Et tunc nulla erat IV
(And Then There Was)
In ‘Et Tunc Nulla Erat III’, we left off during the late Devonian (382.7-372.2 mya) with the appearance of the truly tetrapodal labyrinthodonts arising from lobe-finned fish. These creatures radiated out in speciation on land with some species members becoming ancestral to reptiles and lissamphibians. Tetrapodal essentially means possessing four limbs.
To addend a bit where we finished up last time, during the Late Devonian 370 mya there was massive effort in vertebrate speciation to radiate out onto land due primarily from heavy aquatic predation, new food sources and poorly oxygenated shallow waters.
During this late Devonian period, the seas and even rivers and lakes were teeming with life of both invertebrates and vertebrate. The land however was relegated to plants and insect arthropods until our ancestors first crawled onto land.
Before we dig into the amphibian line it would be worth mentioning the animal, Bothriolepis (Bo-three-o-lep-is). The creature lived from the mid to the end of the Devonian 387-360 mya and was from the placoderm line of fish although it didn’t look much like a fish. At 20.3-30.4cm/8-12in (although one species B. maxima had a carapace measuring 100cm/39.4in), Bothriolepis was heavily armor plated and had long dagger-like spines for fins that went the length from just behind the head to the trunk of the body.
|Bothriolepis during the Devonian
One reason I’m dwelling on Bothriolepis is that it was one of the first shallow marine animals to populate freshwater environs. The gills were relatively short but broad allowing more surface area to make transitions and adaptation to freshwater more reliable. The dermal skeleton was made of cellular bone making it one of the first animals in utilizing bone for support and protection to transition onto land environments.
|Pandericthys chomping on Bothriolepis
In resultant descendants of the sarcopterygian lobe-finned fishes, natural selection transitions produced Eusthenopteron (Phonetics: You-sten-op-teh-ron) and although it was still relegated to an aquatic life, its adaptations began the progress to life on land with fin bones verging to limbs. Pandericththys (Phonetics: Pan-der-ik-thees) was well suited for muddy shallow bottoms with lobed limbs supported by digits. Acanthostega, which was still dependent to watery environs, had developed webbed feet with eight digits that could pull the body onto shore. Ichthyostega (Phonetics: Ik-thee-o-stega) possessed larger limbs with adults relying more on lungs rather than gills for obtaining oxygen.
|Acanthtostega/Ichtyostega comparison Click to Enlarge
Finally, around 372-370 mya, there was Tiktaalik (Phonetics: Tik-ta-lik) that exhibited the link between fish and amphibians. Its webbed feet were weight bearing, possessed wrists, crocodilian-like shoulders and elbows, simple rays reminiscent of fingers and had gills plus primitive lungs. It had no fish bony plates in the gill area to restrict up or down and sideways head movement. This trait gave rise to one of the first necks ever to be held by an animal. In addition, Tiktaalik’s ribcage was robust enough to support body weight once outside of aquatic environs.
Tiktaalik’s physiological endowments were to lead the way to all future terrain tetrapodal lifestyles in which labyrinthodonts were later to begin making their appearance to continue carrying the tetrapod torch.
The genus, Tulerepton (Phonetics: Tu-lur-ep-tahn) of the Late Devonian was from the Ichthyostegalia order, just as Acanthostega and Ichthyostega, but in contrast, had a much more strengthened limb structure used however more for paddling than walking. This creature though, in adult form had lost its gills and was totally dependent on lungs for acquiring oxygen and with a disconnection of the head from the pectoral girdle, allowed for much more head movement.
The tetrapod genus Pederpes (Phonetics: Ped-er-pees) was the first tetrapod to have five digits on each limb, although there was a sixth vestigial nub on the forelimbs. The feet were not paddle-like and were fashioned to accommodate walking on land. Pederpes thus far, is the first of the fossil record in exhibiting true terrestrial locomotion.
Labyrinthodont skeletal structures differed from true reptiles in having teeth that exhibited complexly in-folded enamel surfaces. These teeth were grooved strongly reinforcing the whole dentition structure enhancing the seizing/holding of prey. The dental arrangement was borrowed from Late Paleozoic Era trending amphibian-like labyrinthodonts, who in turn borrowed it from lobed-finned fish. The term Labyrinthodontia (Phonetics: Lay-be-rinth-o-don-chee-ah) in itself is Greek meaning ‘maze tooth’.
Also borrowed from the early amphibian type ancestry were eyes placed at the top of the skull; a holdover from watery ancestral origins for better field sight on a flat head when floating on the surface.
