Et Tunc Nulla Erat V

Et tunc nulla erat V
(And Then There Was)

Et Tunc Nulla Erat IV led us through the history of amphibian evolution from very early labyrinthodonts up to the modern or extant lissamphibians. In the attempt to continue on in the saga of life’s further evolution, we’re going to now introduce the reptile line that arose from diapsids slugging through extinct, lizards turtles, amphisbaenians and marine reptiles the euryapsids, while ending with some extant groups; but first, a better introduction to cladograms.

Cladogram is from Greek with clados meaning ‘branch’ and gramma emphasizing ‘character’. We will be using the cladistic diagrams more with reptiles in tracing ancestral lines instead of graphs of family trees. Cladograms do not show direct ancestral relationships to species descendants, nor do they display evolved changes as evolutionary trees do, but a cladogram does point to common ancestors of a group. Cladograms can also utilize relations morphologically (et al: invertebrate/vertebrate, prokaryote/eukaryote, skull types: synapsid/diapsid) or as a molecular unit in DNA sequencing.

As an aid in understanding cladograms, the first graph shows that it is the baseline as always being the ancestral original root with speciation commencing from there. The second graph depicts two simple forms of cladograms with speciation always arising from the base stem. The third and fourth cladogram graph gives more detail in showing the ancestry base’s descendants and in what order was the earliest to the latest by numbering or naming the lines normally from oldest to younger as from left to right or from top to bottom.

Simply put, cladograms are straight lines with a main common ancestral line amidst other lines branching off from it in representing different clades of the common ancestral main line. 

Towards the end of the Carboniferous and into the early Permian, atmospheric oxygen levels peaked at ~32% while displacing higher oxygen levels into the oceans serving as sinks. This high level created some of the largest insects with Meganeura, a dragonfly reaching lengths of 66cm/26in. That’s the size of some sea gulls. The millipede, Arthropleura armata reached lengths of up to 2.3m/8.5ft. Surviving into the middle of the Permian, this arthropod was the largest invertebrate to ever walk Earth.

The higher oxygen levels benefitted larval metabolism more so in reaching gargantuan sizes since the aquatic larvae cannot control oxygen intake in breathing through their skin. The terrestrial adults breathe through spiracles that can be voluntarily closed. 

A model of Arthropleura

The Permian Period lasted from ~299-252 mya. Although the beginning of the Permian was cool while ending the Carboniferous Karoo Ice Age, the end of the Permian experienced a super mass extinction that almost annihilated all marine and terrestrial animals due to heated climatic change. It appears the culprit was a combination of events in the last million years of the Permian that culminated into the wholesale slaughter.

The current Siberian basalt flood plains ~ 6,500m/21,325.5ft thick, is indicative of a million years of intensive volcanic eruptions during the end of the Permian. During that time frame, there was an extensive release of volcanic carbon dioxide and other obnoxious gaseous releases that in turn warmed the atmosphere and ocean waters releasing an immense amount of methane into the oceans that was tied up in methane hydrate reservoirs. The methane in turn consumed free oxygen levels making ocean zones uninhabitable for life by creating widespread anoxic/dysoxic regions, in particular along coastlines. 

Using a new method to estimate the oxygenation of ocean water in the past, scientists found that the ratio of thorium and uranium isotopes changed dramatically at the time of the extinction.

Meanwhile with acidification of the oceans due to the upper threshold release of greenhouse gases, the atmosphere warmed so much it resulted in ocean level increases and hot climatic zones that made it lethally too hot for life to exist along the equator and dry interior portion within the Pangaea continent.

Today, Global Warming deniers will tell you that warming of the atmosphere is a natural event and has occurred many times over Earth’s history. Yes it has, but they leave it at that. What they don’t include is its following impact and effect on life.

Fortunately, most of the earth’s Global Warming events were small scale in the dispersion of carbon dioxide allowing the climate and life’s adaptations to compensate and recover. Unfortunately for life at the Permian/Triassic boundary, the change was rapid and intense killing off most of life. Whether you agree or disagree if the current carbon dioxide emission climatic warming is natural or manmade, it doesn’t matter, for in also being a rapid transition, even more so in concentration than the Permian ending witnessed, life is going to have a hard time adjusting.

The warmest NASA /NOAA global year on record was this past 2015. It beat the previous warmest global land/oceanic surface record, which was just the year before in 2014.  To put these back-to-back records into context, if temperatures hadn’t been rising recently, previous normal back-to-back record high temperatures correlate to a chancy every 1500 years. With prevailing temperature rises that is now correlating into every ten year intervals.

Struggling to survive with this latest event where carbon dioxide atmospheric concentrations have already surpassed 400 ppm, a sixth massive extinction is poised. Present carbon dioxide concentrations have more than doubled the 200 ppm concentrations that current life has adjusted to.      

Now, with that over with, let’s dig into reptiles…               

The Road to Reptiles:
Reptiliomorpha (phonetics: Rep-til-e-o-mor-pha), otherwise sometimes listed as Anthracosauria (phonetics: An-thra-co-sor-ee-ah), is a stem clade order deriving from a temnospondyl line. They are an ancestral lineage, therefore more related to reptiles, birds and mammals than they are to lissamphibians. The laying of the first terrestrial egg occurred some 340 mya in the Mississippian Period of the Carboniferous by a reptiliomorph and most likely one that still spent most of its days in aquatic environments, but could definitely roam land possessing stiffening knee joints geared for a more pronounced gait in favoring walking.

Scalation was clearly a convergent parallel evolutionary trend from both groups of labyrinthodonts. Convergence is morphological similarities that were acquired independently in different species that are not related as opposed to homology. When morphologies are homologous, it is due to two species having a common ancestor that the morphology was inherited from.

In many of the lepospondyl microsaur species, such as Trimerorhachis (phonetics: Try-mer-or-ah-chis), overall body dermal scale patterns are not unlike primitive reptile scalation. Trachystegos (phonetics: tray-key-stay-ghos) of the latter Carboniferous and Saxonerpeton (phonetics: Sax-unh-ner-pe-tawn) of the Early Permian had an extensive scale covering from head to tail.

This terrestrial egg or amniote was completely self-sufficient from desiccation in a dry environment, therefore a huge advantage for the continuing survival of a species in arid conditions. At the end of the Permian Period which ended the Paleozoic Era, the continents Euramerica and Gondwana became one forming the supercontinent Pangaea. The interior, far from coastal regions became very arid, so the steamy swamplands of the latter Paleozoic gave way to drier conditions.

Amniotes had replaced buccal pumping for breathing relying instead on a complex and an in-folded pair of lungs. Early day amniotes also possessed a longer, more flexible neck devolving the skull’s otic notch in making for a more terrestrial lifestyle.

As mentioned above, known as the ‘Great Dying’, the Permian/Triassic extinction resulted in the largest mass extinction ever causing ~72% of all tetrapods to go extinct. Thus ended the reign of the amphibian-like labyrinthodont, which was still wholly dependent in laying jelly-like egg masses in watery environs. The amphibian’s environment had gone from widespread to isolate. During this extinction episode, even insects were scathed losing up to 83% of all insect genera. However, this left room for a small indiscrete animal known as a reptiliomorph to raise a legion into dominance…the reptile…whose egg was suited for the transition. As a result, reptiles began to rule the land, arboreal settings, air, and waters.

Reptilian amniotic egg vs. Amphibian egg

Going from outside inwards, the amniote egg consists of an outer shell, albumen, another protective layer called the chorion where inside it rests the yolk sac (nutrients reservoir), allantois sac (respiration facilitation/waste receptor) and finally within the amnion, rests the embryo. In theriodont eutherian and metaeutherian animals extinct and extant, the placenta affords the protection served by the shell, so in mammal species, the chorion is the first protective membrane.

Animals such as most fish and amphibians that lay non-amnion eggs are considered in contrast to amniotes as anamniotes.

Amniote Clad

The exact line where reptiliomorphs evolved from labyrinthodonts is very blurred as the reproductive units, including eggs, fossilize very poorly. There is however fossilized evidence of amniote reptiliomorphs occurring during the Carboniferous 340 mya. The fossil remains are from a transitional animal containing both amphibian and reptilian features that were laid down in very arid conditioned sediment. This animal was the small Casineria (phonetics: Cass-in-eh-ree-ah).

With a total length of only 15cm/5.9in, Casineria lived and laid eggs in dry conditions hunting down the multiple invertebrates for food and possibly finding moist spots to lay its buried eggs. Its fossil remains have only been found in dry imposed bedrock.


Casineria would have appeared as a small lizard in life complete with a scaled body and possessed on its five toes the first known claws. Indeed, it is the first truly known fully terrestrial tetrapod, if not the first reptilian animal.


Another early reptiliomorph is Westlothiana (phonetics: West-lo-thee-an-ah) that occurred 338 mya. Its overall anatomy is also small at 20cm/7.9in. In the body plan there were short legs as in labyrinthodont temnospondyls, while reptilian characteristics were unfused ankle bones and non labyrinthodont infolding of dentin and it laid leathery amniotic eggs.

Both these animals had no otic notch and in being small they laid small eggs. The smaller eggs experienced a much lower volume to surface ratio. This was an advantage in developing primitive amniotic eggs which first arose from non-amniotic terrain egg laying labyrinthodonts that sought out moist environs such as moss covered ground or stumps that trapped moisture.

Westlothiana is most likely a stem clade for diadectomorphs. Diadectidae (phonetics: Di-uh-dec-tuh-day) was a family of reptiliomorphs that although still retained long sharp teeth protruding from the mouth had evolved from an insectivorous diet to vegetarian. The most primitive diadectomorphans like limnoscelids were omnivorous. Eating an insect and the plant that it was on, over time, the latter diadectomorphs developed a plant diet until the last of the group, the diadectids were strictly herbivores and most likely the first tetrapod to subsist solely on plants. Diadectids were also the largest land animals of their time in the Early Permian with the genus Diadectes (phonetics: Di-uh-dek-tes) reaching 3m/9.9ft; thus, the measure for larger carnivores to appear.

The fact that diadectomorphs developed an herbivorous diet was revolutionary, for before there were only piscivores, insectivores and carnivores due to limitations in being tied to water margins. The herbivore opened up the field of range pushing and radiating tetrapods out into other ecosystems.  

Eggs from these trending reptiliomorph labyrinthodonts housed non-feeding tadpoles that hatched not as tadpoles, as they fully metamorphosed as miniature adults within the egg. Moisture from the eggs’ ambient environment protected the eggs from desiccation through infusion. The fetus now developed all its larval stages within the watery domain of the egg instead of in bodies of water like the tadpole was and is still today chained to.

About this time in the Mid-Late Carboniferous, the reptiliomorph Gephyrostegus (phonetics: Gee-fy-ro-steg-us) had already veered away from temnospondyls creating its own order of Gephyrostegida. It had tiny circular scales covering the body, large eyes and insectivorous pointed teeth. Gephyrostegus had fused small ankle bones into one large bone known as the astragalus; a trait found in all later reptiles. This small creature was leading to convergent reptilian forms, but died out during the Late Carboniferous so is more distantly related.

What the family Gephyrostegidae offers us is not a vast amount of species, as there were only two genera, but the first real proof of amniote eggs going back to 347 mya in Visean strata of the Middle Mississippian. In a 310 million year old fossil find in the German, Westphalian formation was the well preserved remains of Gephyrostegus watsoni. The Westphalian layers are a regional part of the middle Pennsylvanian stratigraphy.

Gephyrostegus skull
The fossil remains of G. watsoni was a female, in which due to the small size was first considered to be a juvenile of the latter species Gephyrostegus bohemicus. The fossil shows she had a deep pelvis capable enough to carry and lay amniotic eggs. Also, in the lack of posterior dorsal ribs, a gravid female could carry a large cache of eggs. The clincher here that G. watsoni was an amniote is that in the fossil remains there were found seven distinct crushed spherical shapes just above the hips. Although there is no evidence of a harder outer shell, there is a distinct amnion membrane enveloping the spheres. These are the first known remains of a tetrapodal amniote egg.

