross up into Central and North America. These are the immediate ancestors to the Didelphis American opossum species of today.
With a temporal range of 112.6-109 mya, Pappotherium (Pap-poe-thear-e-um) is representative of an earlier lineage close to the metatherian/eutherian divergence commencement. It was most likely arboreal and an insectivore. The name itself is ancient Greek coming from ‘pappos’ meaning grandfather and ‘therium’ for beast. Its fossil find comes from Texas’ ‘Glen Rose Formation’.
Most Mesozoic metatherians have been found in North America and Asia living during the Late Cretaceous primarily between 90 and 66 million years ago. Whether metatherians originated in North America or Asia is largely dependent on the prevalence of Cretaceous forms from North America and recent evidence from Asia; for example due to the absence of lower latitude Early Cretaceous tropical faunas. Nonetheless, the oldest metatherian fossil thus far found is in the ‘Yixian Formation’ of Liaoning Province, China. This 125 mya fossil is, Sinodelphys (Sign-o-dell-fees) that is an exquisite slab/counter-slab exhibiting skeletal anatomy fur and soft tissue. It was only 15cm/5.9 in in length, while weighing just 30g/1.1oz. Due to pes (foot) structure it was scansorial and arboreal and most likely was an insectivore dieting on insects and insect grubs.
In being scansorial, Sinodelphys’ curved claws and foot structure for grasping branches have similar proportions to modern-day climbing animals. This also suggests that it was arboreal spending significant amounts of time in trees. Once evolving climbing abilities, it was able to take advantage of a new habitat and the arboreal life of untapped resources.
|Credit: BBC Sinodelphys|
The order, Deltatheroida (Dell-tah-thuh-roid-ah) is an extinct group that is distantly related to marsupials as basal metatherians that lived from 125 mya to what was thought 66 mya until the fossil remains of the deltatheroidean, Gurbanodelta (Grr-ban-o-dale-tah) was discovered. It lived during the Late Paleocene some 56 mya and is the tiniest of Deltatheroideans at 12cm/4.7in in length. Deltatheroideans carnivorous dental anatomy was not tribosphenic; instead it possessed tritubercular dentition in only having three tubercules (cusps) on the lower mandible molars and not on the upper molars as in tribosphenic. This is what most know of as tricuspid molars and is a more primitive mammalian dental stage, but is what led to tribosphenic dentition. For deltatheroideans, the last molariform deciduous premolar is not replaced in adulthood, with the tooth position referred to as M1/m1. This morphology is shared with marsupials, but not with eutherians.
|Artist: Midiaou Deltatheridium vs Archaeornithoides|
The deltatheroidean, Deltatheridium (Dell-tah-thuh-rid-e-um) had a temporal range of ~ 84.9-70.6 mya. It was small at 15cm/5.9in, but was a vicious predator attacking any prey its size or a bit larger. In fact, the juvenile fossil of the 75 mya dinosaur, Archaeornithoides (R-kay-or-nif-oy-deez) had the skull bearing the exact teeth marks that make-up the dentition of, Deltatheridium. The skull and rest of the fossil appear to have been passed through the digestive tract. Archaeornithoides is estimated to be ~ 55cm/21.7 as an adult in which is around 3.5 times larger than Deltatheridium was. This is the second known case of a mammal consuming a dinosaur, only to be predated by the 125-123.2 mya fossil of Repenomamus (mentioned earlier under Mammalia) that fed on a Psittacosaurus hatchling found in the mammal’s fossilized stomach area.
|Deltatheridium head features|
Deltatheridium had large canines for its size and some deltatheroideans possessed large enough canines that they were dubbed as junior saber-tooths. Of course their canines are strictly convergent evolution in comparison to the eutherian feline groups. But with another group of sparassodont metatherians, an order closely related to marsupials, had saber-toothed canine qualities. Thylacosmilidae (Thigh-la-cos-mill-ah-day) was a family of sparassodonts that inhabited what is northern Argentina ~ 9-3 mya. Some grew up to 1.5m/4.9ft long and weighed up to a maximum of 150kg/330.7lbs. For sure their canines were of saber-tooth quality as they were blade-like and used as daggers to down prey once piercing the neck and arteries bleeding out the victim. The canines grew throughout the animals’ lifetime. It appears however, that thylocosmilids were more of scavengers, indicated by an uncanny ability in pulling back with the canines through craniodental features, along with the unique lateral ridge of the canines adding strength to this function.
|Artist: Viergacht Thylacosmilus|
The clade, Marsupialiaformes (Mar-sue-pee-e-lee-ah-forms) are a transitional stage between basal metatherians and marsupials and are more derived than the deltatheroideans. Arcantiodelphys (R-con-tee-o-dell-fizz), a marsupialiform with a temporal range of 99.7-94.3 mya is thus far the oldest known therian coming from Europe. From France’s basal Cenomanian deposits of the ‘Font-de-Benon Quarry’, at Archingeay-Les Nouillers, Charentes; Arcantiodelphys, with some endemic European evolvement suggests its dentition crushing specializations is a stemming from Asian ancestral metatherians.
Just mentioned above under Deltatheridium, the marsupialiform order, Sparassodonta (Spar-as-o-don-tuh) had a temporal range of 84.9mya with the appearance of the scansorial omnivore, Varalphadon (Vah-ral-fah-don), to the youngest in the aforementioned, Thylacosmilus (Thigh-la-cos-muh-luss) from 3 mya. Some distinguishing anatomical features of sparassodonts are: most had canines protruding the mouth, a pronounced bulge in the snout surrounding the canine teeth, highly reduced epipubic bones that are convergently shared with the extinct marsupial, Thylacine (Thigh-la-sain) and all members were carnivorous with most having a dental formula of 22.214.171.124.
In expressing the dental formulation, the top numbers represent the amount of upper jaw (maxillary) teeth, while the bottom numbers represent the lower jaw (mandible) teeth. Teeth types are also represented in left-to-right ordered addition. The first far left numbers above and below represent the incisors (I); the second set of numbers above and below represent the amount of canines (C), the third above and below numbers represent the premolars (P) while the far right above and below numbers represent the molars (M). So, in using tooth symbols for the upper teeth we have 4I, 1C, 3P and 4M. For the lower teeth the count is 3I, 1C, 3P and 4M. Now further, the mouth is divided into quadrants with half of the quadrant consisting of teeth in the upper dentition and the other half of the quadrant in the lower dentition. So, one must multiply each number by two to get the total teeth count. This gives a total of 24 upper teeth and 22 lower teeth for a grand total of 46 teeth for sparassodonts.
You will find literature stating that sparassodonts are marsupials, but that is not the case, for as marsupialiforms they are a bit less derived than marsupials. However, as a sister taxon to marsupials they are indeed close relatives. Except for the North American, Varalphadon mentioned just above, which was omnivorous, all other sparassodonts were carnivorous and endemic to South America.