All labyrinthodonts possessed two otic notches, one behind each eye orbit that invaginated as the posterior margin of the skull roof. The otic notch was an auditory structure supporting a tympanum much like as found in modern day frogs.
|Labyrinthodont skull with otic notches
Varying from the size of a small salamander to a modern day crocodile, labyrinthodonts possessed stocky lizard-like bodies with short limbs. The subclass, Labyrinthodontia is not monophyletic in that the group or clade does not include all ancestral and descendent species, but is rather paraphyletic consisting of the common ancestor, but not all of the descendants. In this regard, a lot of researchers have abandoned the Labyrinthodontia subclass, but due to their direct relationship to modern amphibians and even to the first reptiles, I still include the subclass.
Supporting a massive skull possessing dentine armor, numerous labyrinthodonts were one of the first known land animals to utilize scutes or scales for protective covering. They also maintained topside eye openings, a pair of nostrils and a parietal eye that could sense photoreception. Today, the primitive tuatara, some anoles, iguanas and juvenile bullfrogs also exhibit a distinct parietal eye located between the normal eyes.
In the more primitive water-bound labyrinthodont species along with the more advanced land species, their fossil records show an otic notch behind each eye that served as an open spiracle or water breathing tube in the earliest specimens, which later evolved into a tympanic (ear) membrane in more evolved land forms. The otic notch evolved away allowing for stronger bites by jaw muscle rearrangement reducing stresses and deformation during jaw movement.
During the last 20 million years of the Devonian, ~72% of all animal life became extinct in particular marine organisms were hardest hit. But through this, the earliest of tetrapods survived to be the first vertebrate to walk on firma terra.
Labyrinthodonts survived and radiated outwards onto land from the early water restricted Late Devonian forms to the more advanced land forms in the Early Mesozoic Era with all becoming extinct around 210 mya during the Triassic.
One thing to take away from all this before we dig deeper is that saltwater fish, in order to regulate proper blood salinity drink saltwater retaining the water while getting rid of the excess salt as waste. The first freshwater lobed-fin fish, in order to adapt reversed the process in maintaining blood salinity by drinking water continuously and getting rid of the copious amounts of water through urination while retaining the lower percentage of salts.
This retaining of salt in blood amounts to roughly 9 g/l or 0.9% salt solution in the blood. Further, this is about one-third the salt of seawater. All vertebrates, even today, whether on land or in water, maintains this exact same percentage of essential salts. This also includes man in bearing vestiges of his very earliest fish origins in carrying a little bit of the seas within us.
What is now called ‘early amphibians’ or ‘primitive amphibians’ or ‘basal amphibians’ were in all actuality aquatic or semiaquatic labyrinthodonts. Composed of the superorders, Osteolepiformes (Phonetics: Os-te-o-lep-iss-for-mees), Elpistosteglia (Phonetics: L-piss-toe-steg-lee-ah), and Ichthyostegalia, the class, Tetrapoda (Phonetics: Tet-rah-po-dah) is the crown jewel of all land animals with four limbs including snakes, cetaceans, amphisbaenians and others that have since lost limbs through Hox gene evolution. Out of the three, all lines of evolutionary species became extinct, except for Ichthyostegalia which is the root base for all tetrapod animals past and present.
Ichthyostega and the more primitive Acanthostega were still dependent upon an amphibious lifestyle, but both had a defined osteology formed for tetrapodal locomotion on dry land just beyond shorelines. Limbs with digits had evolved in the waters.
|Another Devonian Ichthyostega scene
From Ichthyostega serving as the basal ancestor, three main branches of labyrinthodont orders arose with side orders becoming extinct leaving no present day survivors. The three main orders were Diadectomorpha (Phonetics: Die-uh-dec-toe-mor-pha), a reptiliomorph labyrinthodont, Lepospondyli (Phonetics: Le-pos-pon-duh-lie) and Temnospondyli (Phonetics: Tem-nos-pon-duh-lie).
Stereospondyli (Phonetics: Steer-e-os-pon-duh-lie), with a simpler backbone composed of a single intercentrum, had branched off from temnospondyl labyrinthodonts around the Permian/Triassic border 255-251 mya leading to a dominant but now extinct lineage. The capitosaurs as viewed from the otic notch sketch were stereospondyls that produced the first crocodilian morphology in losing the otic notch, although they were not an ancestral line to crocodiles.
|Capitosaur trend in losing the otic notch
The order, Nectridea (Phonetics: Nec-tree-day-a) branched off early around 300-299 mya from the newly evolved Lepospondyli labyrinthodont producing the fairly-well known genus, Diplocaulus (Phonetics: Dip-low-cawl-us) only for this genus to die off by the time the Triassic arrived. But lepospondyls weren’t finished in experimenting with new clades and families that were going to succeed up to the present.