Tetrapod amniotes had securely made their presence and no longer limited to water bodies for fetal development, could spread throughout the lands where later isolated from geological events, would evolve into more species in evolutionary responses to ambient ecologies. Thus, the amniote accelerated their speciation.

The amniote class, Reptilia (phonetics: Rep-till-ee-ah) has been divided into four main subclasses based on the number of temporal fenestrae/openings positioned posteriorly behind the eye and below or above the postorbitals as laterally on each side of the skull. These holes served as jaw muscle attachments.

1.  Anapsida ~ no openings present; ‘proto-reptiles’, turtles
2. Diapsida ~ 2 skull openings below postorbital on each side; ‘majority of reptiles’, bird progenitors
3. Synapsida ~ 1 skull opening below postorbital on each side; ‘mammal-like reptiles’ mammal progenitors, in which mammals retained synapsid skulls
4. Euryapsida ~ 1 skull opening above postorbital on each side; ‘extinct marine reptiles’ derived from diapsids losing one lower temporal fenestra

This is a simplified view, but is an orderly scheme. No less, due to phylogeny in monophyletic and paraphyletic realms, there are much more complicated cladistics one can view and study.

Anapsids include the earliest forms of reptiles along with all turtles. Early anapsid reptiles are sometimes referred to as ‘parareptiles’; the term means “at the side of reptiles.” Just as diapsids, synapsids came from the amniote line but were the first to arrive in diverging before the diapsids’ arrival as witnessed in the cladogram below. Synapsids include the extinct pelycosaurs and mammal-like reptiles (therapsids) in which mammals evolved from. Mammals are the only extant synapsid group. Diapsids include dinosaurs, crocodilians, tuataras, snakes, lizards and birds. Even though birds no longer have any temporal fenestra, they derived from the group phylogenetically. Euryapsids are extinct marine reptiles such as ichthyosaurs, plesiosaurs and the lesser known nothosaurs.

Before we continue here, I’d like to relate a side note on a parareptile. The superorder Sauropterygia (lizard flippers) includes all reptiles evolving from land reptiles only to take to the seas as euryapsids. These creatures show up in the middle of the Triassic to become all extinct by the end of the Cretaceous.

The interesting oddball we’re to discourse a bit on here is the mesosaur, who by all intents and purposes is a reptile but are now grouped as ‘parareptiles’. They first appear in the fossil record 299 mya which was 57 million years before the first sauropterygian. Mesosaurs (not to be confused with mosasaurs) were amniotes, but instead of being anapsid, or even a diapsid derived as a euryapsid, they had a synapsid skull. Mesosaurs had already left land to swim in the salty sea shallows or the inland hypersaline watery environment in what is now the Mangrullo formation of Uruguay while amphibians still ruled the land. Mesosaurs are one of the first or very first tetrapod to return back to the seas. All other synapsids would remain terrestrial and with a few, eventually leading to mammals.

Drawing of Mesosaurus fossil embryo

Fossilized mesosaur embryos exhibit pachyostosis in having thicker ribs than land tetrapodal embryos. This suggests embryos in utero were ovoviviparous being born live in the water as numerous sea snakes currently do. Mesosaurs may turn out to be the most basal marine reptile, or a stem ancestor to true reptilians, or a basal and stem to both groups of parareptiles and synapsids. Or, it may simply continue to be an oddball synapsid that diverted away from all other land synapsids before mammals to live a life in the seas.

During the early stages of the Permian, marine life took a big hit with it being rare to find any marine fossils. Calcareous algae and calcareous sponges became the primary shallow water reef builders. Trilobites were declining and had gone extinct by the end of the Permian, while lobe-finned fish that had given rise to tetrapods and spiny fishes were giving way to bony fish. Sharks maintained a stable population. On land, fungi had finally figured out a way to consume bark 60 million years after trees had evolved it during the Carboniferous. That is why the Carboniferous sediment has so much coal deposits, as the tree, bark and all were covered over. Glossopteris (an extinct order of seed ferns) came into existence along with clubmosses, horsetails and various ferns still making their presence as holdovers from the Carboniferous. Swamp forests began giving way to conifer forests such as the short palm cycads and the gingko of today.

The Reptile:
The Early Triassic Period started off hot with vast interior deserts and warm Polar Regions supporting lush forests and warm shallow seas.

Thus far, we have reptiliomorphs that had evolved from the labyrinthodont temnospondyls. This led to the reptiliomorph sister clade, the amniote that had the ability to lay viable eggs on land. From amniotes, the stem reptiliomorph genera Casineria and Westlothiana as explained above, had emerged creeping ever closer to a true reptile.

Casineria, was one of the first to have toughened feet for land equipped with claws for grasping. It was also one of the first to develop sex as we know of it. For eggs to be protected on land the shell formed, but sperm cannot penetrate the shell to fertilize the egg, so internal fertilization first arose with Casineria to intercourse fertilization before the shell formed.


Paleothyris (phonetics: Pay-lee-o-thy-ris) occurred in the mid-late Pennsylvanian of the Carboniferous from 310 to 302 mya. It most definitely was a reptile, but a very primitive one with an anatomic reptilian body, but also supported a temnospondyli-like, though advanced skull. It was large eyed with sharp teeth most likely hunting insects nocturnally. As in all former labyrinthodonts it was an anapsid with no fenestrae. The evolutionary key to this creature is that it derived from an amniotic lineage inching ever closer to a fully reptilian trait.


From here the first true reptile evolved in the form of the genus, Hylonomus (phonetics: Hy-lon-o-mus). It had the typical anapsid boxed head with skull openings only for nostrils and eyes.

Anapsida Cladogram

Hylonomus, a romeriid anapsid lived during the Carboniferous in the mid Pennsylvanian 312 mya. It may even be older at 315 mya as fossilized footprint impressions have been attributed to this animal after careful study of its fossilized foot bones. At 20cm/8in, it appeared as a small lizard living and hiding in tree stumps as most fossils have been located in fossilized club moss stumps. Hylonomus most likely ate small invertebrates with its small sharp teeth supported by its anapsid skull.

Surely a more ancient reptile fossil will be found, but for as it stands, Hylonomus is the basal ancestor to all reptiles including dinosaurs, extant crocodiles and all mammals and birds.

Captorhinis agouti
As a close predecessor to Hylonomus, the first reptile herbivores in the family, Captorhinidae (phonetics: Cap-toe-rye-nee-dae) appears in the fossil record at the Carboniferous/Permian timeline 302-285 mya. An anapsid, the captorhinids ate vegetation. One of the largest captorhinids was no more than 17.8cm/7in. The earliest and basal most captorhinid was Thuringothyris (phonetics: Thu-rin-go-thy-ris), whom possessed single rows of teeth. The preceding and most evolved, Captorhinus (phonetics: Cap-toe-rye-nus) had double rows of teeth. All species had teeth adapted to masticating and meshing tough plants. Along with this, their skull was robust and had thickened vertebrae.

An anapsid pareiasaurid holdout that made it towards the Permian/Triassic timeline 252 mya is the herbivore genus, Bunostegos (phonetics: Boo-no-stay-ghos). Bunostegos genera had knobby skulls with an armored back. The most attributable trait of this early reptilian was its upright gait in being one of the first animals to use limb joints for better mass maintenance and running endurance. 

Late Permian bunostegos species

Aside from still lingering argument that turtles came directly from anapsids, there are no extant anapsids. Modern day turtles do possess an anapsid skull, but it now appears they originally came from a diapsid ancestral lineage. As a collage of earliest primitive reptiles, anapsids are not a monophyletic group, but rather simply a collection of the earliest reptiles that were distantly related. Currently, all genetic molecular studies pinpoint turtles well within the early diapsid clade.

After reanalysis, Testudines (turtles, terrapins and tortoises) have been assigned to the lepidosauromorph diapsid group that contains the lepidosaurs (snakes, lizards and tuataras). Turtles diverged away from lepidosaurs somewhere between 200-275 mya. The turtle’s skull began reverting back from a diapsid to an anapsid skull as the lineage began losing teeth replacing the dental ware with a chitinous beak.


Synapsids had already diverged from the amniote tree during the Late Pennsylvanian 308 mya devolving away from true reptile morphology. The divergence formed the Synapsida line that eventually led to mammals. The Sauropsida line would eventually lead to diapsids which includes crocodiles, pterosaurs, marine reptiles, turtles, lizards/snakes and dinosaurs/birds.

The undisputed oldest synapsid fossils are Echinerpeton intermedium (phonetics: Eh-she-nair-pah-tawn inter-med-e-um) and Archaeothyris (phonetics: Are-kay-o-thy-ris) from 308-306 mya respectively. Echinerpeton, though only the size of a small lizard, just might be the base ancestor to the later much larger synapsid sail-backed dimetrodons that reached up to 4.6 m/15ft. Echinerpeton had articulated vertebrae with high neural spines supporting a dorsal webbed sail.

Echinerpeton intermedium

Synapsids may even have an earlier origin in the ~314 mya fossil remains of Protoclepcydrops (phonetics: Proto-klep-sigh-drops). Unfortunately the single skeletal remains are fragmentary, but the disarticulated remains do allude to a synapsid skull type. Early synapsids begin digressing away from eventual true reptile evolution leading eventually to mammals.

The synapsid line led to pelycosaurs including the dimetrodons that formed the basal ancestry to therapsids (mammal-like reptiles) that eventually gave rise to mammals. For this treatise, we’re only going to mention a few facts about pelycosaurs where more in depth discussion will be given in a future ‘Et Tunc’ series concerning synapsids to the rise of mammals.

Pelycosaurs are one of the earliest most primitive of synapsids. First appearing in the Upper Pennsylvanian ending, pelycosaurs became extinct by the end of the Permian.

Bone growth types: A.low vascular B. highly vascular C. fibrolamellar

Warm blooded animals (endothermic/homoeothermic) like birds and human grow at a faster rate than cold blooded (ectothermic) animals do. With this fast growth rate process found only in homoeothermic animals is a bone growth pattern termed fibrolamellar bone (FLB). The early pelycosaur, Ophiacodon (phonetics: O-fee-ah-co-don) also included FLB in its physiology. One of the greatest opportunities in evolutionary devices is the regulation of internal warm bloodedness. Although Ophiacodon was not directly related to mammals and even more remotely related to birds, it had accomplished at least some form of thermoregulation nearly 300 mya. Living 296.4-279.5 mya, it also had the largest skull of any contemporary animal of its day reaching lengths of 50cm/20in.

Compare head sizes Ophiacodon in middle; Cotylorhynchus in back

In direct contrast to Ophiacodon’s large skull was the pelycosaur caseid, Cotylorhynchus’s (phonetics: Co-tee-lor-reen-cuss) minute skull. With a barrel shaped and heavily set body at 6m/19.7ft in length while weighing in at ~1 ton, the head was only around 87.5cm/34.5in. The nasal openings and cavities were also large for the skull, perhaps to take in more oxygen or functioned as a higher surface area to moisture. Nonetheless, its girth allowed for larger organs in more functional digestion of plant material. Being the largest herbivore during the Permian, with the massive body used as a weight force, the front legs were used for digging up roots. Regardless of the animal’s small head, its total size also acted as a predator deterrent as from 279.5-272 mya it was quite common in many Permian environments.

A miniature Cotylorhynchus was Casea (phonetics: Cah-say-ah) at 1.2m/4ft. Its ribcage was highly expanded to make way for a large gut to properly digest plants like horsetails and ferns. It also was successful with fossils being found from Texas to France.


Varanops (phonetics: Va-ran-ops) has the distinction to being one of the very last pelycosaurs to die off becoming extinct at the end of the Permian. Perhaps this was due at least in part to competition against an established rising number of therapsids during that time.