The sparassodont, Lycopsis (Lie-cop-sis) had a temporal range of 16.3-9 mya. Its fossils discovered in Argentina from the ‘Arroyo Chasicó Formation’ and Santa Cruz Formations’ and the ‘Honda Group’ of Columbia, South America, Lycopsis has three species in L. longirostrus (lawn-gee-ross-truss), L. torresi (tor-e-see) and L. viverensis (vie-ver-in-sis). Belonging to the sparassodont family, Borhyaenidae (Bore-hi-en-ah-day), Lycopsis averaged around 40.64cm/16in in length. Coming from arboreal ancestry it was fully terrestrial but a bit clumsy in speed judging from its fossil anatomy. So, instead of stalking and running down its small to medium intended prey, it most likely waited in ambush to pounce onto its victims.
|Artist: Velizar Simeon Lycopsis pursuing Prolagostomus|
Below this paragraph is an illustration of the 98.6 mya, Kokopellia’s upper and lower molars. I’m showing you this because for an innumerable amount of mammal fossils are only composed of teeth. One can see how the components can be difficult to interpret, but paleontologists are well versed in this and can readily interpret whether a jawbone of teeth are cynodonts, multituberculates, monotremes, metatherian, or eutherian. Even due to wear they can tell exactly how the animal chewed. I’ve intentionally left out dentition surface features, such as a hypoconulid and postprotocrista because it is a specialty science in the study of mammalian teeth. I’ve only merely touched base with meta-, para- and protoconids simply because I don’t know much about it either. I mean that’s a ‘mouthful’ of nomenclature for a tooth.
|Artist: Richard Cifelli upper lower molars|
In getting back to Kokopellia, it belongs to a group of metatherians that due to the dentition was at first considered a primitive marsupial, but due to further analysis, it is now considered a transitionally evolving marsupialiform into basal marsupials. Kokopellia comes from Utah, USA’s ‘Cedar Mountain Formation’ in its uppermost member of the ‘Mussentuchit Member’. Along with other evolving marsupialiforms species like: the 105.3-94.3 mya Adelodelphys (Ah-dell-o-dell-fizz), the 99.7-94.3 mya Sinbadelphys (Sign-bah-dell-fizz) and the 84.9-70.6 mya Iugomortiferum (I-u-go-mor-tif-er-um), Kokopellia presents the case that marsupials first evolved in North America, then radiated down into South America and the Pacific Rim eventually reaching Australia and New Zealand via Antarctica. But once, the migration took effect, due to being out competed by eutherians, marsupials became extinct in North America only to return once the Pleio-pleistocene Central American bridge was formed.
|Credit: 20th Century Fox Alphadon|
On an ending marsupialiform note here, with nine species, the genus, Alphadon (Al-fah-don) was a derived marsupialiform from China. With a temporal range of 99-66 mya, it was around 30.5cm/12in in length. In life, it most likely looked like a modern day American opossum even though it wasn’t closely related. As an omnivore, as evidenced from its teeth, it foraged for fruits, invertebrates and smaller vertebrates.
Marsupials have a temporal range of 66-0 mya and take their name from the pouch which is called a ‘marsupium’. Today, there are ~ 272 marsupial species with around 72 species living in the Americas and 200 species in Australasia that are diverse in body form, ecological niches and in radiated environments. From tiger, wolf, rat, and shrew eutherian body forms, most marsupials range in shape and size from a squirrel to a medium sized dog, but there are marsupials outside this range that are tiny to enormous.
|Artist: Laurie Beirne Diprotodon|
Marsupials range in size from the extant Planigale (plah-nee-gail), otherwise commonly known as the ‘marsupial mouse’, in barely reaching a length of 12cm/4.7in; while on the other hand, the Australian Pleistocene wombat, Diprotodon (Di-pro-toe-don) that lived 1.6 mya-44 kya, was the size of a rhinoceros reaching the length of 3.3m/9.9ft, with a height of 2m/6.6ft weighing up to 2,790 kg/6,150 lbs. The smallest known kangaroo is the extant, Hypsiprymnodon (Hip-see-peer-yim-no-don). It’s known as the musky rat-kangaroo and from head to body, only averages 210mm/8.3in in length. Procoptodon (Pro-cop-toe-don), one of Australia’s Pleistocene giant short-faced kangaroos that lived 774-15 kya stood on its haunches at 2m/6.6ft, but in standing upright on its feet would’ve reached a height of 3m/9.9ft.
One interesting feature concerning Planigale is that in the five species, about the only way to tell them apart is in the different shapes of the respective species’ footpads. A facet tied to Diprotodon was that it was seasonally a migrant; an unusual feature of Australia’s colonized marsupials. Hypsiprymnodon is a kangaroo that has an omnivorous diet consisting of fruit, fungi and invertebrates such as insects. Procoptodon is the only kangaroo that didn’t hop as its leg tendons couldn’t handle the extra weight stress in hopping. Instead, they shuffled around on large one-toed feet bipedally, much like human mobility. Protocoptodon even had large musculature buttocks as humans do, which is absolutely key if an animal is to balance while lifting one leg at a time off the ground.
The extinct marsupialiform family, Herpetotheriidae (Her-puh-toe-thear-i-day), as mentioned earlier under, Metatheria are the ancestral sister clade to all marsupials. With a temporal range of 66-13 mya, they survived the K-Pg extinction but succumbed to extinction soon after the Middle Miocene Disruption’s sudden cooling effects in climate temperature.
|Artist: Jorge A. Gonzalez Herpetotherium|
Herpetotheriids were a successful terrestrial group originating in Europe with the 66 mya oldest member in, Maastrichtidelphys (Mass-tree-hit-e-dell-fizz) of the Cretaceous Netherlands to the 20-13 mya youngest in, Amphiperatherium (Am-fee-pear-ah-thear-e-um) from Miocene Europe with fossils found in Belgium, Bosnia/Herzegovina, the Czech Republic, France, Germany, Switzerland and the U.K. portion of England and Scotland.
The oldest known marsupial is the 63.3-33.9 mya, Peradectes (Pair-ah-deck-tees). Originating in North America’s Paleocene Colorado Mason pocket beds, this marsupial was a well-travelled little fella, for its fossils have also been found in Paleocene California, Montana, New Mexico, North Dakota, Utah, Virginia, Wyoming, Canada, Bolivia, and Peru, but also in the Eocene USA, Canada, Peru France and England. Peradectes fossils have actually been found in 38 locations.
It was a scansorial carnivore, but fossils have been found in dry terrestrial environs, in swamps, alluvial flows and in wet fluvial lacustrine and river channel fills. The genus name is Greek for ‘pouch biter’. It had a distinct marsupial jaw and tooth anatomy along with a prehensile tail and most likely gave rise to didelphids.
This read is not for listing every marsupial, especially the extant crown group marsupials, but we will mention a few below:
|Artist: Peter Schouten Malleodectes|
Living during the Miocene of Australia some 17-15 mya, the extinct, Malleodectes (Mao-e-o-deck-tees) was a marsupial at ~ 38.1cm/15in in length looking much like a house cat. Its Latin and ancient Greek name implies ‘mallet biter’ due to its huge hammer-like teeth that has not been found inside the mouth of any other animal extinct or extant. Only the snail eating skink, Hemisphaeriodon gerrardii, also endemic to Australia has remotely similar dentition. With the teeth evidence, it has been proposed that Malleodectes was durophagus in that it dieted on hard-shelled animals such as snails and molluscs throughout the wet forest habitat it roamed in. Fossil remains were found in the ‘Riversleigh World Heritage Area’, Queensland.
Its huge premolars along with the teeth’s hammer-like shape could have easily cracked and crushed the hardest of shells. Malleodectes appears to have been related to the dasyurids ˗ carnivorous marsupials due to the details of its canines and molars. Although it may have specialized in dining on escargot, it might have supplemented its diet with small vertebrates.
|Artist: Peter Schouten Nimbadon|
Living 25-12 mya Nimbadon (Nim-bah-don) was arboreal, but not only did it climb trees, it is the largest arboreal marsupial ever known. Discovered also in the ‘Riversleigh World Heritage Area’ of NW Queensland, Nimbadon shared the fate of many other animals that fell into a cave hole and couldn’t escape, for it had no exit but for the hole in the ceiling. Eventually the cave’s rock walls and ceiling had eroded away leaving only the floor with its rich array of doomed fossilized animals.