Early amphibian labyrinthodonts were the first anchored side of the bridge. The bridge span itself was lepospondyls, temnospondyls and reptiliomorphs. The bridge spanned to the other end of the anchored bridge that produced the first true lissamphibians (modern day amphibians) and reptiles. Let’s journey over that bridge.
A side note before we begin is an oddity that I feel unique in the sequencing of evolution. It concerns tetrapodomorph fish (aquatic vertebrates possessing four lobed limbs but no feet). Osteolepis (Phonetics: Os-te-o-lep-is), a lobe-finned fish basal to all tetrapods (extinct and extant land vertebrates with four limbs ending in feet) are more closely related to four limbed land vertebrates than they are to present-day lobe-finned lungfish. Even though lungfish retain numerous ancestral characteristics of lobe-finned fish going all the way through the Devonian 425 mya, through verified DNA analysis, the ancient/present day lungfish is far more distantly related to osteolepids than say camels are. This is due to lungfish early side branching.
|Osteolepis ~ closer to a camel than to lungfish
Lepospondyls are characterized in having spool-shaped vertebra that did not ossify from cartilage as lungfish currently possess. The upper portion of the vertebra known as the neural arch was fused to the centrum which was the main body of the vertebrae. Most had smooth skin and were clawless. Lepospondyls ranged from the end of the Devonian 350 mya to Late Permian 255 mya.
|Amphibia Tree of Life
In the photo below is a rare fossil of the temnospondyl Scelerocephalus which choked to death over too big a meal.
|Scelerocephalus choked to death (Note the eaten animal's head within the body cavity)
Both groups had aquatic, semi-aquatic and terrestrial representatives where some temnospondyls were totally terrestrial and could run. Again, the fairly known lepospondyl was Diplocaulus while the genus Eryops (Phonetics: Eh-ry-ops) were temnospondyl specimens.
There are physiological and morphological lending evidence that supports this. Yes, all amphibians today derived from labyrinthodonts, but were the result of divergence within the labyrinthodont class. Frogs and toads came from temnospondyl labyrinthodonts that were losing their tails and replacing socketed or rooted teeth for pedicellate teeth or none at all. Salamanders and caecilians came from those lepospondyl labyrinthodonts that retained tails and teeth. Most caecilians have very few tail vertebrae which support very small tails or hardly any tail at all. The reason for this will be explained shortly, but first we’re going to discuss the order Anura (frogs and toads).
The origins of present day lissamphibians are a route of divergence. No lissamphibian group came solely from one branch or clade, but rather, while all shared an ancient labyrinthodont basal ancestor, there were further numerous branching that each modern day lissamphibian group evolved from as separate from one another.
In other words, where salamanders and caecilians are kissing cousins, frogs are their third cousin.
Leading to Anura:
With frogs and toads coming from the labyrinthodont temnospondyl line, they are only distantly related to the more closely related salamanders and caecilians that arose from two divergent lepospondyl groups. This has been validated from an intensive molecular phylogeny study on a 2005 rDNA analysis.
For lissamphibians, the more distant relationships of frogs to the more related salamanders and caecilians has a lot to do with geological events that took place during the very late Paleozoic and very early Mesozoic 250 mya. The event occurred just before the breakup of the continent Pangaea, but after temnospondyl and lepospondyl divergences from true lobe-finned species. So, although the groups originally shared a more basal ancestor like Gerobatrachus (Phonetics: Geh-row-bah-track-us) that appeared 290 mya in the Permian, the breakup of Pangaea isolated amphibian phylogeny where divergent evolution proceeded in coming up with the three modern day amphibians.
In the fossil record, frogs first appeared on Pangaea land in what is now India and Africa when western India was conjoined to central eastern Africa. Salamanders have eastern Asian origins when China and Mongolia were conjoined to eastern Russia and Kazakhstan, while the later origins of caecilians appeared in the tropical jungles of the Pangaea Triassic. All of this is corroborated with eight mitochondrial genomes of current lissamphibians to the phylogenies of amphibian sequencing.
Slightly more evolved away from its contemporary temnospondyl cousins, a creature known as Amphibamus (Phonetics: Am-phi-bay-mus) showed up in the Late Carboniferous during the Pennsylvanian 300 mya. This animal had begun the process of a frog’s anatomy with much larger hind limbs than forelimbs along with a larger pelvis, while the ribcage and tail became shortened. This animal no longer possessed scales.