Conceivably the most famous of the pelycosaurs are the dimetrodon and edaphosaur, the sailbacks of the Permian most often times mistaken for dinosaurs. But no, man shares more genetic material with the dimetrodon than the dimetrodon does with dinosaurs. Dimetrodon and edaphosaur were sphenacodonts, the fossil family showing a direct lineage to mammals.

As the prime carnivore, dimetrodons were the top predator during the Permian, while edaphosaurs were mild herbivores. Although Pangaea exposed more continental dry land than that of oceanic area at any time during Earth’s history, these two pelycosaurs preferred roaming near swamplands or near bodies of waters as opposed to the more inland arid lands.

Through Sauropsida (phonetics: Saur-op-see-da) lineage, two major stem-based clades diverged forming the Eureptilia (phonetices: U-rep-til-e-ah) and the Anapsida/‘Parareptilia’. Out of Eureptila, the Diapsida clade evolved.

Eureptilia Clade

Around the Carboniferous/Permian boundary, two diapsid groups diverged known as the Neodiapsida (phonetics: Nee-o-die-app-sah-duh) and Araeoscelilidia (phonetics: Ah-ray-oss-kell-ah-lee-dee-ah). One early diapsid araeoscelilid was Petrolacaosaurus (phonetics: Pet-row-lak-oh-sor-us).


Petrolacosaurus appears in the Late Carboniferous 302 mya and as a direct line had died out by 275 mya. Superficially similar to modern day lizards, this rather small extinct reptile at 40cm/16in is the first known amniote diapsid and possessed canine-like secondary teeth as latter forms of therapsids and mammals will. It even possessed a heart template as all modern mammals do, including humankind. Even though definitive evidence is not in yet, petrolacosaurids just might be a common ancestor to both diapsids and synapsids as a basal diapsid amniote. The well-formed two temporal fenestrae in its phylogeny might have had one deleted as the turtle line had deleted both. In evaluating its teeth, most likely Petrolacosaurus was an insectivore.

Neodiapsid Clade

Another araeoscelilid was Spinoaequalis (phonetics: Spy-no-aye-kwal-iss). Occurring 300 mya it had longer and stronger hind limbs. Its symmetrical spine supported a laterally flattened tail used for propulsion in water. Spinoaequalis was at home on land as well as water using its tail for propulsion. This reptile was semi aquatic and may have not only frequented freshwater, but transitioned to marine as well for some of its fossil finds are in the same sediment as some well-preserved fish fossils.  


Around 260 mya, Neodiapsida further diverged into the 60cm/2ft Claudiosaurus (phonetics: Claw-dee-o-saur-us) and Sauria (phonetics: Sawr-ee-ah). One of the earliest neodiapsids forming the basal diapsid group, claudiosaurids had a poorly developed sternum, more cartilage than bone, a relative long body/neck and swam marine shorelines frequenting rocks above the surface to sunbathe living much like marine iguanas today in sunbathing on land and swimming in sinusoidal undulating fashion with legs tucked to the sides. Although it had developed limbs, but with webbing, due to the amount of cartilage in its osteology and poorly developed sternum, it most likely rarely frequented land.

With the elongate body/neck, same paddle-shaped distal limbs, lack of a lower skull temporal bar and transvers flange of the pterygoid, a closed condition of the palate and reduced suborbital fenestrae, claudiosaurs are seriously considered as a basal ancestor to nothosaurids and plesiosaurids. Other than that the claudiosaurid line became extinct towards the end of the Permian 253 mya.

Sauria (Archelosauria branch) cladogram

The other neodiapsid divergence, the Sauria clade was once relegated only to the lizard line, but since the introduction of molecular genetics, it has been broadened to include the most recent common ancestors to lepidosaurs and archosaurs. So, all lizards, snakes, crocodilians, birds, tuataras and numerous extinct animals such as pterosaurs, marine reptiles and dinosaurs are saurians. This new rearrangement has aided in distinguishing stem saurians such as in the synapomorphy characters of cephalad (pertaining to anterior or posterior ends) regions, the trunk, pectoral, pelvic and limb sectors.

Sauria (Ankylopoda branch) cladogram

The saurian clade represents the divergence between lizards and crocodiles while the clade itself is subdivided further into Lepidosauromorpha (phonetics: La-peed-o-sor-o-mor-pha) meaning “closer to snakes than birds” and Archosauromorpha (phonetics: Ar-ko-sor-o-mor-pha) meaning “closer to birds than snakes.” An early period saurian group was in the genus, Coelurosauravus (phonetics: See-lore-oh-sor-ay-vuss). This genus was one of the first lizard groups to take to the air ~260.5 mya. Coelurosaurvus will be discussed later under lizard evolution.
Coelurosaurus skeletal
Lepidosauromorphs lead to the Lepidosauriformes (phonetics: La-peed-o-sor-e-forms)  representing lizard traits such as a sprawling gait more in tune to sinusoidal trunk and tail movement, longer mobility strides due to a sliding joint between the coracoid and sternum and primarily possess pleurodont teeth as opposed to thecodont teeth witnessed in the crocodilian archosauromorphs. In addition to thecodont teeth, archosauromorphs in fossils and current species possess a parasagittal gait (legs positioned entirely more under the body while parallel to vertebral column during mobility) and a reduction or total loss of the sternum.

Teeth type plays important roles in evolutionary niches. Thecodontia teeth once even played a role in taxonomic grouping, although it now is obsolete. Thecodont means ‘socket tooth’ and represents animals with teeth socketed through the gums into the jawbone.

Tooth Implantaions: A. acrodont B. pleurodont C. subthecodont  D. thecodont 

Pleurodonty (pleurodont ~ side tooth) exhibits teeth ankylosed (fused) by their sides to the jaws inner surface. Acrodonty (acrodont ~ summit tooth) exhibits teeth mounted in a slight depression atop the gums and are not socketed with roots. Sphenodonts, pleurosaurs and certain extant lizards will utilize this teeth arrangement.

Youngina red possible nascent antorbital fenestra

Youngina (phonetics: Yun-gin-ah) is a basal lepidosauromorph that occurred during the Late Permian. This lizard-like reptile was 1.47m/4.82ft long and with its conical teeth, was most likely an insectivore, but also carnivorous to small vertebrates. The photo below is a Youngina skull. The red highlights a possible nascent antorbital fenestra.

From here, the diapsid clade Archosauromorpha came about 275mya.  The skull opening between the nostrils and eye sockets is known as the antorbital fenestra. In the archosauromorph family, Protorothyrididae (phonetics: Pro-tor-o-thy-rid-a-day), the genus Protorosaurus (phonetics: Pro-tor-o-sor-us), which means ‘first lizard’ is the earliest known archosauromorph thus far. Archosauromorphs would lead to Archosauriformes (Phonetics: Ar-ko-sor-e-forms) that eventually led to true archosaurs that would further lead to the only extant archosaur lines, which is of course the crocodilian and avian groups.

Most of these transformations from Archosauromorpha to Archosauriformes and Lepidosauromorpha to Lepidosauriformes occurred right before or just after the Permian/Triassic extinction. The Archosauriformes and Lepidosauriformes survived, the archosauromorphs and lepidosauromorphs didn’t.

The Path to Turtles:
Turtles were one of the first diapsid archosauromorphs to branch off the archosaur line through the Pantestudines (phonetics: Pan-tess-tu-dee-nees) group, via the lepidosauromorph infraclass. While in the process, turtles reverted back to an anapsid skull; so in the proceeding course, they did not evolve directly from the early reptilian anapsids.

Pantestudines Clade
In turtle genomics there are diverse chromosome numbers and chromosomal rearrangements. Before developing a shell that is fused to the ribs, turtles first had to have broadened ribs; this is evident in the Pantestudines clade.

The order Placodontia (phonetics: Plaque-o-don-chee-ah) occurring during the Triassic 245-200 mya, were in the Pantestudines clade and due to convergent evolution, later forms appeared much like turtles but were only related as cousins and not basal or in a direct line to turtles. Placodonts were euryapsids. Besides the synapsid mesosaurs, these were some of the first reptiles to enter the seas filling a predator void that had been wiped out during the Permian/Triassic extinction where only sharks had successfully endured.

The earliest placodonts looked much like a modern day iguana, only much larger. Placodus (Phonetics: Plaque-o-dus) at 1-2m/3.3-6.6ft consisted chiefly upon a durophagus diet such as shellfish and animals with hard exoskeletons like crabs. Placodus possessed chisel-like front teeth and broad flattened back teeth for crushing prey. It must have been a shore animal frequenting shallow waters to obtain its shell-fished prey. Although the short limbs ended in claws, in between the digits webbing was present and with a flattened tail, these features aided in propelling the animal in water.

Placodont Henodus
Latter placodonts such as Cyamodus (phonetics: Sigh-am-o-dus) and Henodus (phonetics: Hen-o-dus), due to convergent evolution looked superficially like a turtle with a carapace and plastron. Most likely this armory was developed as protection due to an increase in larger marine reptile predators entering placodont shallow coastal water environments. Cyamodus even possessed a second shell protecting the rear pelvis area. A second advantage to the shells was their increased weight that was just below the level of neutral buoyancy allowing the animal to paddle in reaching the shallow bottoms where the shelled bivalves were.   
Occurring 265.8-251 mya, Eunotosaurus (phonetics: Eu-no-toe-sore-us) was an early Pantestudines still retaining the diapsid skull. This reptile had a broad body formed by nine pairs of broadened ribs. The upper surface of the ribs was concave giving the body a rounded broad shape. Most of the ribs were fused to the vertebrae. Sharpey’s fibers aid in anchoring muscle to bone in most animals. On the ribs’ anterior sides, Eunotosaurus lacked Sharpey’s fibers suggesting the animal lacked intercostal muscles. Turtles also lack intercostal muscles.

Another Pantestudines arising 240 mya during the Middle Triassic, was Pappochelys (phonetics: Pa-po-sha-lis). This animal had exhibited pachyostosis (expanded broad ribs/vertebrae) and had gastralia, which are dermal bones found in the ventral body walls. The gastralium appears to be a precursor to a shell. It was still a diapsid in possessing two temporal fenestrae skull holes. 

Occurring 220 mya, the oldest known anapsid true turtle with a shell, although partial with shortened ribs was Odontochelys (phonetics: O-don-toe-kell-ees). The plastron (ventral shell) was complete, while the carapace (anterior shell) was incomplete covered by skin as in the modern softshell. This turtle also possessed teeth in both the upper and lower jaws. Along with these primitive features, in Odontochelys’ tail, the transverse process was not fused as in modern turtles and the scapulae lacked acromion processes (bony projections extending from scapula on shoulder blade) as evident in modern turtles. Nonetheless, Odontochelys is a transitional turtle.


With the acromion advent, turtles moved their shoulder blades and pelvis (hips) underneath their ribs. The monophyletic sister clade to Odontochelys is Testudinata (phonetics: Tess-tu-dee-nah-ta) forming the two modern turtle groups in the genus, Proganochelys (Phonetics: Pro-gan-oh-kell-ees) and the order of extant turtles, Testudines (Phonetics: Tess-tu-dee-nees).

Add caption
Proganochelys, dating to the Late Triassic 210 mya are extinct. Proganochelys is the oldest stem turtle species thus far found to have a complete shell. It was around 1m/3.3ft long and heavily armored with bony shell plates and a tail with spikes terminating in a club. Its head could not be retracted into the shell and although it had no teeth, denticles existed on the palate. As in all extant turtles its scapulae (shoulder blades) sat underneath the rib cage instead of outside as in all other vertebrate animals. This also makes for a contorted arrangement of torso muscles.

In the embryonic stage, the chicken, mouse and turtle are all similar resulting from a common ancestor. But, as the embryo develops with the ribs extending down the embryo’s flanks surrounded by the myotome (muscle plate), where the chicken and mouse’s ribs, scalpulae muscle plate stay in place as the embryo matures, hatches and grows into an adult, the turtle embryo folds the myotome forward while the ribs, as shorter, never reach down into the flanks. 
The oldest known turtle to show the modern features of extant turtles was in the sea turtle Desmayochelys (Phonetics: Des-may-o-kell-ees) occurring 120 mya in the Mid-Cretaceous. However, this ancient turtle evolved independently from modern sea turtle lineage. This suggests that there were multiple instances of land dwelling turtles evolving into sea turtles.