Nimbadon, like the aforementioned Diprotodon was a diprotodontid and wombat-like, but unlike the other diprotodontids, it was the only one that was koala-like and arboreal. It was an herbivore chewing on soft tree leaves and stems. To aid in walking on land, it could retract its claws. So far there are three Nimbadon species that had an average length of 1m/3.3ft with a hefty weight of 70kg/154.3lb.
|Eastern long-nosed bandicoot|
Showing up in the Late Oligocene between 27.8-23 mya to the present, bandicoots are found in Australia, New Guinea along with the endangered Seram bandicoot of the largest Indonesian island also known as Seram. The bandicoot order, Peramelemorphia (Puh-ram-el-mor-fee-ah) consisting of 14 genera are unusual as marsupials, in that the fetuses are enveloped in a marsupial-type choriovitelline placenta, but in addition, bandicoots also have a eutherian-type chorioallantoic placenta that connects the fetus to the uterine wall. Although the chorioallantoic placenta does provide some nutrients for the fetus, it is too small to wholly sustain the fetus in lacking chorionic villi. Bandicoots are nocturnal omnivores subsisting on plant sprouts and roots, insects, worms, and smaller vertebrates like lizards and rodents. Bandicoot hind legs are longer than the front limbs giving them the ability to hop. Species range in size from 15-56cm/5.9-22in in length.
|Bandicoot with adopted duck family|
I like stories like this one above as a western Australian rancher noticed that his momma duck had adopted a stray orphaned bandicoot. She began raising the bandicoot with her ducklings. The rancher began daily checks, where one day he noticed that the little bandicoot was missing. However, the very next day it was with its adopted duck family again. He really has no idea what truly happened, but feels either the bandicoot mother had discovered it and taken it back to her nest where the infant bandicoot then found its way back to its duck family, or the momma duck brought him back after the bandicoot strayed off.
|Southern marsupial mole|
The marsupial mole belongs to the family, Notoryctidae (No-tor-rick-tah-dye) with the one genus, Notoryctes (No-tor-ah-tees) consisting of two extant species, and the one genus, Naraborcytes (Nah-rah-bor-ah-tees) consisting of one extinct species. With a temporal range of 20-0 mya, the only marsupial mole fossil find is during the Miocene 20 mya. With a leathery shield over the snout, forefeet possessing two greatly enlarged spade shaped claws for excavating sand/soil and hind feet that are flattened bearing three small claws for shoveling the loose sand/soil behind it and out of the way, the marsupial mole can literally swim through the subsurface where they spend most of their time. Sizes are variable from 12.1-15.9cm/4.8-6.3in with head and body while only having a tail 21-26mm/0.8-1.0in in length.
The tiny eyes are fused shut by the eyelids. Marsupial moles have no ear lobes, but instead just two small holes, one on each side of the head covered by thick fur. On the other end the short tail is stubby and naked. Both the covered ear openings and short tail evolved convergently to full-term placental mole features to fit their fossorial environmental lifestyle. The last five cervical vertebrae are fused giving the head greater rigidity in digging. As in all marsupials there is a pouch on the females, but unlike other marsupials the opening is on the bottom instead of the top to keep sand/soil out when the mother is tunneling.
|Southern marsupial mole dining on lizard|
The scientific notation for the southern marsupial mole is N. typhlops (tye-flops) while the northern marsupial mole is N. caurinus (core-een-us). These two extant marsupial moles live in the deserts of Australia with the southern marsupial mole inhabiting central Australia where the northern marsupial mole inhabits further northern central Australia bordering the southern marsupial mole’s extent northern boundary. The extinct Naraborcytes actually lived in the NW Corner of Queensland during Australia’s Miocene period not in desert, but in an environ of lush tropical rain forests. All three species are insectivores/carnivores.
|Northern marsupial mole dining on centipede|
There has been some questioning if Naraboryctes is truly a marsupial mole due to differences in synapomorphies (derived anatomical characters) the two extant marsupial moles feature anatomically. The two Notoryctes species have considerable polymorphism in tooth number between both species and as well within the same specimen. Nonetheless, reflecting on dentition, the consensus of older studies like the Archer et al. (2011) report, the presence of four molars (typical of marsupials) in each quadrant is also standard in both the extant, Notoryctes and the extinct notoryctid, Naraboryctes.
Their hearing is exceptional and about the only time they will surface is to capture the prey after being located through the sounds the intended victim makes while transporting across the ground’s surface. Occasional flooding will also make marsupial moles surface.
Below is a quick video of N. typhlops digging:
The relative Cretaceous paucity of placentals from North America and their greater abundance in Asia suggests the earliest flowering of the Eutheria clade in an unspecified region of the Old World. Until further discoveries of eutherian fossils are found, this is the prevailing thought. The closest living relatives to eutherians are the marsupials.
Basal eutherians were small, most no larger than a rat and when considering biological implications, their derived high metabolic rate was more pronounced allowing for a more successful existence. For instance, when one region they inhabit runs out of food, they can easily migrate without being noticed to another region where resources are more abundant. Smaller animals also have relatively higher birth and quicker growth rates accelerating maturation processes. Rapid population increases spur the tendency to move and adapt to new environments around the globe. Giving birth to fully developed embryos has also aided in their speciation and spread.
Under Metatheria we introduced a time frame of 160 mya of the oldest eutherian fossil thus far discovered. That eutherian fossil was Juramaia (Jure-ah-my-ah) with the name referring to the Greek terms for ‘Jurassic Mother’. The fossil discovery was in the western half of China’s Liaoning, Province.
Also, under Mammalia we described a bit about Juramaia, only to refresh it up a bit here. At an average size of 85mm/3.5in, it was a small arboreal animal scampering after insects up in the trees. Its leg and toe anatomy was ideal for climbing and running in trees. Juramaia provides new insight into the evolution of placental mammals by showing that their lineage diverged from that of the marsupials 35 million years earlier than previously thought, while filling gaps in the fossil record to calibrate modern DNA-based methods of dating mammalian evolution. However, even though Juramaia is listed by most as eutherian, it is most likely at best a great, great aunt to eutherians. That’s my conjectured thought, but I’m sticking to it…for now, anyway.
Shocking as it may seem but before we go any further, most basal eutherians were not full-term placentals as the females had too narrow of openings at the base of the pelvis disallowing full-term fetuses to emerge; much like marsupials. They also possessed pubic bones. However, all eutherians shared in the tibia base (the larger of the two shin bones) in having an enlarged malleolus which is the ankle bone that is hammer head in shape. The joint between the first metatarsal bone and the entocuneiform bone (the innermost of the three cuneiform bones) in the foot is offset further back than the joint between the second metatarsal and mesocuneiform bones (the middle cuneiform bone). Various anatomies of the jaws and teeth are distinct only in both primitive and modern eutherians.
To refresh, we introduced Eomaia under, Multituberculata that arose 125 mya. I personally list it as an advanced multituberculate due to its age and not as a eutherian or even therian. The main current consensus is that it is a basal eutherian and therefore a therian. The name itself implies ‘dawn mother’. But there is a trend brought forth by the esteemed paleontologist Maureen O’Leary and colleagues that it is only a stem group of therians and at best a sister clade as basal to non-placental eutherians. However, I feel it is too much in age to be a genuine therian, much less a eutherian, but for sure it is more closely related to the therian groups.
Eomaia not only has eutherian traits, but it also has anatomical characteristics of metatherians as well, such as possessing an epipubic bone. Therefore, it is more closely related to therians than the O’Leary team suggests, but too primitive to be a eutherian as most all paleontologists suggest that it is. Hence I feel it is closer to the common ancestor of all therians, just not quite there to be a therian. You be your own judge…
There are other mammals that could be considered as basal therians, or even as stem eutherians such as Prokennalestes (Pro-ken-nah-less-tees) that also occurred around 125 years ago with a temporal range of 125.45-99.7 mya. Coming from Early Cretaceous Mongolia, Prokennalestes had a dental formula consisting of three molars and five premolars. The lingual cingula (singular: cingulum ~ incisors and canines; lingual: palatal developmental lobe of anterior teeth) are present, but as very short and obscure. Also, a masseteric foramen along with a mandibular foramen that opens in a prominent longitudinal ridge are present.