Utegenia, (Phonetics: U-tuh-gin-e-a) a basal Seymouriamorpha of Late Carboniferous to Early Permian, is a probable sister taxon to Amphibamus, thus a predecessor to frogs and is the lineage point where lissamphibian frogs split from reptiles as Utegenia is also a basal reptiliomorph. Utegenia lived in the latest of the Carboniferous 300 mya down into the Permian 290 mya.
From the Early Permian during a time of very humid biomes, Utegenia along with Doleserpeton (Phonetics: Doe-le-sir-pe-tawn), although were aquatic dependent in larval stages, ambient humidty allowed the adult to roam land. Phylogenetically, these two preceded Gerobatrachus (Phonetics: Ger-o-bah-trak-us) and were transitional from the usual Seymouriamorpha morphologies.
Again, Utegenia is the species that gives the true split of lissamphibian frog lineage from living reptiles. Doleserpeton had four digit forelimbs and five digit hind limbs; the formula virtually all modern day frogs follow.
Utegenia and Doleserpeton had narrower snouts than modern day lissamphibian frogs, but were more broadened than other contemporary Seymouriamorpha. Today’s frog leap lengths are due to this evolutionary dropping in weight and broadening of the overall skull with no tail to lessen counterweight and drag.
Gerobatrachus, swimming the Early Permian swamps and frequenting the humid jungles 290 mya had a frog head and salamander tail. This creature was without doubt a temnospondyl and had only borrowed the salamander-like features from ancestral labyrinthodonts. With a shorter vertebral column than lepospondyls and even other temnospondyls existing at the time, along with a shortening vomer facial bone and as in most frog mouths exhibiting the palatine bone as a narrow strip along the side of the palate, Gerobatrachus known as the ‘frogamander’, was well on its way to being the closest basal ancestor to modern frogs and toads.
By 250 mya in the Triassic, Triadobatrachus (Phonetics: Tri-ad-o-bah-trak-us) had made its appearance in what is now known as Madagascar when the current island was sandwiched between India and Africa. It appears that this animal lived partly in water and on land as both aquatic and land plants have been found with the fossils.
Triadobatrachus, with fourteen vertebrae had six of them supporting a small retained tail. Of course, modern day frogs have only four to nine vertebrae with no tail. Triadobatrachus had large hind legs but without the ability to hop or jump, they were used for kicking in swimming.
Triadobatrachus is a basal ancestor to true frogs giving way to the first known frog, Prosalirus (Phonetics: Pro-say-le-rus) that lived 190 mya during the Jurassic. This primitive frog had long hip and hind limb bones that indeed were made for jumping and possessed a skeletal structure to absorb forces resulting from landings. Prosalirus is aptly named coming from the Latin word, ‘prosalire’ meaning ‘to leap forward’, for it truly was the leap into modern day frogs.
During the early Jurassic 190 mya, the tiny (1in/2.54cm) Vieraella (Phonetics: Vee-eh-rye-ell-a) had all the characteristics of an extant frog with a typical frog head possessing large eyes. Its hind limbs may have been tiny as well, but were well equipped to conduct long jumps.
Although it is now an extinct species, Callobatrachus (Phonetics: Call-lo-bah-trak-us) is one of the first known lissamphibian showing up 125 mya in the Early Cretaceous. In every way it is like all extant frogs with pedicellate teeth where the crown is separated from the root by fibrous tissue.
The term toad has no taxonomic value, as a toad is simply a special type frog that hops more than jumps and is usually encased in warty skin with a pair of parotoid glands located on the sides of the head that manufacture the steroid lactone toxin bufotoxin. Toads independently lost their teeth from extant frogs that also exhibit the absence of teeth. Toads are rather late arrivals first appearing in the Upper Paleocene 62 mya.
As easily witnessed today, frogs have enormous skulls and hind limbs as opposed to the rest of its skeletal structure. While the hind limbs are powerfully built, the skull had to be lightweight while still being relatively massive. In order to lose weight, frogs forego the fenestrae while reducing all the other skull bones to a bare minimum in broadening the overall skull. This skull trending, along with shortening of the tail can be traced back all the way through the aforementioned extinct species.