Testudines is further divided into two suborders, Cryptodira phonetics: Crypt-o-deer-ah) and Pleurodira (phonetics: Pleur-o-deer-ah). All extant turtles have eight neck vertebrae, but in Pleurodira the cervical vertebrae are spool-shaped in cross-section with biconvex centra on one or more of the cervicals. Pleurodires include the side neck turtles that withdraw the head into the shell by bending the neck into a horizontal plane. Cryptodires include all other turtles, terrapins and tortoises that pull their heads straight back into the shell.

Two really large extinct turtles were the pleurodires, Carbonemys (Phonetics: Car-bon-em-iss) and Protostega (phonetics: Pro-toe-stay-gah). Carbonemys existed 60 mya in what is now Columbia. This terrestrial freshwater turtle’s shell reached a length of 1.72m/5ft 8in. Though not definitive, it was a carnivore and could easily consume a small crocodile with super strong jaws.

Protostega occurred 83.5 mya with its fossil remains being found in the Smoky Hill Chalk formations of western Kansas. With a soft shell for light weightiness and flippers for limbs, this turtle was a tireless swimmer during its search for an omnivorous meal. This marine turtle reached lengths of more than 3.1m/10ft, but it was not the largest turtle ever. That title belongs to the sea turtle, Archelon (phonetics: R-kell-on) that grew to lengths of 4m/13ft.

The pig-nosed Arvinachelys

Just named Arvinachelys (phonetics: R-vin-ack-ell-iss), a recent 2015 fossil find is a .61m /2ft long turtle that once roamed the bayous and lazy rivers of present day Utah ~76 mya in the Late Cretaceous. It’s pig-like nostril snout makes it a very peculiar turtle indeed baffling scientists on what exactly was its function. 


In 2012, the fossil remains in what is now Columbia, Puentemys (phonetics: Pwen-twem-iss) was a turtle with a carapace that extended 1.5m/5ft in diameter. In addition, the shell was almost a perfect circle. Puentemys lived 60 mya along with enormous snakes and crocodilians. The carapace size would have made the turtle much less susceptible to being on the larger reptiles’ menu.

The last embrace

Please note the above photo. One turtle fossil that is more important than the turtle itself is of Allaeochelys (phonetics: Al-ah-ee-ock-ell-iss). The male, smaller than the female were both caught in a loving embrace that’s forever preserved in fossilization. The pair died mating, with scientists surmising that the couple started the reproductive ritual near the surface of a lake and once embraced sunk to the bottom where a layer of poisonous gases existed perishing the couple. In the German Messel Pit, not only one fossil has been found of this turtle mating, but no less than nine have been found, although in varying time zones during the Eocene 57-36 mya. Perhaps as suggested, a more befitting name would be ‘Coitus interruptus’.      

The Path to Sphenodonts/Lizards/Snakes/Amphisbaenians:
As mentioned above, during diapsid appearances, the mainline was the Archosauromorpha infraclass which first appeared ~255 mya. A few million years later diverging away from archosauromorphs ~ 247.5 mya was lepidosauromorphs, in which comprised all diapsids closer to lizards than to archosaurs. Lepidosauriformes, as predecessors to the lepidosaur extant squamate line of lizards, snakes, amphisbaenians and the sphenodont tuataras were already lizard-like in physical appearance.

Taking up where we left off in discussing lepidosauromorphs under ‘Diapsids’, lepidosauromorphs had produced the Lepidosauriformes clade. This group is almost there as a lizard. The supratemporal of the skull is reduced while migrating backwards replaced in its original position by the squamosal that rims the upper temporal fenestra as witnessed in true lizards.


In the Late Permian ~ 260.5-251mya and in the Late Triassic ~210 mya, two lepidosauriforms emerged in the aforementioned Coelurosauravus (phonetics: See-lor-o-sor-ay-vuss) and the Kuehneosaurus (phonetics: Keen-e-o-sor-us) genera respectively. Both possessed longer tails as opposed to their predecessors and had a streamlined rigid body. They also had longer claws for grasping onto the barks of trees in an arboreal lifestyle.

The more ancient Coelurosauravus (40cm/16in) had a skull crest where Kuehneosaurus (72cm/2.3ft) didn’t, but both had thin hollow skeletal structures for aerodynamic flight. The winged assembly of Coelurosauravus was held by newly evolved dermal bone extensions ending in posterior pseudo-ribs supporting a skinned membranous web known as a patagium. In the case of Kuehneosaurus, the ribs had been thinned and extended. These were one of the first vertebrates to take to the air. 

Coelurosauravus was a true glider lofting from branch to branch or from tree to tree much like a flying squirrel today does, where Kuehneosaurus most likely parachuted instead of gliding to get from higher limb to lower limb or from tree to ground. This wasn’t a slow parachute descent. According to 2008 aerodynamic studies by K. Stein, descending at a 45° angle, speeds could’ve easily been reached up to 10-12 m/33-39.6ft per second.

Derived as a niece so to speak from a sister group of Coelurosauravus, but from a sister of Kuehneosaurus was the genus, Icarosaurus (phonetics: Ick-ah-ro-sor-us). This little gliding reptile, no more than 10cm/4in long, in addition to membranous skin webbing down the sides of its body, also possessed smaller webs along the backside of the hind legs.

Icarosaurus first appeared 228 mya. Rather than developing its long rib wings in the embryonic egg, it most likely developed them after it hatched and matured. Its pedal proportions of the feet were similar to Coelurosauravus, but more in line of terrestrial lizards in every other aspect.

Sharovipteryx (phonetics: Shar-ov-ip-ter-ix) occurring 225 mya had increased the Icarosaurus’ hind patagium to foot and leg wings, however there were no front limbed patagia (plural for patagium) freeing the forelegs. With large hind limbs making the much smaller forelimbs appear diminutive in comparison, Sharovipteryx most likely was not a quadruped, but hopped around on its hind limbs when not gliding during mobility. 

Xianglong (phonetics: Zang-long) is, in the fossil record, the last of these gliding reptiles occurring in the Early Cretaceous. Besides the torso extended membranes, it also had two small neck membranes used as rudders.

These flying reptiles are mentioned because they ushered in the beginnings of what I like to call, the proto-lizard although technically, they’re proto-squamates, but nonetheless, both terms express lizard-likeness in physiology and morphology.

In the Late Permian 255 mya one of the first proto-squamate appears in Lacertulus (phonetics: Lah-sur-tull-us). It had a wider premaxilla, vomers and palatines than lepidosaurs preceding it and has been described as a facultative bipedal animal with longer hind limbs.  

Another basal lepidosauriform was Santaisaurus (phonetics: San-tah-ee-sor-us). Occurring during the Early Triassic, it differed in having small sub pleurodont teeth; an augury to fully developed sphenodont teeth. Thus, begins the Sphenodontia (honetics: Sfen-o-don-chee-ah) order as an offshoot away from the evolutionary line of their sister group, the squamates (lizards). The only remaining extant sphenodont is the tuatara of New Zealand with two species.

Basal proto-sphenodont

Splitting 220 mya in the Triassic, proto-sphenodonts as the likes of Marmoretta (Phonetics: Mar-mo-ret-tah) began diverging from lepidosaurs in anatomically having distinct characteristics while superficially resembling lizards.

One of the earliest sphenodonts was in the basal genus, Gephyrosaurus (phonetics: Jeff-er-o-sor-us) appearing in the Early Jurassic then becoming extinct.

Synapomorphies (unique shared characteristics) among sphenodonts are located in the mouth such as an enlarged palatine tooth row with acrodont dentition and a posterior extension of the dentary. Sphenodonts for the most part also displayed some form of propalinal (forward and backward mastification) jaw action.

The five taxa that sphenodonts are divided in are the aquatic pleurosaurs and sapheosaurs, the specialized scissor-like teeth carnivorous clevosaurs, the Mid-Late Cretaceous highly specialized eilenodontines and the sphenodontines which are solely represented by the two existing tuatara species. 

Pleurosaurus goldfussi
Down through the Triassic and into the Jurassic, there were various proto-squamates leading ever closer to true lizards until the Early Cretaceous when one of the last proto-squamates in the genus, Meyasaurus (phonetics: May-ah-sor-us) morphologically appeared more lizard than not.


Meyasaurus had typical lizard traits such as the scapulocoracoid pectoral girdle arrangement and a pelvis opening called the thyroid fenestra. It also had tail vertebrae as shallow keels representing possible caudal autonomy, foot bone metacarpals and phalanges capped by cartilage, heavy ossification of limb bones and the specialized lizard foot and ankle structure of the fused astralago-calcaneun. The primary difference is that the teeth of Meyasaurus were not homodont, but were heterodont in possessing anterior small pegs with posterior bicuspids.

Sphenodont clad

Lacertilia: Most extant lizard groupings had their beginnings in the Cretaceous. Today’s lizards are arranged into five infraorders; they are:

1.       Iguania (among others iguanas, crotaphytids, agamids, chameleons)
2.       Gekkota (including geckos, legless lizards, blind lizards)
3.      Scincomorpha (including among others skinks, whiptails, cordylids)
4.      Diploglossa (anguids, anniellids, xenosaurids)
5.      Platynota (monitors, helodermatids, mosasaurs ~ extinct)

Proto-lizards with essential lizard characteristics branched out in forming the various lizard groups. From the late Jurassic ~155 mya, Euposaurus (phonetics: U-poe-sor-us) with shared sister taxa traits of skull features appears to be the basal most to Iguania, Gekkota, Scincomorpha and Diploglossa. On the basis of frontal morphology and limb proportions, Euposaurus nested as ancestral to Scandensia (Phonetics: Scan-den-see-ah) and Liushusaurus (Phonetics: Lee-oo-shu-sor-us).

Found in Early Cretaceous rock layers from ~130 mya, Scandensia possessed trenchant claws in leading an arboreal life. But this squamate came down from the trees giving rise to the basal line of Liushusaurus that is basal to the suborder, Scleroglossa (Scl-ro-gloss-ah) which includes iguanas, geckos, skinks and varanoids. It also includes the new clade, Bifurcata (Phonetics: By-fur-caw-tah) which proposes Iguania as a sister taxon to anguimorphs. Due to the latest phylogenetic analysis on molecular data concerning extant squamates, Bifurcata was instituted to make Scleroglossa valid in distinguishing the bifurcated tongued lizards from the iguanian muscular full tongue used for lingual capturing of food.    

Yabeinosaurus fossil

The split between Iguania and lizards contained in Scleroglossa is represented in Yabeinosaurus (phonetics: Yah-be-in-o-sor-us) as one of the earliest splits within lizard evolution. A Yabeinosaurus fossil found in 2011 with fifteen developed embryos is the oldest fossil of a live-bearing lizard. In addition, physiologically, Yabeinosaurus was trending towards a varanid body shape.

Estesia skull

Estesia (Phonetics: A-tees-e-ah) showing up in the Late Cretaceous is basal to varanus and ancestral to Heloderma, the Gila monster genus. Estesia’s dentition alludes to the fact that it was venomous. The teeth were all sharp and recurved as in current varanoids.
Estesia drawings

There is one varanid lizard from the Pleistocene that went extinct just 50,000 years ago; it was Megalania (phonetics: Meg-ah-lane-ee-ah). From what is now Australia, this is the largest lizard known reaching a body length of 5.5m/18ft and weighing 575kg/1,268lbs. With the tail it could’ve reached a length of 7m/23ft.  