From Prokennalestes, direct descendants were the Mongolian, Kennalestes (Ken-nah-less-tees) from 75 mya and the Siberian, Murtoilestes (Mur-toi-less-tees) from 113 mya. More derived non full-term placentals were Japan’s Sasayamamylos (Sass-i-ah-mam-e-los) from 112.6-109 mya, Montana’s, Montanalestes (Mon-tan-nah-less-tees) from 113 mya and Uzbekistan’s, Bobolestes (Bo-bo-less-tees) from 99.7-94.3 mya.
|Artist: Richard Cifelli|
For the above illustration the numbered symbols stand for: (2) coronoid bone, (3) Meckelian sulcus, (4) premolars, (5) back premolar, (6) molar count.
Concerning the mandible in most mammals, there is a circular opening where the mandible nerve runs through. This is called the labial mandibular foramen. Also, the shallow depression near the base of the coronoid and angular processes is the masseteric fossa. In some mammals the masseteric fossa actually perforates the dentary; in those forms the resultant opening becomes known as the masseteric canal. The reason these traits are being mentioned is because most all Cenozoic placentals including humans have them. Eomaia did not possess a labial mandibular foramen and masseteric fossa, but the more derived Prokennalestes did. Eomaia had undeveloped conules on the upper molars, but the more derived Murtoilestes had more pronounced conules just as Cenozoic placentals do.
As well, Eomaia differs from both Prokennalestes and Murtoilestes in having an anteroposteriorly shorter molar trigonid and a longer talonid basin. Eomaia differs from Montanalestes in having a paraconid lower than the metaconid. Eomaia also differs from Montanalestes and all other Late Cretaceous therians and full-term placentals in retaining the primitive Meckelian sulcus (groove) on the mandible, although Prokennalestes retained the sulcus as well.
The Meckelian sulcus is structurally associated with a Meckel’s cartridge middle ear connection in many Mesozoic mammals. The two older stem eutherians, Prokennalestes and Eomaia, which are related to modern placentals, have clearly preserved the Meckelian sulci. Similarly, the stem metatherian Kokopellia (mentioned under Marsupialiaformes) also has a distinctive Meckelian sulcus. Enlarged Meckelian sulci are also present in the immediate outgroups of the metatherian–eutherian clade, such as the cladotherians. Independent studies of these stem eutherians, metatherians and respective outgroups have all determined that the Meckelian sulci in these forms were grooved from the Meckel’s cartridge; not from other soft-tissue structures. Thus, it can be inferred that Meckel’s cartridge and ear were connected in basal eutherians and metatherians in that Meckel’s cartridge breakdown occurred separately in the eutherian/placental and metatherian/marsupial lineages through at least partially distinct cellular patterns.
With Meckel’s cartridge showing up as early as the Early Cretaceous is a classic example of evolutionary trends. The evolutionary origin of the definitive mammalian middle ear is transformative. As we expounded on much earlier when discussing mammalian ear development, reptiles and pre-mammalian synapsids possess multiple bones in the jaw but only a single bone (stapes) in the middle ear. By comparison, all mammals possess a single dentary bone in the jaw, and multiple bones in the middle ear, namely the malleus, incus and ectotympanic, while not to forget in addition the stapes.
This developmental mechanism for the separation of the mammalian middle ear ossicles from the reptilian jaw has been traced and verified in step by stepped stages genetically and embryonically by a group of U.S. and U.K. researchers led by Karen E. Sears. Using marsupial fetuses and undeveloped pups, they were able to pinpoint the trending migrations of the 3jawbones’ migration from the jaw to the making of the ear bones through the various stages of the fetus and undeveloped pups as the bone migrations are still retained in early marsupial development.
All these basal therians/stem eutherians show that their clade had originated in Asia and successfully radiated out across northern Europe and into America. They all were Cretaceous inconspicuously small scansorial insectivore tree dwellers and were most likely nocturnal. The reason being for eutherian roots existing in an arboreal niche was most likely to stave off dinosaurian predation. Of course when the avialan dinosaur evolved and utilized trees, it put a wrinkle in these early mammalian strategies although the small size would help in hiding. But before they were to leave the trees and venture forth onto terra firma, their progeny would have to grow in size.
A couple of more advanced group members belonging to non-full-term placental eutherians are the extinct adapisoriculids and cimolestans. The family, Adapisoriculidae (Ah-dap-uh-sore-uh-cull-ah-day) had a total of eight genera that covered a temporal range of 66.043-40.4 mya. The order, Cimolesta (She-mo-les-tah) had members ranging in temporal ages of 70.6-55.8 mya.
When it comes to biological reproduction, since reproduction components do not fossilize well, all we can definitively say is that adapisoriculids were dioecious in that there were distinct male and female genders for sexual procreation. Yeah right, like that is almost for all vertebrates from fish, amphibians, reptiles and mammals. Adapisoriculids were small (no more than 15cm/5.9in long) scansorial insectivores and arboreal. Their fossils have been found in Europe, India and northern Africa.
Cimolestans were a large successful group and were non-placental stem eutherians. The anatomy is so common with other eutherian groups in looking like rodents, weasels, squirrels, otters and opossums that cimolestan species originally were confused to be related to other extinct and extant eutherian groups. However, they are a unique eutherian group leaving no descendants after going extinct ~ 33 mya. Part of the larger superorder Didelphodonta (Di-dell-foe-don-tah), cimolestans are remotely related to the eutherian, Carnivora (Car-nee-vor-ah), along with the creodonts, an extinct carnivore group. Cimolestans are more related to full-term placental mammals than they are to marsupials.
|Credit: Internet Cimolestes|
With a temporal range of ~ 68-56 mya in surviving the K-Pg extinction Cimolestes (See-mo-les-tees) was a cimolestan inhabiting woodlands of Morocco, Belgium, and North America. Being arboreal with a mouthful of dentition geared for consuming insects, in all probability it was an insectivore. In fact, the name refers to ‘bug thief’. With a body length of 20.3cm /8in, it also possessed a prehensile tail that was a bit longer than the body.
Another group of cimolestans was in the family Pantolestidae (Pan-toe-less-tuh-day). Living 63.3-28.4 mya that were otter-like in form geared for a semiaquatic lifestyle. They were piscivorous, as confirmed from fossilized stomach contents, while also consuming other aquatic forms like frogs and freshwater mussels/snails. But, due to the particular dentition patterns they were also most likely insectivores, snatching invertebrates on land.
With strong powerful forelimbs and hind limbs and a rudder-like tail, pantolestids were geared for swimming with ease. The forearms had the radius and ulna bones articulated providing wide rotational movements. With a tail length of nearly 35cm/14in, the body was on average 50cm/20in long.
Latter pantolestids possessed prominent cranial crests combined with strong spinal processes indicating the presence of strong neck muscles where they could constantly hold their heads above the water surface while swimming.
With the oldest pantolestid fossil finds coming from the Paleocene’s Saskatchewan, Canada, Montana and Wyoming, USA’s ‘Green River Formation’, they then spread to Eocene Europe’s France, Belgium and the German ‘Messel’ pit 48-40 mya. From Europe’s dispersal to the youngest fossils found in the ‘Ergilian’ deposits of Mongolia and Kazakhstan 37-33 mya; pantolestids became one of the few examples of European mammals dispersing into Asia during the Grande Coupure, which was an extinction event 33.5 mya between the Eocene and Oligocene marking large scale extinctions and floral/fauna turnovers.
One last cimolestan we’ll discuss here is, Kopidodon (Koe-pid-ah-don) from 47 mya. Its fossils from Germany’s ‘Messel’ pit are very detailed and articulated; even retaining fossilized ear lobe impressions. Originally the ‘Messel Pit’ was the site of a lake bed surrounded by a tropical forest that with slow deposition of mud and dead plant debris formed a slow anoxic stratification in forming oil shale. This process formed very detailed fossils of animals that died in the vicinity.