Leading to Caecilian:
Lepospondyls came in all physiological body forms that have been categorically put into five main branches. The five groups recognized are: 1) Adelospondyli ~ (aquatic elongate bodies with short but well developed limbs) arriving and dying out during the Early Carboniferous’ Mississippian period; 2) Aistopoda ~ (aquatic limbless snake-like bodies) first appearing in the Early Carboniferous’ Mississippian then ending in the early Permian; 3) Lysorophia ~ (elongate aquatic bodies with very small limbs) arising during the Late Carboniferous’ Pennsylvanian becoming extinct by the Early Permian; 4) Nectridea ~ (aquatic urodele-like in appearance) arising during the Late Carboniferous’ Pennsylvanian while becoming extinct by the mid Permian; 5) Microsauria ~ (aquatic and fully terrestrial forms possessing short tails, four small limbs and feet) first appearing in the Late Carboniferous’ Pennsylvanian while becoming extinct by the Early Permian.
As one can see, lepospondyl species spans are only from the Carboniferous to the Early Permian, but these amphibian-like animals are the ancestors that gave a direct rise to caecilians and urodeles (living salamanders/newts). Perhaps, even caecilians are a survivor in the direct line of lepospondyls that lost their legs due to a fossorial lifestyle.
A firm conclusion is still out on whether caecilians evolved from lepospondyls or temnospondyls with the debate scooting more towards temnospondyl lineage. However, here they will be treated as from the lepospondyl line and attempts will be added to reinforce this path. But, also included will be the possible temnospondyl line.
The reason for the continuing caecilian argument is that until caecilians had first made their appearance, there is no clear cut ancestry in the fossil records just yet leading to caecilians. Indeed, large morphological and topological gaps in the caecilian fossil record owe to the ongoing debate.
To live underground, adaptations had to be met such as a more cylindrical body plan, numerous vertebrae, a reinforced skull or forearms for efficient tunneling and hemoglobin uptake to extract more oxygen from poor oxygenated environments. In a dark and narrow environment, other physiological adaptations to conserve energy in dropping once important surface features would be the degenerative loss of eyes and the drastic shortening of long tails and limbs.
No tail or a highly shortened tail facilitates caecilian locomotion. In leading a strict fossorial lifestyle, caecilians have developed a body hydrostatic mechanism for burrowing. This kinematic mobile force is dependent on skin to vertebral independence, where a longer tail would interfere with this type of mobility, for performing this whole body and internal form of concertina locomotion, a long tail would be a hindrance and add drag.
Lepospondyl microsaurs and lysorophians had species that were already trending to a fossorial lifestyle in osteological (skeletal)/physiological caecilian traits. It is within one of these two orders that direct caecilian lineage is derived. This may become a mute issue as Microsauria is now considered paraphyletic which includes lysorophians. Since argument is convincing for either one, we will discuss both. So first is the microsaurs proper followed afterwards by lysorophians.
Microsauria is paraphyletic in being a crown base and along with all its suborders and their families, Aistopoda, Lysorophia and Nectridea are nestled within the order. These three lepospondyls in clade format appear to have arisen from the microsaur genus, Rhynchonkos (Phonetics: Rin-chon-kos) which was very salamander-like.
All microsaurs had short limbs and short tails and Rhynchonkos was no exception, but it also possessed an elongate body with degenerate limbs that was uncommon for other microsaurs.
In leaning as well toward reptile physical characteristics, Rhynchonkos, for about the only difference between the early true reptiles and numerous microsaurs skeletal structure is that microsaurs have two side-by-side condyles where reptiles have one. However Rhynchonkos is only a very distant relative to reptiles. It can be argued though as appearing to be the basal ancestor to the order, Gymnophiona (caecilians).
Rhynchonkos carried with it reptilian features found in other groups such as Eocaecilia (E-o-say-see-le-ha) that both groups had borrowed from an earlier linked lepospondyl ancestry. In other words, it was not convergent evolution where both carried the similar traits due to independently evolving them in adapting to analogous environments. We will get back to Eocaecilia shortly.
Whether in paraphyletic (consisting of an ancestor, but not all of its ancestors) groupings, or phylogenetic analyses, the microsaur clade, Recumbirostra (Phonetics: Re-cum-bir-os-tra) appears to be the ancestral base to all caecilians. Altenglanerpeton (Phonetics: Alt-en-glan-er-pe-tawn) belonged to this clade 299 mya with a temporal range at the very end of the Carboniferous and very beginning of the Permian.
Thus far, there is only one species to date of Alteglanerpeton which is A. schroederi. If for sure other fossil finds uncover another direct link to caecilians, it will be a very close relative to Altenglanerpeton.