There is monophyletic evidence that platynoans (genera: Varanus lizards, Heloderma lizards and mosasaurids) are related to snakes. The extinct marine mosasaur lizard, in reference to their terrestrial lizard origins, is actually more related to snakes than snakes are to extant varanoid monitor lizards. Both mosasaurs and snakes have reduced, vestigial or absent limbs, four or fewer premaxillary teeth, free mandibular tips, a vertically straight splenio-angular joint, the double hinged jaw’s ability to unhinge in creating a wide mouth opening  and the absence of epiphyses on the axial skeleton of the skull. All in all, there are forty morphology characteristics prompting scientists in introducing a new mosasaur/snake clade called Pythonomorpha (phonetics: Pi-thon-o-mor-pha).

Mosasauroidea clade

Mosasaurs are a suborder of lizard going back to the seas while becoming an apex predator in being as dominant in the waters as Tyrannosaurus was on land (Sea rex vs. T rex).

Roaming the shallows and ocean depths for any sized meal, mosasaur species traveled the world’s open oceans and inland seas. Nonetheless, mosasaurs did evolve from terrestrial origins replacing feet and toes with flippers.

Mosasaur after a shark

Mosasaurs breathed air so had to surface to breathe periodically and were efficient rapid swimmers possessing a strong rudder designed broad tail for propulsion with limbs as flippers to steer and cut through water. Mosasaur limbed flippers have the same skeletal structure as their terrestrial lizard ancestry, but instead of feet and toes, membranous webbing overtook the whole foot as mosasaurs evolved encasing the foot skeletal elements. These hydropedal limbs evolved at least twice independently in mosasaur species.

Semi-aqautic Dallasaurus skeleton
The more primitive and smaller mosasaurs could still walk on land with their limbed flippers known as plesiopedal, but as later hydropedal species grew larger, they also grew heavier and would not have been able to traverse land due to weight. Being relegated to only an ocean existence, mosasaurs gave live birth with one fossil showing four fetuses in what once was her womb region. Neonate mosasaurs were one to two meters long at birth with the mother giving birth anywhere she was when the birthing began to commence. There were no shallow nurseries where soon-to-be moms collected. A few newborn fossils were 483km/300mi from the nearest shores at the time.

Mosasaur skeletons
Distinct from the four other synapsid mesosaur and the three euryapsid pliosaur, plesiosaur and ichthyosaur marine reptile subgroups, mosasaurs evolved from lizard anatomies.

Occurring in the Middle Creataceous, 100-95 mya, Aigialosaurus (Phonetics: Eye-gee-ah-lo-sor-us) lived along the Cretaceous river/lake shores, river deltas and coast lines. It most likely ate aquatic animals living along shores.

Aigialosaurus was 1.4m/4.5ft long and was a slender animal. It is intermediate to varanoids in having varanoid terrestrial limbs. Thus far, being the basal most representative of the Mosaurinae subgroup, it did have toes with claws, but also as representative to mosasaurs, possessed mosasaurid characteristics of fused frontal bones, a developed hinge joint between the angular and splenial bones, a circular configuration of the quadrate bone and a reduction of the transverse processes and zygapophyses which are one of the two paired processes of a vertebra that interlock it with the adjacent vertebrae.      

Appearing a few million years later in the fossil record was Dallasaurus (Phonetics: Dal-lah-sor-us) occurring 92 mya in the Middle Cretaceous. It is the intermediate between Aigialosaurus and mosasaurs in the Mosaurinae subgroup.

At .91m/3ft long it did have an indistinguishable innateness of Aigialosaurus and varanoids’ trunk and tail vertebrae, while still carrying the mosasaur bone specifics described in the afore mentioned Aigialosaurus description.  It is the basal stem for the primitive mosasaurs and the intermediate progenitor to mosasaurs like Tylosaurus (Phonetics: Ty-lo-sor-us) and Mosasaurus (phonetics: Mo-za-sor-us) with both reaching lengths in excess of 15m/49.2ft.

Dallasaurus’ posterior maxillary teeth were varanoid in being strongly recurved. Along with moderately webbed feet, this lizard was semiaquatic frequenting the waters along shorelines to appease its piscivorous diet. Dallasaurus also had the distinctive mosasaur fused haemal arches, a humeral postglenoid process and an elongate atlas synopophysis to name a few.

I’m attempting to be just wise enough in mentioning here, Coniasaurus (Phonetics: Ko-nee-ah-sor-us) that appeared in the fossil record 97-84 mya. Although there is still debate due to the incomplete fossil finds of two species, phylogeny analysis and cladistics suggest that Coniasaurus is a sister group to an Aigialosaurus and mosasaur clad.


Mosasaur evolution experienced a rapid diversification and global distribution during the Late Cretaceous. To break it down in a generalized form:

100-94 mya ~ Mosasauroidea; Aigialosaurus     
92-90 mya ~ Mosasaurinae; Dallasaurus and other mosasaur precursors
89-85.8 mya ~ first wave of primitive but true Mosasaurs; Clidastes, Platecarpus
85.8- 83.5 mya ~ much larger/distributed worldwide Mosasaurs; Tylosaurus
83.5-71.3 mya ~ second wave of Mosasaurs; Mosasaurus, Globidens, Halisaurus
71.3-65.4 mya ~ greatest diversity even invading freshwater; Pannoniasaurus
65.4 mya ~ all mosasaurs become extinct

Mosasaurs were the first vertebrate animal group to populate the entire world. The map below, adapted from paleontologist S. Suzuki’s original work (1985) shows the Late Cretaceous distribution of mosasaurs 84-66 mya. The dashed lines represent the extent of epicontinental shallow sea encroachment during the time period.

Mosasaur global fossil sites

The reason for mosasaur extensive migrations is most likely due to food competition and in seeking new niches. During this period sharks were becoming abundant [such as Cretoxyrhina (Phonetics: Kreh-tox-see-rye-nah), a 7.6m/25ft long prehistoric shark] and would eventually rule the seas after the K-T extinction.

With a flexible skull and a head contributing to anywhere from 10%-14% of total body length, from molluscs, fish to large euryapsids and sharks, mosasaurs ate anything they chanced upon.

Tylosaurus keel scaled skin fossil

Tylosaurus mosausaur
There were also freshwater mosasaurs towards the end of the Cretaceous and just like in the mass ocean migrations, most likely it was due to predator competition and pressures from lack of prey that induced certain mosasaurs to move upstream from river deltas further inland.

A Plioplatecarpus (Phonetics: Ply-o-plat-ee-car-pus) unnamed species was found in Alberta and Saskatchewan, Canada in 85-3-83.5 mya overbank deltaic deposits with no evidence of opened marine environments letting out into the ocean. This evidence confers that Plioplatecarpus had indeed invaded estuarine and river environments. The older genus, Platecarpus (Phonetics: Plat-ee-car-pus) had evolved into the genus, Plioplatecarpus. From shallow coastal seas, Plioplatecarpus moved inland into rivers.

Several mosasaur species of juveniles and adults were found far from the seas in inland rivers deposits. The Hungarian finds show the animal had webbed limbs with terrestrial lizard-like feet, so apparently was in a state of evolutionary reversion back to claw-toed limbs. This mosasaur species was named, Pannoniasaurus inexpectatus (Phonetics: Pan-nown-nee-ah-sor-us N-ex-pec-tah-tus).


Pannoniasaurus lived between 85-3-83.5 mya surviving in a freshwater river system composed of an island chain that sat between the African and Eurasian landmasses in what was once the Tethys Ocean. With a size of 6m/19.7ft and a mouth set full of sharp teeth, it was the rivers’ apex predator even though prehistoric alligators shared the rivers it inhabited.
Still maintaining mosasaurid skull characteristics, Pannoniasaurus further had flattened the skull much like a crocodile’s which aided in ambush strategies on aquatic and bank frequenting animals.

The mosasaurid Tylosauarus

Lizards, as the case with mosasaurs were and are one of the most successful groups radiating outwards into differing species with today exhibiting over 3,000 species. One of the main components that added to this success was cranial kinesis allowing for skull element flexibility.

Lizard cranial kinesis

Cranial kinesis deals with skull motility and is the presence of moveable joints within the cranium. All vertebrates have some form of movable skull parts whether it’s simply the lower jaw joint only or the flexible unhinging of skull elements.

Mesokinesis, involving the frontal and parietal bones, experiences more jointing in the rostral region of the cranium, while metakinesis is experienced jointing between the dermatocranium and occipital segment. Hypokinesis is a decreased contraction within cranium parts where jointed.

Streptostyly is defined as a rotation of the quadrate at its dorsal articulation against the squamosal and/or supratemporal bones. The axis of rotation is transversely oriented, so that the quadrate swings through an anteroposterior arc. Like other forms of cranial kinesis, streptostyly involves quantifiable movement of cranial elements applying some kind of force against each other, whether it be tension, compression or friction. Streptostyly is the fore-aft movement of the quadrate about the otic joint (quadratosquamosal joint), although transverse movements may also be possible.  

Amphikinesis is the occurrence of mesokinesis and metakinesis in a coupled and coordinated manner. For any type of kinesis to occur there must be neurokinesis addressing movement between the braincase and palate at the basipterygopterygoid joint in the vicinity of the sphenoid bone.

Cranial kinesis characterizes distinctive skull movement features. Kinesis features were slowly adapted improving the modes of capturing and holding prey prior to swallowing. By the time snakes evolved, cranial kinesis, along with neurokinesis were already well evolved and genetically inherited.


Serpentine: The big battle over snake evolution on whether it evolved from a fossorial (burrowing) lizard, or an aquatic lizard is finally solved. Snake osteology is very fragile and is not a good candidate for fossilizing. But with new fossil finds and DNA sequencing, the riddle has been solved and I am glad to say it was the route I always wagered on and that is...snakes evolved from burrowing proto-lizard varanoids, not just once but multiple times.

Snake evolution

Snakes inherited the ability to produce venom as most extant platynoans do in various volumes. Venom is primarily derived proteins whereas snake venom evolved a single time from its lizard lineage before divergence in speciation. The derived venomous protein further evolved in snakes through gene encoding of a normal protein from regions of other regulatory bioactivity processes. For example one pancreatic protein would be recruited into a venomous proteome (a genome expressed entire set of proteins) while protein from another body area is inducted also into the venom gland.

Venom clade

This protein mixture eventually resulted in a species unique venomous cocktail specific for affecting and subduing a particular prey. Venom was developed for apprehending prey. Its secondary advantage is in defense.

There may be no real way to know if mosasaurs had the ability to inject venom. Although they came from a terrestrial ancestor with venom injecting capabilities, the type mode of injection is not from a hollow fang, as is the case for most recent evolved venomous snakes, but from grooved fangs that require chewing to stimulate venom to flow down the grooves and into the wound. In an aquatic environment mosasaur venom would simply have washed away.

So perhaps, mosasaurs lost venom injection capabilities, but it was no real loss as their jaws and size would eventually allow them to rule the seas.

The most primitive extant basal snake group is scolecophidians (blind snakes). One of the largest genetic reptile dataset assemblages carried out by Associate Professor, John J. Wiens in the Department of Ecology/Evolution at ‘Stony Brook University’ verifies almost all extant snake groups arose from this primitive burrowing blind snake group. This is highly suggestive in alluding to the fact that snakes had fossorial origins and from this evolvement, all extant snakes still carry (no matter if they are terrestrial, arboreal or aquatic) a retained small tail. Small tails are evident in all burrowing fossorial vertebrate animals.      

Hox genes govern the boundaries of the neck, trunk, lumbar, sacral and tail regions of vertebrates. Hox genes play the role of a switch-box controlling the modules of genes at specific regions along the axis of the body. Hox genes encode a class of transcription factors and are pivotal in specifying regional identity in body plans. Differences in their expression explain the evolution of animal phyla. For vertebrates, Hox genes function to regulate the regionalization of the axial skeletal structure. In snakes, Hox genes have not turned off but have slowed the development of limb structure, while greatly slowing it down in the forelimbs from the scapula (shoulder) down to the phalanges. The slowing process has continually occurred until in modern snakes, there is no evidence left of the forelimbs and only vestigial remnants of the hind pelvic girdle in the more primitive snakes such as the blind snakes and boas. This is totally the opposite effect in mammalian Hox gene expression where the trending has been accelerated in limb lengths.