Kopidodon’s molars were adapted for chewing plants but had huge canines that were most likely used for defense. At 1.2m/3.9ft, it is one of the largest arboreal European mammals ever for the time. The legs and long claws allowed Kopidodon to scamper up and down and through trees with ease. Due to the exquisite preservation of its fossils, its fur impressions were preserved showing a bushy tail making it look either like a squirrel or a red fox in life.
|Phylogeny of Crown Group Mammals|
Explanation of the above figure: The phylogeny of extant placentals. (a) Topology of extant placentals indicating branches with high (red) or low (blue) ratios of morphological/molecular evolution. Node numbers indicate clades; as follows: 1, Placentalia; 2, Atlantogenata; 3, Boreoeutheria; 4, Xenarthra; 5, Afrotheria; 6, Laurasiatheria; 7, Euarchontoglires; 8, Eulipotyphla; 9, Scrotifera; 10, Chiroptera; 11, Artiodactyla; 12, Pegasoferae; 13, Perissodactyla; 14, Carnivora; 15, Glires; 16, Lagomorpha; 17, Rodentia; 18, Euarchonta; 19, Scandentia; 20, Primates. (b) Averaged molecular partition branch lengths. (c) Averaged morphological partition branch lengths. Source: Royal Society Article ~ Rapid Morphological Evolution in Placental Mammals Post-dates the Origin of the Crown Group.
There is a lack of definitive Cretaceous full-term placental mammals and may be a result of morphological similarities among stem and early crown eutherians, but there is an avenue for reconciling the fossil record with molecular divergence estimates for full-term placental mammals (Placentalia). Although both molecular data and fossil occurrence patterns predict that placental mammals should be present in Cretaceous beds, as yet no demonstrably Cretaceous placental fossil has been found. However, there is Protungulatum, known from the latest Cretaceous and earliest Paleocene of North America, as mentioned earlier toward the end of Theria, is the best candidate in having been recovered as a relative of extant ungulates in much research analyses. But a few other paleontologists have contended that it too is a stem eutherian. Only more fossil finds and time will tell.
Regardless of how rare Cretaceous full-term placentals are, once the Cenozoic time frame kicked in, prototherians, metatherians, eutherians and in particular the extant crown group mammals radiated out and kicked fanny perpendicular in taking over all Earth’s habitable niches. The mammalian class consists of 5,000 species broken down further into 27 orders. The Cenozoic, which is the core of today’s crown group mammals’ origins, really supported a large fauna of extinct mammals. It would’ve been an ordeal to list them all in this treatise. We will not go into extant mammals and their ancestry, so to add to the extinct mammal list provided earlier, here is a list of extant mammals if you want to preview further:
But, we will end with some fun facts, so here goes…We start first with an easy one, for if ya read the whole article ya would know what this animal is. Below is a skull photo and illustration of an extinct mammal; can you guess what it is?
|Artist: Viergacht Thylacosmilus|
If ya guessed it is a saber-toothed cat, like Smilodon, (Smile-o-don) ya guessed wrong, for it is not anywhere close to being a full-term placental felid (cat). Above is, Thylacosmilus (Thigh-lak-o-smile-us) that, as a sparassodont metatherian, was much closer to marsupials than to full-term placental mammals. The saber-like canines in both Thylacosmilus and the felid saber-toothed cats/tigers possessed the extra-large canines through convergent evolution, therefore independently of each other as they weren’t related.
|Artist: Sorin Buluciano Smilodon|
Besides, Thylacosmilus most likely wasn’t a killer like, Smilodon, in which it used it canines for stabbing, then killing prey. Due to the mouth musculature strength of Thylacosmilus, the canines were best for horizontal tearing and pulling, rather than for vertical piercing, it most likely was a scavenger utilizing the canines for tearing and ripping apart dead carcasses. Nevertheless, I wouldn’t have wanted to have ticked either one of these two mammals off. Also, saber-toothed cats/tigers could open their mouths up to 128 degrees wide, where Thylacosmilus and felid conical-toothed cats couldn’t and cannot.
|Smilodon mouth gape|
Then there was Barbourofelis (Bar-burr-o-fee-liss) that lived 16.9-9 mya. Originally roaming Africa, it crossed over into Eurasia, then from there to North America when an Early Miocene land bridge opened up between Africa and Europe. Barbourofelis was a filiform carnivoran sister group to the saber-toothed cat family of, Nimravidae (Nim-rav-ah-day) or the subfamily, Machairodontinae (May-care-ah-don-tah-nay) but also possessed the mandible ventrally extended mental process (bony extensions on either side of the lower jaw) just as, Thylacosmilus had.
|Artist: Roman Uchtyel Barbourofelis|
Since we’re on the topic of cat-like animals there was the family, Thylacoleonidae (Thigh-la-cole-e-in-ah-dee) that roamed Australia 15.97-0.012 mya. Although this marsupial family transitioned to an omnivore to being wholly carnivorous it evolved from herbivorous vombatiforms, the lineage that includes the extant wombats and koalas. As widely distributed across Australia, the family consisted of four genera that ranged in size from a large housecat to a leopard with size increasing from the older to the more derived younger genera.
With very unusual dentition, thylacoleonids could take down prey much larger than themselves. Held by a very wide short stout skull, the jaws contained bucktooth-like incisors that acted as chisels digging into flesh then pushing it back toward the secateur third premolars that performed like shears tearing and ripping up food. Studies conducted deem that for their size, thylacoleonids possessed the most powerful bite of any known mammalian carnivore. As well, this family had powerful claws that were retractable suggesting they were not only used for defense, but also allowed a scansorial existence. The thumb had an extra-large claw hidden in a skin sheath when not in use and could have been utilized for disemboweling prey or enemies. The teeth could easily make mincemeat out of flesh and even bone.
Since we just spoke of cats, how about speaking of dogs as another group of carnivores. Do the two illustrations below represent the origins of dogs or bears?
|Artists: Julio Lacerda-Roman Yevseyev Amphicyonids|
The animals in the above illustrations are amphicyonids and if ya guessed dog or bear you’d be right either way. In fact, the common name for amphicyonids is ‘bear-dogs’. The family, Amphicyonidae (Am-fee-seen-ah day) with a temporal range spanning over 40 million years of 42-2.6 mya, consisted of three subfamilies with a total of 34 genera. The earliest and smallest ones were no more than 23cm/9in, while the more derived younger ones reached sizes of 2m/6.5ft. As they became bigger, amphicyonids went from in appearance to being more dog-like to bear-like.
Recent research now bears out that amphicyonids did not originate in Eurasia, but first appeared in N. America then crossed over into Europe during the Miocene. Also, an amphicyonid migration occurred via the trans-Beringian route into Russia, then Asia. Although amphicyonids are distantly related to canids (dogs) and ursids (bears), they are very basal caninforms arising from lineages much older than the origins of both bears and dogs.
Can anyone guess what the animal below represents as far as an extant mammal lineage goes?
|Artist: Leonardo Alannis Indohyus|
The illustration above is the mammal, Indohyus (In-doe-hi-us) which belongs to the family, Raoellidae (Rah-l-ah-day). Raoellids lived 48.6-40.4 mya during the Eocene of Pakistan, India and China. They were terrestrial digitigrade (walked on toes while the heel does not touch the ground) artiodactyls, but as well were highly semiaquatic. Just like the capybara of today, they were herbivores that didn’t dine on water plants, instead preferring to graze on terrestrial plants and most likely used the watery environs for escaping predators. What is the amazing significance of this family is that they are closely related as a sister taxon to the completely aquatic cetaceans, which includes porpoises, dolphins and whales. Indohyus possessed an auditory bulla formed from the ectotympanic bone exactly as in whales, porpoises and dolphins. It also had a middle ear space called the involucrum that is only found in cetaceans today.