Altenglanerpeton fossil remains come from the Carboniferous/Permian timeline 299 mya. It had all the ear markings of transitioning to a caecilian body structure. In life, this animal sported a very long slender body though with a shortened tail. The limbs were greatly reduced. The triangular skull was robust with an upper labial snout overbite. The skull also supported widely spaced eye sockets with the jugal bones extending well in front of the socket orbits, while the body support consisted of ~ 30 spool shaped vertebrae.
Altenglanerpeton possessed lungs, but was not totally terrestrial or fossorial. It used its elongated body to undulate while swimming and its triangular shaped skull to wedge into aquatic bed sediment and debris; a precursor to tunneling. The degenerative limbs aided in gaining access to tight squeezes. Its fossil in fact was found in lake sediment, but this does not mean the animal did not venture onto land.
Both Batropetes and Pelodosotis belonged to families as sister taxons within the Recumbirostra clade, so therefore were earlier relatives of Altenglanerpeton, Rynchonkos and thus eventually producing the family Eocaecilia originated in.
There is an approximate 100 million year gap between Altenglanerpeton and Eocaecilia, the first known caecilian that shows up in the Jurassic and perhaps future fossil finds will link the two due to their common morphologies.
Eocaecilia is essentially a caecilian with vestigial tiny limbs. Fossil finds dating in the Jurassic from 199.6-175.6 mya shows characteristics that it also shared a few similar traits with extinct microsaurs and extant salamanders. Although Eocaecilia was not totally fossorial it lived an insectivorous lifestyle foraging under forest floor litter and debris.
The Eocaecilia braincase analyzed from computed tomography gave a more reliable phylogenetic indicator than simply studying peripheral skull regions. In the tomography analysis, E. micropodia, the only species representative of Eocaecilia, showed that the animal possessed long anterolateral processes on the sphenethmoid (an unpaired skull bone on the neurocranium), paired olfactory nerve foramina and ossified nasal septum along with an ossified anterior wall of the sphenethmoid. All these traits are now known to only exist in extant caecilians.
Caecilians, unique among other animal groups have evolved a dual jaw closing system where the upper maxillary bone pulls up on the lower jaw mandible much like a third order lever system. This mouthing process is served by more developed posterior interhyoideus jaw muscles in closing the jaw by pulling back then down on a process located just behind the lower jaw hinge. These muscles are strong giving the animal a greatly strengthened biting force.
This is a novel function found only in caecilians, where before in caecilian ancestry jaw components served more as a ventral constrictor. Only in the most primitive of caecilians, the rhinatrematids is this lever system mouthing process poorly developed.
What led to this more evolved caecilian component is the fully solid roofed skull in modern caecilians to facilitate a fossorial life of burrowing and tunneling. The skulls of caecilian ancestry all had a temporal fossa which became fully closed in more recently derived caecilians.
Caecilian independent evolvement of stegokrotaphic skull features, according to the most revered and respected herpetologist specializing in caecilians, R.A. Nussbaum, directs evidence to open temporal region lysorophid microsaurs as the more likely direct ancestors to lissamphibian caecilians.
|Caecilian stegokrotaphic skull features
Lysorophians are closely related to microsaurs; in fact just might be a microsaurid and if not, most certainly an extension. However, they were highly specialized creatures for their time existing from the Middle Pennsylvanian to the very Early Permian. With very early specialization features such as reverse evolutionary engineering in reducing limbs rather than extending them, fenestrate skulls bearing short mandibles and sutured neural arch halves at the body’s midline, their early appearance and exit makes this problematic in comparison to the microsaur line in general.
|Lysorophians Left: Brachydectes Right: Lysorophus
Those who argue that caecilians arose from temnospondyls claim that the temnospondyl family Amphibamidae shared similar skull structure, dental arrangement and auditory structure. Phylogenetically, this can be pointed out, but the order Temnospondyli literally means cut vertebra because each vertebra is divided into several parts; there is no morphological evidence of caecilian backbones evolving from such a vertebral column.
Leading to Urodele:
Morphology would suggest that microsaurs could have been directly ancestral to urodeles, but with backbone structure exhibiting the transverse process as located on the anterior end of the dorsal vertebrae such as in the microsaur, Cardiocephalus (Phonetics: Car-di-o-ceph-ah-lus); they are not.
However, with both nectridians and aistopods arising from microsaur lineage, microsaurs share an indirect relationship to urodeles. Having the transverse process located near the middle of the vertebrae, nectridians and aistopods share a unique common feature with urodeles in which the bony projection is also located near the middle of the backbone. Phylogenetically, it appears that urodeles (all modern day salamanders) are the off-shoot branch of nectridians serving as the crown ancestor with aistopods serving as the stem branch.