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With recent fossil finds, the Hox gene limb development slowdown has been nicely presented while pushing snake evolution back another 70 million years from some 97 mya to 167 mya. Between 167-100 million years, snakes began radiating and evolving under their increasing elongate and loss of limb body plans.  In leaving a fossorial lifestyle and going back to the surface, loss of limbs was an advantage for snakes in rocky/bushy terrain, climbing and in swimming.  Also expressed in snake evolution is a reversion back to sight; although it isn’t the most dependent sense snakes rely on.

Evolution of the elongated cylindrical snake body reveals homoplasy in amniote Hox gene function. What this means is that the snake body form is convergent being a shared characteristic between differing species and not acquired from a common ancestor. Regionalized expressions of the Hox gene code in the fossil record supports a subtle morphological gradient along the anterior to posterior primaxial (along or closer to) axis in stem member species of extant lizards and snakes.

So, even though the lack of Hox gene expression in creating smaller and smaller limbs is apparent, the osteological origin of the whole snake form is due more from the decoupling of primaxial and abaxial (situated away from axis) domains in conjunction with increased somite numbers. Therefore, primaxial Hox gene function does not play a part in the overall snake skeletal structure.

Paleontologists now realize that the snake overall body form was not due simply to the lowering of Hox gene expression, for if that was so, then there would be fewer regional differences in the shapes of vertebra down the vertebrae column. As it turns out, snakes have the exact same number of differing vertebrae regions as lizards possess.

The oldest known snake fossil thus far is the four-limbed, Eophis underwoodi (phonetics: E-oh-fize Un-der-wuh-dye) occurring 167 mya. Only 25.4cm/10in long, they’re not sure if it was a juvenile or adult. With hind legs and no fossilized evidence of forelimbs, it most likely possessed them in smaller vestigial form than the hind legs; they just did not preserve. This proto-snake had a head more like extant snakes than lizards, subdental lamina, no fangs, a far shorter body than extant snakes, but the neck was already elongated. Eophis was found in England strata that was once a swamp. Being semi aquatic it most likely ate minnows, small crustaceans, tadpoles and insects.

Living between 157-152 mya at the end of the Jurassic, Portugalophis lignites (phonetics: Por-tu-gal-o-fize Lig-nye-tay) fossils were found in Portugal coal deposits. Its mouth was loaded with sharp, fully recurved conical teeth. This four legged proto-snake was 1.2m/4ft long and most likely ate smaller vertebrates like frogs, lizards and perhaps even a small or juvenile dinosaur.

155 mya fossil remains of Diablophis gilmorei (phonetics: Dee-ab-lo-fize Gil-mor-eye) were found in Colorado riverbed deposits far inland from any Jurassic seas. It had well developed subdental lamina in which formed a medial border of the subdental gutter and had a neural spine tail. Diablophis possessed ‘trefoil’ organized neural canals that are typical of modern snakes. This four legged snake’s limbs were still functional, but the forelimbs were degrading towards vestigial and due to size were becoming a hindrance and obsolete.

The stem-snake genus, Parviraptor (phonectics: Par-vi-rap-tor) now includes just one fossil species in P. estesi found in the very late Jurassic to very Early Cretaceous 147-143mya England limestone bedrock. P. gilmorei, was initially designated in the genus Parviraptor, but under more phylogenetic scrutiny has been placed into the new genus Diablophis as explained above. Parviraptor appears to have been restricted to lagoonal shallow subtidal swampy marine environments.

These four snake finds fills a morphological, phylogenetic and ecological data gap that was predicted by molecular phylogenetics. It occurred during the final Pangaea breakup into Laurasia and Gondwana co-occurring with early anguimorph lizards. The finds also bear out that the evolution of characteristic snake skull elements appeared long before the loss of limbs.

Ophidia clade

Ophidia (phonetics: O-fid-ee-ah) is a suborder for all extinct and extant snakes. The cladogram below gives perspective in relationships to the ancient stem snakes or proto-snakes to extant species.

Tetrapodophis fossil
Appearing 120 mya, Tetrapodophis amplectus (phonetics: Tet-rah-pod-o-fize am-plek-tus) was still sporting four limbs but still utilizing the serpentine locomotion belly crawl as all previous four legged snakes had done. To aid in crawling, it had the typical broad oblong ventral scales, along with other extant snake features such as a short snout but long braincase, curved jaws and sharp recurved teeth. Having small neural spines suggest a fossorial burrowing lifestyle. 

Found in northeastern Brazil’s Cretaceous fossil rich limestone Crato Formation, Najash rionegrina (phonetics: Nah-jas Rye-un-eg-ree-nah) proto-snake had lost its forelimbs, but had functional forelimbs outside the ribcage including a well-defined sacrum supporting a pelvis. It was found in 90 million year old terrestrial deposits in Patagonia Argentina. Measuring up to 1.52m/5ft, it was a burrowing snake.

Also during Najash’s time period 90 mya, snakes entering the shallow seas had only hind limbs but were becoming vestigial as tiny legs. In the three genera, Eupodophis (phonetics: U-pod-o-fize), Pachyrhachis (phonetics: Pac-e-rake-iss) and Haasiophis (phonetics: Ha-see-o-fize) found in the Middle East’s West Bank Cretaceous sediment, had an aquatic lifestyle capturing mostly aquatic prey.

All three of these snakes’ have left fossilized paddle-like tail fin impressions much like modern seas snakes. In addition, the 1m/3.3ft long Pachyrhachis had very thick and dense ribs and vertebrae aiding it in diving.

Lft: Eupodophis Middle: Pachyrhachis Rt: Haasiophis
Lft: coniophis Rt: extant snake
Having no evidence of cranial kinesis, Coniophis precedens (phonetics: Con-e-o-fize Prek-e-dens) appears to be in a sister group to all extant snakes. Showing up around 70 mya in the Late Cretaceous and measuring 70cm/27.6in, it had elongate jaws, but they were fixed unlike extant snake jaws that unhinge. Although it couldn’t capture larger prey, it appears, due to the teeth type, Coniophis was suited more to seizing and holding onto small soft vertebrate prey instead of insects or worms. The vertebrae and shape of the jaw suggests Coniophis was a burrower.

Snake cranial kinesis evolution

Coniophis was a transitional snake in showing the evolution of snake cranial kinesis. In considering snake skulls, it represents the most primitive snake line, but is not the oldest snake. In the clade above, each circled number indicates cranial kinesis evolution with 1 representing the most primitive ascending to 10 as the most advanced form.


Superimposed Titanoboa skeleton

Titanoboa (Phonetics: Ty-tan-o-bo-ah) was a monster among predators reaching a length of 14.6m/48ft and weighing in at 1,134kg/2,500lb. This boid occurred 60-58 mya in the Paleocene just after the extinction of dinosaurs at the end of the Cretaceous. With multiple remains found (28 individuals) at the Cerrejon coal mines of Columbia, this snake lived in a warm tropical wet environment. although due to the size of Titanoboa, the environment could not have been too warm, for the giant snake’s metabolism generated a lot of heat. Too hot a climate, the snake would’ve overheated and perished. however, the warm temperatures did allow for the larger size as it does today. Large boids (boas and pythons) today live in the tropics where the smaller snakes inhabit more temperate climes.

Although Titanoboa could easily consume the largest crocodilians present at the time, evidence alludes to the fact that the snake was piscivorous in primarily preying on fish.  This snake was much thicker in its midsection than in the more tapered anterior and posterior ends and most likely was a drab color for better camouflage as it was an ambush predator.

An adult L. carlae

Now compare the 14.6m/48ft Titanoboa to the extant snake, Leptotyphlops carlae (Phonetics: Lep-toe-ty-flops Car-lay) which could comfortably curl up on a U.S. quarter at 10cm/3.9in. 

Snakes and amphisbaenians evolved from burrowing lizards. Both had fossorial origins, but where snakes went back to the surface away from the tunneling and burrowing underground lifestyle, amphisbaenians continued to exist in a fossorial environment. Instead of primarily invertebrate prey found in tunneling, snakes found larger prey on land, so the skull elements became more flexible and unhinged, where amphisbaenians remaining underground retained more fused skull features to aid in tunneling.

Amphisbaenian clade

Amphisbaenians have been very difficult to quantify, for although they indeed came from a lacertid lizard ancestry, there too occurred much parallelism and convergent evolution between the two groups having nothing to do with genetic sharing. 

While amphisbaenians are mostly limbless, three extant Bipes (Phonetices: By-pees or Bi-pez) species have reduced forms of front limbs and rudimentary internal hind limbs. Morphological data shows that species with front limbs form a sister group to those that are limbless. This means that the amphisbaenian loss of limbs occurred only once.

Tamaulipasaurus (phonetics: Tah-moe-lee-pa-sor-us) was indeed a burrowing lepidosaur diapsid and also has numerous sister taxa. Occurring in the Early Jurassic 189.6-183 mya and Middle Jurassic 165mya, two fossils have been found in Mexico’s La Boca formation material. Extant amphisbaenians and ambamids are closely related. One of Tamaulipasaurus’ sister groups gave rise to amphisbaenians while another to dibamids.

Tamaulipasaurus fossilized skull
One feature Tamaulipasaurus possessed that is absent in all other squamates is that it had an autapomorphic (possessing a distinctly anatomical derived trait unique only to one taxa) quadratojugal. Besides this novel bone, it appears its orbits confluence with the temporal fenestrae. This is a feature also found in the extant two-legged amphisbaena genus, Bipes. It also shared with amphisbaenian lineage an emarginated quadrate with a tympanic recess and a lower temporal bar.      

Named Crythiosaurus (phonetics: Crith-ee-o-sore-us) this ~90 million year old lizard may well be a basal predecessor to amphisbaenians. Only the skull has been found and is longer and narrower with a larger occipital as is the case in lacertid lizards. However, extensive cranial fusion of skull elements are evident with the temporal reduced to just a piece of bone. Even though the skull was more elongate and narrower like lizards, the fusing of the skull elements suggest a fossorial lifestyle and it most likely had reduced limbs and an elongated body for burrowing. The teeth indicate an insectivorous diet.    

Chalcides (Phonetics: Chale-side) is an ancient and extant three-toed skink genus where embryological evidence points to the lessening size and number of digits are due to Hox gene d11 expressing digits 1, 2 and 3 as digits 2, 3 and 4. This suggests that digit reduction, as in amphisbaenians, is a more frequent consequence in convergent evolution such as in snake, dibamid lizards and avian evolution.

Skull comparison
From an ancient sister group to Chalcides derived the genus, Sineoamphisbaena, which, although there is strong debate, is one of the first stem amphisbaenians. This animal’s skull reveals a mosaic of lizard and amphisbaenian like characters. A main feature is cranial consolidation adapted for a fossorial lifestyle. However, the fossils show skink-like modifications of the palate and temporal regions of the skull. The relative primitive morphology of Sineoamphisbaena is an indication that it did not devote its whole life to being fossorial and most likely frequented the surface far more often than extant amphisbaenians do. This most likely is due to the fact that during the animal’s time in the Late Cretaceous, there was a transition from a perennial lacustrine environment to a semiarid aeolian environment 75-71 mya in what is now present day Mongolia.


Sineoamphisbaena caused a quandary for paleontologists, for the assumed basal and dispersion of amphisbaenians was rooted in North America from 22 assigned Rhineura (Phonetics: Ree-noor-ah) species and not from Inner Mongolia. From the 22 species, they were whittled down to 9 extinct and the only extant N. American in the species R. floridana. The debate still lingers, but Rhineura stills holds onto ground when considering extant amphisbaenians. We’ll touch on that in a bit.

Cryptolacerta (phonetics: Crip-toe-lah-sir-tah) appearing 47 mya is considered a lacertibaean as it had an amphisbaenian fused head but, though much smaller, lacertid-like limbs. This fossil, as a holotype supports molecular genetic studies that amphisbaenians and lacertids are related.