In whale lineage drawings you may well see Indohyus as the forerunner in the cetacean lineage however; Indohyus is not the direct common ancestor to cetaceans as its temporal range is too late, for the first true oceanic whales appeared 50 mya. The direct common ancestor hasn’t as yet been discovered, but Indohyus surely is a sister to whatever was the direct common ancestor. Its fossils possess an involucrum which is a thick bone covering over the middle ear space to enrich underwater auditory vibrations. Before discovering Indohyus’ fossils and some of its immediate predecessors in the cetacean lineage, all other animal groups, extinct or extant, do not possess an involucrum. Indohyous also had heavier limb bones to keep it from floating while wading or submerged in water.
|Artist: Lucas Lima Indohyus|
So, in quick fashion, we went from Indohyous, a small deer-like artiodactyl that was quite comfortable in being semiaquatic in freshwater to…
|Credit:Mariam Moftah Pakicetus|
…Pakicetus (Pak-ah-see-tuss) that was more transitional from land even-toed ungulates to whales with developing webbing between toes, but more closely related to hippopotami wandering shorelines in search of small animals on land and in water for freshwater fish to…
|Credit: WwB Ambulocetus|
…Ambulocetus (Am-be-lo-see-tuss) who by now was nearing total saltwater by inhabiting estuaries and inland bays with limbs that were geared more for swimming strokes than walking, but would still come ashore to ambush terrestrial animals with whale-like teeth to…
|Artist: Pavel Riha Rodhocetus|
…Rodhocetus (Rod-hoe-see-tuss) that by now was almost totally hairless and adept to saltwater environs with the nostrils beginning their migration up the snout and able to swallow underwater, although it could still walk on land to…
|Credit: WwB Basilosaurus|
…Basilosaurus (Ba-zuh-low-saw-ruhs) and Durodon (Dur-ah-don), belonging to the family Basilosauridae (Ba-zuh-lo-sawr-ah-day) are the last of whale species to possess vestigial hind limbs and be totally independent of sea environs, but afforded them the chance to spread out into open oceans to...
|Capera pygmy right whale|
…Caperea (Kay-pre-ah), the extant pygmy right whale that is the sole member left of the oldest oceanic whale family, Cetotheriidae (See-tah-the-ree-day) from ~ 28 mya during the Oligocene, which actually looks like what we would call a whale. The rest of cetothere species went extinct by 2 mya.
That’s right…as a page-by-page review in a history book, the rock strata filled with the evolution of whales gives in detail account just how the gigantic 30.5m/100ft blue whale through its ancestry started out and evolved from a terrestrial 60cm/23.6in herbivorous artiodactyl four-legged mammal. Below is a quick 4 minute video by Chris Thompson on whale evolution:
What ancestral lineage does this mammal below represent?
|Artist: Daniel Eskridge Eohippus|
This is Eohippus (E-o-hip-puss), the first horse. The name infers to ‘dawn horse’ as it was one of the first horses in the family, Equidae (Eck-qui-dee). It belongs to the horse family and is described as the first horse living around 56 mya. As one the first horses, Eohippus with four padded toes on the forelimbs and three on the hind limbs dwelled in North American forest habitats and was no more than 76.2cm/30in in total length. But soon, it had company with other, little forest horses evolving, like the 50-45 mya genus with the five species and three subspecies, Hyracotherium (Hi-rak-o-the-ree-um), while in turn was ancestral to the 50-46.2 mya genus, Orohippus (Or-o-hip-pus) that the derived 46-38 mya genus, Epihippus (Ep-e-hip-pus) evolved from which was in size ~ 61cm/2ft tall.
There was, for a time, strong consideration in ranking Eohippus into the genus, Hyracotherium under the family, Palaeotheriidae (Pay-lee-o-ther-e-ah-die), but scientists instead felt that with its morphology differences, Eohippus is truly a monotypic species in, E. angustidens (an-gus-tuh-duns).
The above diagram uses Hyracotherium as the basal horse, which is trending. Please also note that there are many horse species left out of the above family lineages as the illustrations are a simplified version of the horse ancestry.
Except for Epihippus, all of the above early day horses were forest dwellers with dentition of low crowned cusped molars suited for browsers in grinding softer plant material like fruits and leaves. Epihippus dentition was transitioning from lower to higher molar crowns and was primarily supported by the middle hoof (nail/claw), while leaving the dwindling forests adapting to open plains and its vegetation such as grasses. Epihippus probably started subsisting on more forested fibrous vegetation before frequenting open grass fields. Latter plains horses’ dentition showed much higher crowned molars with sharper surfaces for slicing off grasses. Since 4-5 million years ago, the horse diet is primarily composed of grasses. Due to whatever carbon isotope in grasses is trapped in the fossilized teeth and bones is how paleontologists know what diet the horse species were eating.
Savanna grasslands dominated by C4 grasses as they are today, first became widespread much later after horses with high-crowned teeth first appeared. Prior to 7 mya, horses had a C3 based diet and after 7 mya horses started eating C4 grasses. This change in diet occurred when the major drop in the diversity of horses occurred during the late Miocene. The change in vegetation reflected in horses’ diet may be related to a significant reduction in atmospheric CO2 levels toward the end of the Miocene which provided the C4 grasses with an adaptive advantage and led to their expansion at the expense of C3 plants.
The horse essentially evolved in North America, but became extinct there around 10,000 years ago. Once the warmer wetter Eocene climate changed into the colder drier Miocene, flora and fauna, in order to adapt changed as well and this certainly was the case for the equid line. As the forests and woodlands gave way to open plains, the toes as built for soggy forest floors to keep from sinking into muck weren’t as useful in hard packed open plains. So, horse species began changing to accommodate the harder grounds by walking on the middle toe’s hoof (nail/claw) while the remaining outside toes became vestigial remnants. Size also mattered in the open, so began enlarging of the horse’s body size with longer legs and stronger muscles to run away faster from predators while developing a kick for defense. Equus, the only surviving horse genus includes horses, donkeys and zebras.
|Artist: Danielle Byerley Equus/Sifrhippus|
As small as eohippus was, it was not the smallest equid, for that titles belongs to, Sifrhippus (Siff-ra-hip-pus) from the Bighorn Basin of Wyoming, USA 55.8-50.3 mya. Something strange about Sifrhippus is when it first appeared, it was ~ 59.4cm/23.4in lengthwise while weighing 5.4kg/12lbs. However it began to shrink until its length dropped to 46cm/18in long with the weight down to 3.9kg/8.5lbs. Not so strange yet, but towards the end of its existence it had rebounded back up to 74.3cm/29.25in and weighed in at a whopping 6.8/15lbs. What the heck happened?
What happened was heat. The Paleocene-Eocene Thermal Maximum (PETM), a 175,000 yearlong interval of time some 56 million years ago in which average global temperatures rose by about 10 °C/50 °F. Due to the influx CO2 in the atmosphere and oceans, global warming occurred in which endothermic animal bodies respond by decreasing body size in reducing surface area to manage lowering body core temperatures. During this time, around a third of the mammal species experienced a reduction in size. However, towards the end of the PETM’s effects 50 mya as the climate began cooling, Sifrhippus’ body rebounded in length and mass.
|Photo: Sven Tränkner pregnant Sifrhippus fossil|
One exquisite, but sad fossil of Sifrhippus is from the Messel Pit of Germany. Found in 2014, it consists of a pregnant mare with the detailed fetus giving superb information on the horse’s early day reproduction. The 47 mya thirsty pregnant female went to drink from the shore of a lake not knowing that volcanic gases would soon overwhelm her and her unborn baby.