As in caecilians, there is debate that salamanders derived directly from temnospondyls through the Batrachia group along with frogs. Recent discoveries of earliest Late Jurassic-Middle Cretaceous urodele well preserved fossils in China volcanic deposits provides evidence leaning towards the divergence of the lissamphibian Cryptobranchus from the lissamphibian Hynobiidae. This diverging line can be argued as being within the temnospondyl clade. Hynobiids, found primarily in Asia have a biphasic life cycle with aquatic gilled larvae and aquatic or terrestrial adult forms.
|Which lissamphibian route taken in the cladogram?
Also, hynobiids fertilize externally, have a greater degree of ossification, possess an angular bone in the jaw and carry a rather large number of micro-chromosomes. These traits give rise to consideration that hynobiids are the basal common ancestor to all other urodeles. This arrangement would make the order Nectridea a polyphyletic taxon rendering the order as not a true clade in evolutionary grouping. Most researchers agree with this route.
But not bending under that pressure, I’m presenting my thoughts on urodeles as evolving through lepospondyl nectrideans. Amphibian evolution is very complex; it is not cut and dry and it may be later proven that modern day salamanders had many basal evolutionary marks coming from numerous ordered species lines. This is what makes science shine as it is flexible enough to correct a wrong hypothesis no matter how long it was supported.
In going the nectridean path, it is with full intention in my following paragraphs, an attempt in shedding light on this debate.
|Nectridean trunk and tail vertebrae
Nectridea was a diverse small in size group of animals very similar to today’s terrestrial salamanders and aquatic newt forms. Most were 2cm/2.36in-15cm/5.91in in length, although Diplocaulidae (Keraterpetontidae) family members did reach 100cm/39.37in in total length.
|Pennsylavanian scene Cacops
There was an abundance of nectrideans during the swampy forests of the Middle Carboniferous to Middle Permian periods 318-270 mya. It was a diverse group coming up with the bizarre boomerang shaped head in the genus Diplocaulus, but all shared long compressed tails (that is flattened from side-side), well developed interdigitate spinal hind limbs with a set of five hind limb toes and four toes on each forelimb. Nectridean fossils also exhibited symmetrical neural and hemal spines, complex trunk vertebrae articulations and arches constructing the vertebra.
The lepospondyl clade leading to microsaur nectridians had lost their characteristic labyrinthodont teeth infolding patterns of dentin and enamel early opting for paired palatal small fangs. These small fangs could have been precursors in easily evolving into present day urodele vomerine teeth.
|The Middle Pennsylvanian
During the Middle Pennsylvanian, around 308 mya in the Carboniferous Period lived Utaherpeton, (Phonetics: U-tah-erp-uh-tawn) who showed characteristic microsaur features such as small cervical ribs and hind limbs larger than forelimbs but with the hind feet being unusually larger than the rest of the hind limbs. It was also salamander-like in appearance. Utaherpeton’s body length was no more than 4cm/1.6in.
As far as lepospondyls go, completions of phylogenetic and morphological analyses show that Utaherpeton is the most basal member of a separate clade including all lepospondyl members. In addition, with the prefrontal extending to the premaxilla more at the front of the skull than found in other microsaurs, this skull bone configuration leads to hints of indirect nectridean evolvement.
A bit later after Utaherpeton’s appearance, during the Late Carboniferous around 304 mya, Hyloplesion (Phonetics: Hy-lo-pleas-e-un) first made its presence. This microsaurid nectridean was also salamander-like in body form with a total length of 7.7cm/3.03in.
The scapulocoracoid is the pectoral girdle where the scapula links the humerus to the body and the clavicle to the sternum with the beak-shaped coracoid as a paired bone sharing in the overall assemblage. Results from geometric morphometrics show that overall cranial and postcranial growth was isometric primarily and in comparing allometric data to all other Paleozoic tetrapodal taxa where isometric growth instead of metamorphical is an ancestral feature. This shows that Hyloplesion, in morphological change did not go through larval to adult metamorphosis as most modern day salamanders do which includes skeletal reorganization. But as metamorphosis is not evident in any early day amphibian group, it only bears out that metamorphosis is a derived mode of development found only in extant salamanders and their closest caudate relatives.
However, where all temnospondyls never possessed this type pectoral girdle arrangement and while other lepospondyl groups were losing their scapulocoracoid, Hyloplesion was enlarging it, which is present in most all modern day vertebrates except for therians; [marsupial (metatheria) and placental (eutheria) mammals.] Certainly the scapulocoracoid is found in all extant urodeles.