Cryptolacerta fossil
After X-ray computed tomography analysis on the Cryptolacerta well preserved almost complete fossil with only the tip of the tail missing, results bear out that the reinforced thickened skull is in a relationship with amphisbaenians and that both are related to lacertids. This prognosis reveals the fact that amphisbaenians’ cranial akinesis (skull element fusion) came first before the snake-like body and loss of limbs during their evolvement. This also reveals why amphisbaenians adapted to tunneling with the head.

Cryptolacerta, through studies with extant lizards present the lizard as an animal that primarily spent its time with its body held low to the ground (squat bodied) while leafing through forest floor litter, but was also an opportunistic burrower.

Cryptolacerta sits at the base of amphisbaenian branch with amphisbaenians and lacertids branching out from that base on differing, but adjacent lines.

In representing the family, Blanidae (phonetics: Blan-ah-day) is the Miocene Epoch 11.6 mya fossil, Blanus mendezi (phonetics: Blan-us Men-de-zee) excavated in Valles-Penedes Basin in Spain’s Catalonia region. Extant blanids are only represented in Europe, but further subdivided as western and eastern, while as a whole, is one of the three groups representative of the Mediterranean amphisbaenian groups.

B. mendezi skull

The fossil is of a complete 5.8mm/0.23in tiny skull encased in carbonate. Using computed tomography (CT), the scientists developed a 3-D image showing it had 20 teeth and the skull was similar to modern day blanids.

Of the three Mediterranean groups, one is Iberian (western), another is Eastern Europe and the last is Northwest Africa. This fossil represents the western group and alludes to detail that the Iberian and Northwest African groups arose from one western Mediterranean group that only later subdivided.

Currently, there are six families of amphisbaenians and they are:

1)    Amphisbaenidae - Amphisbaenids; tropical worm lizards of South America, some Caribbean islands, and Sub-Saharan Africa (17 genera)
2)    Bipedidae - Only in Mexico; commonly called ajolotes (1 genus)
3)    Blanidae - Anatolian, Iberian, and Moroccan worm lizards (1 genus)
4)    Cadeidae - Cuban keel-headed worm lizards; traditionally amphisbaenids, but shown by DNA to be closest to Blanidae (1 genus)
5)    Trogonophidae - Palearctic worm lizards (4 genera)
6)    Rhineuridae - North American worm lizards (1 genus)           

There are four distinct cranial assemblages and all are highly specialized for a style of utilizing the head as a tool for tunneling. These four modes of tunneling are associated with an inter species stereotyped burrowing behavior.

A basal burrowing amphisbaenian fossil 

Bipedids and blanids use their blunt ‘round-headed’ form for tunneling by ramming the head into the soil and widening the tunnel by uplifting the head and compacting the dug soil to the tunnel ceiling. Bipedids also use their forelimbs to propel forward.

Rhineurids use a depressed ‘shovel-headed’ form by using the dorsoventrally flattened snout and with a strong craniofacial angle, push the loose dirt to the sides where it is then compacted to the walls of the tunnel as the body goes through. 

Trogonophids use a ‘spade-headed’ form by also using a dorsoventrally flattened snout, but with a strong craniofacial angle and further use their lateralcanthi (sharp edging on sides of head) to shave off soil from the front of the tunnel in an oscillatory fashion then pack it to the tunnel sides with sides of the head and body.

Most amphisbaenids (8 genera) and the cadeid utilize a compressed ‘keeled-headed’ form and dig with a laterally compressed head by ramming the head forwards then push and pack the loosened soil rearwards in pushing the head in opposite directions to pack the soil to the floor and ceiling of the tunnel.   

Due to the numerous convergent evolutions within the amphisbaenian group, limited outgroup morphologies and the lack of findings of the tiny and frail subterranean fossils, amphisbaenian evolution has been a booger-bear to track and sort out for scientists.

Extant Amphisbaenian cladeogram

Another quandary concerning amphisbaenians is biogeography in how they became so widespread in living such a fossorial lifestyle, for they certainly didn’t tunnel across the oceans. Amphisbaenians’ global dispersal began around 65 mya. They may have traveled on the third and final breakup of Pangaea and Laurasia ending 60-55 mya, but that breakup began a good 95 mya. What scientists feel is that they ocean rafted amongst soils held together by roots. This also explains why other fossorial species are so widespread.

Anyway, some primitive amphisbaenians did make it to North America and diversified coming up with nine species of rhineurids 60 mya. Eight species went extinct. The extant R. floridana is the only survivor because it was the first to diverge and migrate where milder North American temperatures prevailed. The extinct species’ fossils are found in the northern portion of N. America in strata laid out during the Cenozoic’s Paleocene and Eocene Epochs 66-33.9 mya. The climate was very mild then; there were even crocodiles swimming off the coast of Greenland. By the beginning of the Oligocene 33.8 mya, cooler temperate climates began to prevail while the Antarctic got its first ice cap. The cold was too much for the eight warmer weather loving rhineurids. Not enough cold reached into R. floridana’s more southern range.

In my opinion molecular takes precedence over morphology

For living amphisbaenians, as bipedids are a sister taxon to the trogonophid-amphisbaenid clade, R. floridana as a Rhinenuridae family member is the most basal morphological species among other amphisbaenian families, but is a young genus first arriving in the Pleistocene.

The first diapsid to experiment with euryapsid skull fenestra was Araeoscelis (phonetics: Ah-ray-oss-kell-iss) living 284.4-275.6 mya. This 60cm/2ft long reptile, most likely gave up on its lower fenestrae to create a stronger bite, but it had no direct basal relations to the marine euryapsids.

Euryapsids are lepidosaur reptiles derived from diapsids in migrating the two single openings on each side into the upper temporal as an opening behind each eye on top of the head. Euryapsids had the diapsid lower temporal fenestrae fused over. Ichthyosaurs furthered the euryapsid arrangement with a special set of introduced bones along the margins of the fenestra (opening).

Araeoscelis basal clade
The euryapsid group is considered polyphyletic in that most of their shared characteristics are considered convergent evolution rather than related through ancestral genetics. They first appeared in the early Triassic 245 mya and became extinct at the end of the Cretaceous 65 mya.

There are some that still contend euryapsids were archosaurs instead of lepidosaurs, but from the shoulder girdles to the phalanges, sauropterygian limb anatomies and the lateral undulation of the vertebral column, euryapsids were of lepidosaur origin. The major euryapsid groups are divided into two major taxon superorders; they are:

Sauropterygian clade

Sauropterygia (phonetics: Sawr-op-ter-ridge-ee-ah): meaning ‘lizard flippers’ including the four orders, 1. Placodontia (placodonts) 2. Pachypleurosauria (pachypleurosaurs) 3.  Nothosauroidea (nothosaurs) 4. Pleisosauria (the suborder: plesiosaurs and suborder: pliosaurs).

Ichthyosauria clade

Ichthyopterygia (phonetics: Ick-the-op-ter-ig-ee-ah): meaning ‘fish flippers’ includes the two orders, 1. Grippidea (grippiosaur, gulosaur and chaohusaur) 2. Ichthyosauria (ichthyosaurs).

Tangasaurid Hovasaurus
In sauropterygian evolvement, the major ancestral suspect is the Late Permian neodiapsid belonging to the superfamily, Younginiformes (phonetics: Yun-gin-nee-forms) that also survived into the Early Triassic. It is from the tangasaurid family and further divided into the subfamily, Tangasaurinae (phonetics: Tain-gah-sor-ee-nay). While the other tangasaurid subfamily, Kenyasaurinae (phonetics: Ken-ya-sor-ee-nay) was terrestrial, Tangasaurinae species had become aquatic in a lacustrine (freshwater or salty lake) environment. Adapted to an aquatic life with highly derived amphibious characteristics, they had webbed feet and laterally compressed tails for sinusoidal propulsion through water.

As mentioned earlier under diapsids, the neodiapsid claudiosaurs are perhaps basal to nothosaurs, plesiosaurs and pliosaurs. Considered an extension of the condition seen in claudiosaurs, pachypleurosaurs and nothosaur palates are posterior and medial extensions of the pterygoids, with the interpterygoid vacuities completely closed while ventrally, the base of the braincase is covered.

Primitive sauropterygians like the placodonts first appear in the Middle Triassic 245 mya and begin radiating quickly just 5 million years after the great Permian/Triassic mass extinction (PTME). Normally the bodies were squat and bulky with short legs and fed along shallow coastal sea beds. By the Late Triassic 210 mya, placodonts had become extinct.

Placodonts experienced pachyostosis giving heavier and denser bones for negative buoyancy to aid in being anchored while feeding on shallow coastal sea beds. This still tied placodonts to land to rest after both long periods of holding the breath in feeding and extraneous swimming.     

By 242 mya, pachypleurosaurs made their appearance. These primitive sauropterygians, although small ranging in size from 20cm/8in to 1m/3.3ft appear to be ancestral to plesiosaurs with small heads, long necks and paddle-like limbs. Without doubt though, pachypleurosaurs are a sister taxon to the eusauropterygian clade that both nothosaurs and plesiosaurs belong in.

Nothosaur clade

Soon after the appearance of pachypleurosaurs, eusauropterygian nothosaurs appeared showing a rapid euryapsid radiation and diversification in shallow Triassic epicontinental seas and near-shore basins of the Tethyan seashore fringes. Once flooding from sea transgressions in what is now the continent of Europe, eusauropterygians successfully followed behind the sea inundation filling in the ecological niches as apex predators from what is now Spain to China.

Nothosaurs began to really appear 240 mya, but by the Late Triassic 210 mya, most nothosaurs had died out. This range is with the exception of Nothosaurus zhangi (phonetics: Noth-o-saw-rus Jawn-gee). This 6.7m/22ft nothosaur was found embedded in 247 million year old carbonate sediment in present day SW China. N. zhangi had the largest known jaw among all Triassic sauropterygians. What this precludes is that sauropterygians had Permian ancestral beginnings and as survivors of the extinction, gave rise to apex predators in a fairly quick recovery of shallow marine ecosystems.

Nothosaurs were much like the present day pinnipeds (seals, sea lions, walruses). Dependent upon the sea for much of their food, but would clamor to the seashore for rest and sunbathing.

By the time nothosaurs died out, plesiosaurs began to appear 203.6 mya in the Late Triassic. There is good argument that nothosaurs evolved into plesiosaurs. The intermediate between this nothosaur/plesiosaur transition was quite possibly the related pistosaurids. Resembling nothosaurs, but more derived for a permanent aquatic life with webbed feet replaced by flippers, they also had other plesiosaur features. Like a nothosaur, pistosaurids had the same body form and the same primitive palate with a mouth possessing the same type numerous sharp teeth. Like the plesiosaur, it had flippers for maneuvering through water, a stiff vertebral column and a similar skull structure and head.

Plesiosuar/Pliosaur clades

Plesiosaurs started off with short necks in the earlier species much like nothosaurs. During the earliest part of the Jurassic, a plesiosaur family subgroup, Rhomaleosauridae (phonetics: Row-may-lay-oh-sor-ah-day) had developed into numerous species and proves to be a basal split-off of short necked plesiosaurs. From these subgroups towards the end of the Early Jurassic 180 mya, the long necked plesiosaurs begin to appear.


Early plesiosaur

Neck lengths continued growing until apexing with Elasmosaurus (phonetics: E-las-mo-saw-rus). Evolving and living in the Late Cretaceous 83.6-80.5 mya its neck at, 7.6m/25ft was at least half its 15.2m/50ft total length. Longer necks allowed for prey ambush strategies separating its visually bulky body from its mouth and supported a head with excellent eye vision in water, but watery environments totally supported the neck as it would not have been able to hold it above water.
However the elasmosaurid, Albertonectes (phonetics: Al-bear-toe-neck-tees) reached the largest length in neck and body among all plesiosaurs. At 11.2m/36.75ft with a neck supported by 76 vertebrae, the neck alone was 7m/23ft. Finding the Alberta, Canada fossil with the neck vertebrae detached and how each neck vertebrae were assembled with rigidity, shows the assemblage would not allow neck movement due to the lack of flexible interlockings. Albertonectes simply could not turn its head.

