stem-tetrapod – Nix Illustration

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.

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.

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.

Spathicephalus

Spathicephalus mirus here was part of a group of amphibian-like animals called the baphetoids, a lineage that weren’t quite true tetrapods themselves but were still very closely related to them.

Living in Scotland during the mid-Carboniferous period, about 326 million years ago, this 1.5m long (~5′) stem-tetrapod had an incredibly unusual head compared to its relatives – wide and flat, almost square in shape, with its jaws lined with hundreds of tiny chisel-like teeth.

Most other stem-tetrapods had deep skulls with large teeth, adapted for fish-eating, so clearly Spathicephalus was specialized for a very different diet. Some comparisons have been made to flat-headed ambush predator plagiosaurid temnospondyls like Gerrothorax, but a better ecological comparison might actually be filter-feeders like “pancake crocs“.

Lethiscus

Ever since the earliest tetrapods crawled onto land and developed limbs and digits, some lineages have just… decided the whole “legs” thing was overrated and lost them entirely.

And the earliest known group to do this were the aïstopods. These highly elongated amphibian-like animals had specialized lightly-built skulls with large jaw muscles, and they may have filled a similar ecological niche to modern snakes, hunting small terrestrial invertebrates.

Lethiscus stocki was one of the first members of this snake-like group, living in Scotland during the Early Carboniferous about 340 million years ago. Growing to at least 50cm long (~20″), it was already a very specialized animal despite its basal position among the aïstopods, with eyes set far forward on its face and no trace of vestigial limbs.

CT scans of its skull have shown some surprisingly fish-like anatomy, especially in its braincase, features that were lost very early in tetrapod evolution. This suggests that aïstopods weren’t part of the lepospondyl amphibians like previously thought, but actually originated much much earlier in the tetrapod evolutionary tree – potentially placing them somewhere among the “fishapods” between Ichthyostega and Crassigyrinus.

Eucritta

Eucritta melanolimnetes, an amphibian-like creature from the Early Carboniferous of Scotland (~335 mya). About 25cm long (10″), it had a mixture of anatomical characteristics similar to baphetid stem-tetrapods, temnospondyls, and reptile-like amphibians, making its exact classification difficult. It’s currently considered to be a close relative of both the baphetids and Crassigyrinus, and it was probably close in appearance to what the common ancestor of all later tetrapods would have looked like.

Its name means “true creature from the black lagoon”, in homage to the 1954 monster movie.