tetrapod – Nix Illustration

Anthracodromeus

Anthracodromeus longipes was an early reptile* that lived in what is now Ohio, USA, during the late Carbonifeorus about 307-305 million years ago.

(*or possibly a very reptile-like stem–amniote)

Around 20cm in total length (~8″), it had a rather lizard-like shape with a long body and a short tail. The digits on all four of its limbs were highly elongated with hooked claws, which appears to have been an adaptation for climbing.

It inhabited a coal forest dominated by lycopsid trees and seed ferns, and as one of the earliest known tetrapods to develop climbing behavior it would have had some ecological advantages over its relatives, being better able to escape from predators and access new food sources.

Gaiasia

Gaiasia jennyae was a tetrapodomorph – an amphibian-like relative of early tetrapods – that lived about 280 million years ago during the early Permian in what is now Namibia.

Although it’s only known from incomplete skull and vertebral column material it probably looked quite similar to the colosteids, a closely-related group of tetrapodomorphs with elongated bodies and small limbs. If it had the same sort of body proportions as these relatives it would have been huge, the largest known stem-tetrapod at potentially around 4m long (~13′).

It had a wide flat head with a short boxy snout, and large interlocking fangs on the roof of its mouth and at the front of its lower jaw. It would have been fully aquatic and probably not a particularly fast swimmer, instead likely being an ambush predator using suction from rapidly opening its jaws to pull prey into its mouth before clamping down with its fangs.

It’s also notable for living considerably later than most other stem-tetrapods, and in an unexpected part of the world. While its close relatives are all known from the tropics of the Carboniferous, Gaiasia was in a location that was much closer to the South Pole during the early Permian (~55° S), inhabiting an immense freshwater lake in a rift valley with a cold-temperate climate.

Its presence in this habitat may suggest that other stem-tetrapod lineages survived and thrived in high latitudes for much longer than previously thought, while the true tetrapods were all diversifying nearer the equator – or it might represent a Paleozoic equivalent of Koolasuchus, an isolated straggler lurking in a cold refugium.

Kermitops

Despite being named after the famous Muppet frog, Kermitops gratus here wasn’t actually a frog itself. Instead it was a close relative of the common ancestor of all modern amphibians, part of a grouping of amphibian-like animals known as amphibamiform temnospondyls.

Living in what is now Texas, USA, during the mid-Permian, about 275 million years ago, Kermitops would have resembled a chunky salamander. Only its fossil skull is known, so its full body size is uncertain, but based on the proportions of related amphibamiformes it was probably around 15-20cm long (~6-8″).

Although its discovery helps to fill in the very sparse fossil record of the early evolution of modern amphibians, it’s also complicated matters more than expected. Previously it was thought that the characteristic skull anatomy of modern amphibians evolved in a clear sequence, but Kermitops has a unique mix of features that doesn’t fit this idea – suggesting that there was a lot of convergent evolution going on in amphibamiformes at the time.

Keraterpeton

Keraterpeton galvani here was part of a group of amphibian-like early tetrapods called lepospondyls.

Living in what is now southern Ireland during the Late Carboniferous, about 318-314 million years ago, this 30-40cm long (~1′-1’4″) fully aquatic animal was the earliest known member of the diplocaulid lineage (although its skull was much less elaborately modified than its famous boomerang-headed relative Diplocaulus).

It had a broad short-snouted head with eyes set far forward, and a pair of backwards-pointing bony “horns” at the back of its skull. Its forelimbs were smaller than its hindlimbs, and unlike most other diplocaulids it had five fingers on its hands instead of four.

Its vertically flattened paddle-like tail was also around twice as long as the rest of its body, and was probably its main source of propulsion in the water.

Keraterpeton seems to have been quite numerous in the coal swamps it inhabited, representing the most common species preserved in the Irish Jarrow Assemblage site – a location where fossil specimens were uniquely “cooked” and partially replaced with coal during the fossilization process.

Crassigyrinus

Crassigyrinus scoticus was an early tetrapod from the early Carboniferous Period, known from ancient coal swamps of Scotland, Nova Scotia, and West Virginia between about 350 and 330 million years ago.

Around 2m long (6’6″), it had an elongated streamlined body with tiny vestigial-looking forelimbs, and a pelvis that wasn’t well-connected to its spine – features that suggest it had re-evolved a fully aquatic lifestyle at a time when its other early tetrapod relatives were specializing more and more for life on land.

Fossils of its skull are all rather crushed, and traditionally its head shape has been reconstructed as unusually tall and narrow. But a more recent study using CT scanning has instead come up with a wider flatter shape more in line with other early tetrapods.

Its mouth also had a very wide gape and a strong bite, and it may have occupied an ecological role similar to that of modern crocodilians, lurking in wait to ambush passing prey.

Strange Symmetries #08: A Different Diplocaulus

With its bizarre boomerang-shaped skull, Diplocaulus is probably the most iconic ancient amphibian. (It even inspired the design of the pokémon Dragapult!) It was part of the lepospondyls, a diverse collection of early tetrapods mainly found in Europe and North America between the Early Carboniferous and the mid-Permian, about 350-270 million years ago.

But one species hung on a bit longer into the late Permian, about 259-254 million years ago, and this late-surviving lepospondyl was perhaps the oddest of them all.