Crossotelos (Phonetics: Cros-so-tel-os) is a true nectridean and most likely derived in the early Permian around 295-292 mya. This animal, along with fine abdominal ribs and laterally compressed body, also possessed intervertebral nerve openings that are the foundational layout of current urodele spinal nerve systems throughout the vertebral columns. Various intervertebral nerves of extant salamanders have evolved from within the urodele group from the fossil associated primitive intervertebral nerve passageways of cryptobranchs and hynobiids to the more defined three spinal nerve exits through the posterior foramina in more modern salamanders such as Ambystomatoidea (Phonetics: Am-be-sto-ma-toi-de-ah).
Skull fossils of Cricotillus brachydens and Trimerorhachis leptorhynchus were first incorrectly labelled as temnospondyls which had urodele characteristics. Now though, these two species have been confirmed and properly assigned to the nectridean group as Crossotelos juveniles.
Crossotelos, a urocordylid species were pelagic piscivores being almost wholly aquatic. The creature still retained an abundance of abdomen scales covering its belly. Whether directly or indirectly through close relatives, Crossotelos most likely gave rise to Chunerpeton (Phonetics: Chu-ner-pe-tawn) 172 mya in the Middle Jurassic. It is the earliest known crown caudate and would be listed as a urodele if it had not gone extinct. Chunerpeton in all aspects considered was a member of the suborder, Cryptobranchoidea (Phonetics: Cryp-toe-brank-coid-dee-ah) and being neotenic retained several juvenile features into adulthood such as external gills.
In the photo below, an unidentified larval cryptobranchoid fossil defines soft tissue and a belly full of ingested conchostracans, an extant crustracean first appearing during the Devonian.
|Fine detail of a cryptobranchoid larva's soft tissue
During the Late Jurassic the salamander family, Karauridae appeared 150-147 mya with two genera, Karaurus (Phonetics: Kah-row-rus) and Kokartus (Phonetics: Ko-kar-tus). Karaurus retained a lacrimal bone found in most extant salamandrids and an angular bone in the mandible which all hynobiids and cryptobranchiods still possess. Although an aquatic neotenic salamander with retained external gills, anatomically Karaurus fossil remains resemble urodele terrestrial salamanders.
Please recall that caudates are basically salamanders that have become extinct while urodeles are salamanders that are extant. The first caudate that branched from crypotbranchs arrived during the Mesozoic Era in the Middle-Late Jurassic 164-146 mya. This species called, L. daohugouensis (Phonetics: Dowel-hu-gal-en-sis) is from the newly formed Liaoxitriton (Phonetics: Le-ow-ip-te-rus) genus. Although not quite a member of the Hynobiidae (Phonetics: Hy-no-be-ah-day) family, in full anatomy it was trending that way and most likely is the hynobiid crown group from which most all other modern day salamanders are tied to.
Daohugouensis still retained some of the cryptobranch vestiges such as anterolaterally extended VTR’s (vomerine tooth rows), rostral morphology and a wide/round rostrum. But it also possessed a few trending hynobiid traits.
Although orientation of VTR’s differs between daohugouensis and hynobiids, teeth are of the same arrangement, while the unicapitate ribs expanded proximally in daohugoensis and early hynobiids. Also, vertebrae transverse processes are around half the length of the centra (the vertebra component that supports the arches ~ centrum as singular).
Daohugouensis may not be considered a hynobiid familial base unit, but its sister taxon, L. zhongjiani certainly is one of the first true hynobiids. Arising in the Early Cretaceous just after Daohugoensis 145-124 mya, Zhongjiani (Phonetics: Zun-gee-un-ee), aside from extant cryptobranchs is the crown base for all modern day urodele families.
Through hynobiid speciation, other hynobiid groups evolved that also speciated into the current family groups of salamanders. Zhongjiani possessed gill rakers into adulthood. Neoteny does not show up in all modern day salamanders, but for the ones that independently developed it like the axolotl, are actually reverting back to expressing the Zhongjiani gene they’ve inherited, as markers for gene expression had become passive or inactivated in air breathing salamanders.
Views expressed here under amphibian evolution is mostly my own and certainly should not be followed as gospel. When dealing with 300 million year old animals that could be pedomorphic (retention of aquatic larval form into adult life) or peramorphic (juvenile condition modified from ancestral traits where further adult form substantially modifies juvenile condition), cladistics may virtually become too impossible a task. So there you are, working with a pedomorphic lepospondyl that in all fossilized appearances looks much the same as a peramorphic temnospondyl.
The next excerpt will be diapsids with the path leading to crocodilians and turtles.
In Evolving Form,