Perisodactyla (Peh-ruh-suh-dak-tuh-luh) is the order of all horses, going from Eohippus to the modern day Equus. Since perissodactyls comprise all odd-toed ungulates that bear weight only on the middle 3rd toe, including rhinoceroses and tapirs; all odd-toed ungulates walking on one toe hoof (toenail/claw) are distantly related to these early day horses’ perissodactyl sister taxon, Palaeotheriidae (Pal-e-ah-thear-e-ah-dye). Palaeotherids went extinct during the Grand Coupure 33.6 mya. As perissodactyls, they were much like their early day horse relatives in browsing for soft shoots, fruits and leaves from forested floors. Palaeotherids just didn’t have the adaptive capabilities to adjust to a changing cooling climate. Although one palaeotherid genus in, Palaeotherium resembled tapirs, the European perissodactyl was much closer in relations to the primitive horse lineage.
|Artist: Roman Yevseyev Palaeotherium|
Phenacodus (Fah-nac-o-dus) is under the extinct family, Phenacodontidae (Fah-nac-o-don-tuh-day) from which the family was named after. Consisting of seven genera, phenacodontids were very early ungulates that lived from 61-48 mya. Phenacodontids were large herbivorous mammals for their time with Phenacodus having a length and body weight of a small sheep at 1.3m/4.3ft in length and a weight of ~ 101.8kg/224.5lbs.
|Artist: Heinrich Harder Phenacodus fossil/life|
Phenacodus, from 56-48 mya was primarily an herbivore, but with canine dentition, might as well have been an omnivore. With anatomical similarities and environmental adaptive strategies, Phenacodus appears very much horse-like. These shared traits include a digitigrade gait utilizing the larger middle third toe over the other four and dentition composed of bunodont (low rounded teeth) back molars for more efficient grinding of vegetative roughage. Along with its latter contemporary fellow phenacodontid, Meniscotherium (Mah-niss-co-thear-e-um) from 54-38 mya both of these phenacodontids are considered stem perissodactyls.
|Artist: Roman Uchytel Diacodexis|
Known as artiodactyls, the other ungulate group is the even-toed ungulates that bear weight on an even number of toes such as: sheep, goats, cows, pigs, deer, antelopes, hippopotamuses, camels and giraffes. All extant artiodactyls walk on the third and fourth toed-hooves (nail/claw) with the other three toes degenerated or vestigial. The first known artiodactyl named, Diacodexis (Di-ah-ko-decks-is) still possessed all five toes, but favored the slightly longer middle two 3rd and 4th toes for weight bearing. It existed for 9.2 million years between 55.4-46.2 mya. It was small at 50cm/1.6ft, but the legs were built for speed running (cursorial) and it was widespread first appearing in Pakistan then dispersing out into Asia, Europe and North America. Diacodexis was a forest floor dweller browser consuming leaves, fruits and other soft plant material.
What group of animals is this extinct mammal illustrated below ancestral to?
|Artist: Mehdi Nikbakhsh Moeritherium|
If ya guessed hippopotamus, you are wrong, but that was a good guess nonetheless, for it not only looked like a hippo it also had their habits of hanging out in bodies of water and eating aquatic and terrestrial soft vegetation. However, the mammal above is Moeritherium (Mare-ree-thear-e-um), one of the earlier known true proboscideans that lived 37-35 mya. Nonetheless though, in Moeritherium having an age range of 37-35 mya nearly double the years after, Eritherium (Ear-e-thear-e-um) lived 60 mya which is the first known proboscidean. It was very small as most mammals were then and only measured 20cm/7.9in tall from the ground to its shoulders, while its length was ~ 50cm /19.7in. Due to the small sizes, it is fairly difficult considering that these two mammals were proto-elephant proboscideans.
|Credit: theworldnews.net Latvia Eritherium|
Eritherium’s fossil was found 100km/62.1mi east from Casablanca, Morocco in the ‘Ouled Abdoun’ phosphate basin. It had its mandible’s lower canines jutting outwards about 2.4cm/1in from the mouth; a prelude to elephantine tusks. It is a basal ancestor to all other latter day elephants.
|Credit: Internet Elephant evolution|
|Credit: Encyclopedia Britannica Branching evolution|
The order, Proboscidea includes the two extant Indian and African elephants and their extinct kin, like the mammoth and the more distant mastodon. The word ‘proboscis’ literally means trunk. Although today’s elephants are only found in Asia and Africa, extinct forms also lived in Europe, Siberia and the Americas.
What group of animals today derived from this mammal pictured below?
|Credit: Tet Zoo Entelodont|
If ya had guessed that it was a member of the Suidae (Sue-e-dye) family of pigs, ya guessed wrong as looks are deceiving. The mammal in question belongs to the family of Entelodontidae (In-tel-o-don-tah-day) where its eight genera composed of nineteen species existed for 21.23 million years spanning a range from 37.2-15.97 mya.
Although entelodonts were artiodactyls like swine, making them distantly related to pigs, they are much more closely related to hippos, whales and the extinct, Andrewsarchus (An-dru-sar-kus) of Inner Mongolia, China 43-41 mya from the Eocene’s ‘Irdin Manha Formation’. It possessed nearly a .91m/3ft long jaw with wide cheek bones that strong muscles were attached to.
|Credit: WwB Andrewsarchus|
With a carnivorous and scavenger diet, Andrewsarchus would terrestrially scour the countryside in search of capturing easy prey or consuming carcasses, but as well was durophagus combing beaches for shellfish, crustaceans and maybe even turtles that its particular dentition could easily manage. Any dead animal that washed up on shore would have also been consumed. With the estimation of being 1.8m/6ft high and ~ 4.9m/16ft long while weighing up to 800-1000kg/1,764-2,205lbs, Andrewsarchus is the largest terrestrial carnivore mammal known so far.
|Andrewsarchus skull/feeding Artist: V. Simeonovski|
In going back to entelodonts, they could be vicious looking in appearance and deed as well. With dentition including large canines and heavily built incisors suited for being carnivorous and broad flat molar teeth wear suggesting a diet of plant roughage consisting of roots, tubers, nuts and bark, entelodonts were omnivores. Also as in pigs they most likely would scavenge carrion. So, in not only appearing pig-like, their voracious omnivorous diet was also pig-like.
It’s in the anatomical structure that relates them to other artiodactyl animal groups. The expanded semi-circular forepart of the skull is specific to entelodonts and Andrewsarchus while being very similar to hippos. The ‘pie-crust’ fracture wear on the dentition also suggests bone crunching as also displayed in Andrewsarchus.
|Extant Borean bearded pig/Extinct Entelodont Artist: Max Bellomio|
Entelodonts, with a .91m/3ft skull and a humped neck spine to support the neck had a very strong bite as there are fossilized small camel/llama-like camelids 90cm/3ft long that were bitten in half and cached in what is now Nebraska, USA. The bite marks match-up exactly to entelodont dentition. Speculating a bit here, but I’m surmising the camelids were, Poebrotherium (Po-bruh-ther-e-um).
|Artist: E. Katie Holm Entelodont|
Entelodonts roamed the terrestrial country sides in open plains, sparse woodlands and seasonal floodplains in North America, Europe and China during the Late Eocene to the Middle Miocene. We know they could be vicious, thus the tagged moniker of ‘hell pigs’. Males would have had intraspecific fights that would’ve included gnawing at heads, while females would’ve head butted each other’s ribs. There are plenty of indications in fossil finds of male facial bone tooth scars that match entelodonts dentition and female healed cracked ribs. If any predator was foolish enough to attack an entelodont or its offspring the wrath of fury would come reigning down on the predator.
|Artist: Velizar Simeonovski entelodontid Daeodon vs Dinictis|
The earlier entelodonts were medium sized reaching weights of 150kg/330lbs to the latter larger ones like, Daeodon (Day-o-don) weighing up to 900kg/2,000lbs with a height to the shoulders of 1.8m/5.9ft. With their weight and ominous mouth riddled with slicing teeth of pointed incisors, recurved, pointed, serrated canines and a detachable/movable jaw joint, it could have easily overwhelmed a predator in taking its freshly killed prey.
|Credit: WwB two Entelodont males fighting|
Although there are numerous fossil finds of entelodonts, they are only of individuals, so assuredly entelodonts were not social creatures in grouping into herds preferring instead to strike out on their own living a secluded life. The only time they would come together was in males fighting over females and mating.