Diplocaulus minimus was the only lepospondyl known from the supercontinent of Gondwana, found in what is now Morocco in northern Africa. About 70cm long, around half of which was its long tail, it had the distinctive elongated skull of a diplocaulid – but in a bizarrely asymmetrical shape.

The left prong of its skull was long and tapering, but the right was shorter and more rounded. This doesn’t seem to have been due to individual deformity or distortion of the fossil material, since more than one skull has been found with the same features, but the reason for such a striking amount of asymmetry in this species is unknown.

Diplocaulids’ head shapes are thought to have acted as hydrofoils, providing lift while they were swimming. Perhaps Diplocaulus minimus‘ much more wonky skull means this species wasn’t relying on that hydrodynamic function as much as its relatives, and something else was going on with its ecology.

…Although, that weird head does bear a surprising resemblance to a proposed asymmetric “flying wing” aircraft design from the 1950s, so it might have worked better for underwater flight than it seems at first glance.

Cabarzia

Cabarzia trostheidei here lived during the early Permian in what is now Germany, about 295 million years ago.

Despite its very lizard-like appearance it was actually part of the varanopid lineage, a group of scaly amniotes traditionally classified as early synapsids (distant relatives of modern mammals), but which more recently have been proposed to instead be sauropsid reptiles closer related to early diapsids.

It was around 50cm long (1’8″), and its short arms, long legs, slender body, and long tail suggest it was capable of shifting into a bipedal posture when running at high speeds, similarly to some modern lizards – probably mainly to escape from larger predators, but possibly also used to pursue fast-moving prey like flying insects.

And whether varanopids were actually synapsids or sauropsids, this makes Cabarzia the earliest known example of an animal running on two legs.

Platycepsion

Platycepsion wilksoni was a temnospondyl amphibian that lived during the early-to-mid Triassic (~251-242 million years ago) in what is now New South Wales, Australia.

A single partial skeleton discovered in the 1880s is the only known record of this species, and represents a juvenile that would have been around 15-20cm long (6-8″). We don’t know exactly what it would have looked like as an adult, but it was probably quite similar to other closely-related members of the brachyopid family – mostly-aquatic salamander-like animals with short but wide toothy jaws, eyes set towards the front of the head, small limbs, and paddle-like tails.

A recent re-analysis of the Platycepsion specimen found evidence of soft-tissue preservation of external gills, showing that it wasn’t just a juvenile but a true larva, a sort of temnospondyl “tadpole”.

Distinct larval stages have been found in a few other types of temnospondyls, but this is the first definite example from the stereospondyls, a major Mesozoic lineage that survived all the way into the Early Cretaceous.

Termonerpeton

During the Early Carboniferous, around 330 million years ago, the region that is now the East Kirkton Quarry in Scotland was located close to the equator, with a lush tropical climate and volcanic hot springs dotting the landscape. It preserves fossils of some of the earliest known fully terrestrial tetrapods, and a recent discovery shows how some of these animals were already experimenting with the shapes of their feet to better get around on land.

Termonerpeton makrydactylus is only known from a partial skeleton, and shows a mix of anatomical features that make identifying its exact evolutionary relationships rather difficult – but it was probably a very early reptilomorph, closer related to amniotes than to lissamphibians. It may also have been very closely related to the equally enigmatic Eldeceeon and Silvanerpeton from the same region, but was almsot twice their size with a estimated total length of around 70cm (2’4″).

It would have resembled a rather heavily-built lizard-like or salamander-like animal, with fairly stumpy legs and probably lacking claws on its digits. While it would have had spindle-shaped scales on its underside, and possibly small rounded scales along its sides and back, these were bony structures embedded in its skin and probably weren’t very visible externally in life.

But Termonerpeton‘s most surprising feature was its proportionally large feet with especially elongated fourth toes, which would have helped to extend its stride length for energy-efficient terrestrial locomotion and to stabilize its movement on unstable surfaces – a much more “advanced” amniote-like arrangement than expected in such an early reptilomorph, and convergently similar to to the foot shapes of some modern lizards. Its fourth toe was also unusually chunky, suggesting it may even have been bearing most of its weight on just that one digit when walking.

Whatcheeria

Whatcheeria deltae here was an early tetrapod from the Early Carboniferous, about 340 million years ago, descended from the earlier fish-like forms and closely related to the ancestors of modern amphibians and amniotes.

Hundreds of fossils of this species have been found in Iowa, USA. Most represent juveniles, but rare larger specimens suggest fully-grown adults reached at least 2m long (6’6″).

Its large chunky limbs and flat feet seem to have been well-adapted for walking, with body proportions similar to later temnospondyl amphibians. But its cartilaginous ankles and the presence of lateral lines on its skull suggest it was still primarily aquatic, possibly walking along on the bottom of the ancient lakes, rivers, and swamps it inhabited.

It also had an unusually long neck and oddly-shaped skull for such an early tetrapod – most other known species had rather wide and flat skulls, but Whatcheeria‘s head was instead proportionally taller and narrower. Along with heavily reinforced sutures between the bones of its skull, it would have had a very powerful bite and been able to resist the twisting forces of large struggling prey in its jaws, suggesting it was a specialized crocodile-like predator.