Certain plesiosaur groups gave rise to pliosaurs which first appeared in the beginnings of the Jurassic 199.6 mya.  In reverting back to shorter but stronger necks to support a massive head with strong jaws, pliosaur species became very dominant in the Jurassic and Cretaceous seas radiating out globally and diversifying into numerous species until dying off in the Late Cretaceous 89.3 mya. One of the chief cranial differences between plesiosaurs and pliosaurs is that plesiosaurs tend to have upward facing eyes while pliosaurs are laterally placed. Pliosaurs and mosasaurs superficially resembled each other physically.

Plesiosaur-Pliosaur neck comparison
One of the largest pliosaurs living from 125-99 mya in the Middle-Late Cretaceous was Kronosaurus (phonetics: Crow-no-sor-us) reaching lengths of 10.1m/33ft. A close relative species of Kronosaurus that lived earlier in the Late Jurassic 162-150 mya was Liopleurodon (phonetics: Lee-o-plur-o-don). It was a more modest length of 6.4m/21ft. But a recent pliosaur fossil find (2006-2007) unearthed on the Arctic Norwegian island of Spitspergen was huge. The Late Jurassic pliosaur from 150 mya dubbed, ‘Predator-X’ is bearing out to be 15m/50ft long with the forelimb paddle in itself reaching 3.3m/10ft long. Just recently it’s been named as Pliosaurus funkei (phonetics: Ply-o-sor-us Funk-eye). It’s massive head and jaws had a bite four times stronger than Tyrannosaurus rex.
P. funkei biting force 4x T. rex
Pliosaurus funkei

The first ichthyosaurs looked more like tuna while the latter species appeared more dolphin-like. In either form, all ichthyosaurs were streamlined for swiftness.


Occurring in the Early Triassic so far, Thaisaurus (phonetics: Tie-saw-rus) is the most basal ichthyosaur. Although the forelimbs had become flippers, the hind limbs had not still exposing the feet phalanges and claws. Two other, but more evolved ichthyosaurs showing up a bit later in the Early Triassic were Grippia (phonetics: Grip-pee-ah) and Utatsusaurus (phonetics: U-taht-sue-saw-rus). These two had all four limbs entombed as flippers but did not yet support a dorsal fin as later species would.

S. sikanniensis

Still without a dorsal fin, appearing in the Late Triassic 215-210 mya, Shastasaurus sikanniensis (phonetics: Shas-tah-saw-rus See-kan-nee-n-sis) is the largest marine reptile known growing to lengths of 21m/69ft. How it got that large is still a mystery, for all the species in the genus Shasatsaurus possessed no teeth and it is not known on what it ate or how it captured and ate its meals. Decent speculation is that it grabbed and ate soft bodied cephalopods, but if you truly come up with the answer you will be called a very clever person by many a paleontologist.
Shastasaurus species

Mixosaurus (phonetics: Mis-o-saw-rus) of the Middle Triassic 230 mya has had its fossils found all over the world. As the name implies it was a transitional ichthyosaur from the early smaller and ungainly ones to the later more streamlined. Mixosaurs were also one of the first ichthyosaurs to have a dorsal fin.

By the Middle Jurassic, modern forms had evolved with super streamlined bodies for distance swimming, leaving shorelines for open waters. Ichthyosaur eye development became an asset for vision diving in depths. Temnodontosaurus (phonetics: Tim-no-dawn-toe-saw-rus) at 12m/40ft  is the third largest ichthyosaur while having the largest known eyes of any and all animal groups. Eye diameter measured 20cm/8in. Opthalmosaurus (phonetics: Off-thal-mow-saw-rus) eyes were a bit smaller, but at 6m/19.7ft it was half the size of Temnodontonsaurus, so had the largest ocular diameter compared to body length of any animal ever known.

Largest eye
In the Jurassic, ichthyosaurs reached their apex in speciation diversity then began to decline until becoming extinct around 94 mya before the main Cretaceous asteroid extinction 65 mya. In fact all marine euryapsids became extinct a few million years before the Cretaceous/Paleocene asteroid extinction. It’s not fully understood, but most likely pressures from the more advanced shark rise and mosasaur appearances played a hand in the extreme apex predator competition.

Although fossil eggs or fetuses of placodonts have not been found yet, the evidence from all other euryapsid fossil finds of embryos, fetuses, births and neonates prove most euryapsids gave live births.

A recent ichthyosaur fossil find of a Chaohusaurus (phonetics: Kay-uh-hu-saw-rus) mother dying while giving birth shows the infant ichthyosaur exiting head first. There were three more fetuses also lined up head first to exit. The reason for the death of the mother and her newborns was due to labor complications. The main significance of this fossil find is that the birth was head first.

Chaohusaurus birth
Later evolved tail first births
All tetrapodal marine life (whether extinct or extant) that first derived from a land air breathing lineage then had a latter ancestry of going back to the seas gives live birth in water as tail first to avoid drowning of the newborn. There are innumerable fossil finds of tail first euryapsid births. All extinct marine tetrapods (mosasaurs, plesiosaurs, nothosaurs, etc.) and all extant marine tetrapods (whales, dolphins, porpoises, etc.) give tail first births. This is to allow the maximum time in being able to reach the surface for that first breath of air. When the watery environment hits the head, breathing must occur. That’s why tail first is an advantage by allowing the head to be the last sequence of the birth, giving more time to reach that required first surface breath.

All land animals give birth head first for the same reason, but as an opposite effect. Once the marine newborn is out it must reach the surface quickly, so tail first is an advantage. But in land animal births that breathe air, it is not.  The infant needs instant access to air while being born to keep it from suffocating while still in the birth canal during delivery.

Both strategies have the same effect, to reach air, but with opposite timing; maximum time for a tail first water birth and minimum time for a head first atmospheric birth. What the more primitive Early Triassic Chaohusaurus implication states is that live birth for euryapsids first arose on land then was adapted to watery environs evolving from head first to tail first over a certain period of time.

The video below is an example of how most likely an ichthyosaur birth occurred.

Before we depart, let’s check out a marine euryapsid living dilemma they had to overcome before being as successful as they were.

Fish have no problem maintaining a high level of metabolic energy consumption during locomotory paced activity, for external gills give all the respiratory oxygen exchange needed without being hindered by the fish’s flexing side-to-side movement during mobility.

The first tetrapods to walk on land inherited that flexible side-to-side body motion to travel, but now, in converting air bladders into internal lung respirators replacing gills, an interrupted breathing dilemma arose. This inherited side-flex fish trait equated into a sprawling gait and with the side-to-side body motion in locomotion caused a problem for internal lung function. The first poor tetrapods walked two paces then had to stop just to gasp for air in replenishing its oxygen supply. Running was out of the question for early terrestrial travels.   

Respiration and locomotion use the same set of muscles as derived from fish ancestry. The trunk is flexed to the right and left as land animals walk and run.
Although it may breathe when walking, it cannot breathe when running. This is why.

As a tetrapod land animal steps forward with left foot first, the right side of the thorax including the right lung is compressed while simultaneously the left side of the thorax and left lung are expanded. Of course the cycle reverses when the right foot moves forward.

So this is a big problem, for land animals breathe in air to fill both lungs simultaneously, then exhale to deflate the lungs for another breath of fresh air. But breathing normally cannot commence when one lung is compressed. This distortion interferes with normal thoracic breathing.

When walking slowly, the tetrapod may be able to breathe between steps because the thorax has time to recover from the distortion. But if the animal speeds up, the animal cannot breathe when running. That is why tetrapods must stop in short periods to simply breathe properly. This is called ‘Carriers Constraint’.

Carriers Constraint lung compression

Land vertebrates have solved Carriers Constraint, especially mammals, by evolving a more erect stance with all four limbs vertically supporting the body. This aided moving in a vertical plane when running resulting in less twisting of the thorax, so sparse breathing could be possible during rapid locomotion. With bipedal motion as in humans and kangaroos, the thorax is lifted perpendicular to the ground allowing the ribcage to not flex during rapid locomotion. Still though, a breath can only be taken every other stride or hop.

However a bipedal posture is intrinsically unstable for it requires long periods of high level energy and its consumption, so a high metabolic rate must be maintained at all times, even during rest.

Both quadruped and biped land animals can run for a while without breathing in switching to anaerobic glycolysis. A sprinter can run a 100 meter dash without breathing by switching to anaerobic glycolysis, but this can only be maintained for a few seconds before further circumventing oxygen debt and introducing dangerous levels of lactic acid in the blood in burning lactose instead of oxygen.

Now, with marine reptiles equipped with land heredities like lungs and limbs for mobility, how did they resolve Carriers Constraint in going back to aquatic environs?

These lung possessing marine reptiles competed quite well with gill possessing fish through various choices in swimming styles that did not hinder normal functions of the thorax and lungs within it. Marine reptiles must utilize one breath when diving or subsurface swimming.

Although still dependent on frequenting land, placodonts overcame the constraint in having a leisurely lifestyle going after molluscs and crustaceans on the sea beds of shallow coastal waters. Placodonts also were building epidermal armor that would have made the body more rigid and stiffened the thorax enough to negate twisting from mobile movement.

Pachypleurosaurs and basal pistosaurs were swimming in undulatory anguilliform eel-like fashion without any strengthening or hardening of the thorax. In this transitional mode, pachypleurosaurs had not yet approached a Carriers Constraint solution, so were forced to remain near shorelines.

The Nothosaurs, through a series of species from earliest to most recent, developed larger forelimbs for paraxial locomotion while stiffening the thorax and spine. This nixed undulatory propulsion in the most advanced species utilizing the limbs and not the body for locomotion. The modified limbs anchored to a strengthened ball-and-socket glenoid joint were like hydrofoils negating the body as a necessary element for locomotion.

Plesiosaurs and pliosaurs used paraxial locomotion in which synchronized breathing with surface propulsion. With well-developed massive pelvic and pectoral girdles, all the propulsive force was applied with the limbs directed away from the axis of motion which was of course the body. Plesiosaurians were one of the first to fly through waters.

Ichthyosaurs have not yet fully revealed their key to the unlocking of Carriers Constraint. At a glance in being dolphin-like in appearance, one might think that ichthyosaurs were accomplished long distance and rapid swimmers. Appearances can be deceiving. The torso was not all that rigid and rib cages of ichthyosaurs were lightly built.  

Major ichthyosaur propulsion came from the tail that was anatomically decoupled from the body by a caudal peduncle. They swam in an up and down undulating motion of the tail with a body that doesn’t appear too rigid. With this ammunition, it appears like the ichthyosaur was not prepared to do battle with the constraint.

Ichthyosaur body adaptations with an undulating tail were geared for rapid acceleration and short bursts of rapid speed, but no anatomy shows that it was geared for long distance swimming. Unless, it leaped as dolphins and penguins currently do.

With ichthyosaur tail undulations for rapid acceleration and a pectoral fin for upward force in propulsion, this reptile may have leaped to gain an upper hand over Carriers Constraint. Subsurface cruising expends energy due to waves just below the surface creating a zone of drag. If ichthyosaurs leaped, it would’ve required much less breathing and while in the air giving plenty of time to take one deep breath or two short ones. The subsurface high drag zone could’ve been neutralized by swimming through it at a high angle of 30 degrees or more which would be conserving energy more so than simply maintaining swimming within the drag zone.

In Continuum:
Indeed this was a long article, but it could have been five times longer and still not cover all the bases. Hopefully though there is enough information to remain fluid.

All of the fore, above and future ‘Et Tunc’ series are not concrete fact, but are solidly based on fact. Information derived has most other’s first hand researched input with some sprinkling and dashes of my own stringing together output.
For the next ‘Et Tunc’ series we’ll be discussing the archosaurs that led to crocodilians, pterosaurs, dinosaurs and ultimately birds.

See ya then...     

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