What in the world is this mammal below?
|Artist: Michael Long Ceratogaulus|
The above illustration is, Ceratogaulus (Sah-rat-o-gall-us) from 13.6-5.3 mya. Commonly known in science circles as the horned-gopher, it was a rodent and the only rodent that had horns. At 30cm/11.8in, it’s also the smallest known horned mammal. It was a fossorial (burrower) sciuromorph (squirrel-like) with two horns jutting upwards from the nose. Only one other known fossorial mammal in the 29-11.6 mya South American, Peltephilus (Pel-tiff-auh-luss) had horns. It was a 1.5m/4.9 foot long armadillo. Its two horns on the top of its snout were formed by specially developed scutes.
Ceratogaulus from 11-5 mya is the youngest species in the family, Mylagaulidae (My-la-gall-e-day) with species ranging from 28-5 mya in Nebraska. However, 25 out of the 30species of mylagaulids did not possess horns, only five out the 30 did. It was the smaller mylagaulids that grew horns with the reason being most likely for defense. In the species with horns (and yes, they were true horns made from bone), both male and female had them, so they weren’t enticing mates in mating rituals. With the particular positioning at the top of the snout and jutting upwards the horns would have been ideal to ward off curious predators from their burrow openings with two horns stabbing back and forth at them.
|Artist: V. Simeonovski Ceratogaulus pursued by Pseudaelurus|
In fact there is a mechanical reasoning for signifying the horns were used for defense. With the horns being posteriorly positioned, the height of the occipital plate increases, which in turn increased the leverage for lifting the horns. By positioning the horns more posteriorly, a so-called output lever is shortened while the muscles used to rotate the skull dorsally are also attached at the top of the occipital plate. This lengthened a so-called input lever. Thus, the dorsal strike with the horns would be more powerful as the ratio of output to input would be increased. Predation is a main cause of mortality for small mammals, so the benefits provided by the horns induced mechanism to reduce predation surely offsets the substantial energy costs in the evolutionary time spent of developing the horns in a fossorial mammal.
With the oldest 28 mya mylagaulid in the genus, Trilaccogaulus (Tri-lac-ko-gall-us) originating in America’s Midwest, other species fossil remains have been found in other parts of North America and in Eurasia.
What animal left the above fossilized skeletal remains?
If you said bat, you are correct, but of course the long arms and finger bones gave it away, although with the long tail perhaps perplexing ya for just a moment. Yes, it is a photo of a bat fossil, an early bat from 52.2 mya known as, Icaronycteris (Ick-ah-ro-nik-tur-is). Measuring 14cm/5.5in long and 37cm/14.6in from wingtip to wingtip, Icaronycteris is known from four well preserved fossils found in the ‘Green River Formation’ in the state of Wyoming, USA. However, another Icaronycteris species was discovered in France.
|Credit: Dinopedia Icaronycteris|
Icaronycteris is a true microchiropteran bat in that it could emit ultrasound and echolocate; the earliest bats couldn’t. Thus far, it is the earliest bat that could echo locate. But it still retained primitive features such as a tail that was longer than its legs and the tail wasn’t connected to the body by an uropatagium (skin membrane).
Other than these two primitive features and a few others like a long tapered flat head, it had already been conforming to the modern bat body form, including the absence of claws on the wing fingers except for the first flexible finger in bearing a claw with the second finger bearing a small immobile claw.
|Credit: UCMP.Berkeley.edu Bat wing|
Some fossils showed moth scales where the stomach would’ve been indicating an insectivorous diet. Icaronycteris’ reversed hind feet anatomy also alludes to the fact that it hung upside down during rest stages.
Onychonycteris (O-nic-o-nic-teh-riss) is the most primitive bat known, but not necessarily the oldest as it also lived 52.5 mya in Wyoming’s portion of the ‘Green river formation’. It still bored five claws on each wing finger which harks back to an arboreal ancestry with the claw tipped fingers along with its limb proportions in maintaining good climbing abilities.
Although at the present there is some debate, for researchers found some crushed bone structures in the fossil that could be indicative of an echoing larynx, it is still proposed, due to the absence of an enlarged cochlea in which all extant bats use for echolocation that Onychonycteris could not echolocate.
As a bat that couldn’t echolocate, Onychonycteris most likely couldn’t locate insects during flight, so with the aid of its claws would fly from tree to tree crawl around listening intently for any insect generated sounds like a beetle chewing on a leaf. Two conundrums in considering bats that Onychonycteris doesn’t answer is when bats evolved echolocation for navigation and locating prey and secondly, when did bats actively turn nocturnal, as the eye orbits are too deformed to determine eye size. Larger eyes would perceive better night vision, while smaller beady eyes would’ve hinted to echolocation. Onychonycteris was capable of powered flight, but according to its wing configuration, it would’ve employed an undulating gliding-fluttering flight style.
Although paleontologists are hard at work on it, the origins of bats still remain a mystery, for there is a lack of basal ancestors, stem and proto-bat fossils. Chiroptera (Kai-rop-ter-ruh) is the order of bats and its members fossilized skeletal completeness is the lowest of any previously assessed tetrapod group. Proto-bats are intermediate mammals between the earliest true bats and their nearest stem non-bat relatives.
A couple of reasons behind the lack of bat lineage fossils is that the stem mammal quadrupeds and proto-bats lived strictly an arboreal life in tropical jungles where fossilization is greatly hampered due to very poor ground acidification conditions. In fact preserved bat fossils come only from ancient lake beds that served as a Lagerstätte (deposits that serve host to extremely well preserved fossils due to anoxic conditions). Plus, whatever dies in a tropical rainforest, especially with thin hair and bones are quickly consumed bones and all, by flora and fauna all due to extreme nourishment competition. Also, perhaps due to the evolved gene ‘BMP2’ changes that is a protein coding gene, can create morphogenetic bone diseases, allowing bone to disintegrate quicker upon death of the species.
|Artist: Marcos Paulo Chriacus|
However, there are some proposals of bat origins like from the paleo-artist, David Peters. In it, he hypothesizes that Chriacus (Kree-a-cuss), who possessed limbs built to walk and climb, is a fairly good tetrapod choice to consider even if there is no substantial evidence.
Chriacus, when it comes to classifying is a rebel without a home. Currently ranked as an arctocyonid, which are species belonging to a family of unspecialized mammals primarily from the Late Cretaceous to Paleocene, I just might suppose Chriacus would be a starting choice in being basal to unique flying mammals.
Chriacus was about 45cm/17.7in in body length and 1m/3.3ft in total length with tail. It lived in the Paleocene North America in Wyoming where early bats first emerged. It had many adaptations for living in trees such as long fingers and was an omnivore eating fruits, eggs, small mammals and insects. But most importantly, it had compressed claws connected to feet with flexible ankle joints that allowed the hind feet to turn in reverse behind the body much like bats do.
So in the devised illustration above, what Peters’ interpretation states is that hypothetically, bat ancestors arose from a sister group of Chriacus which was a larger direct ancestor to smaller predecessors.
What lineage of mammals did this animal below ultimately lead to?
|Artist: Xijun Ni Archicebus|
The above illustration is a fossil of a small creature that ventured forth up into the treetops equipped with developed binocular vision to confidently jump from limb to limb and cling to with hands possessing an opposable digit in forests 55.8-54.8 mya. With the generic name of, Archicebus (Arc-e-see-bus) this creature is the first known primate that would eventually through many twists, turns, tributaries and dead-ends lead to mankind.
Archicebus will begin our next and last article in the Et Tunc evolutionary series concerning primates.
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