Month: December 2021

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.

Echinochimaera

Echinochimaera meltoni here was a cartilaginous fish found in the Bear Gulch Limestone deposits in Montana, USA, dating to the Early Carboniferous about 326-318 million years ago.

It was an early member of the chimaera lineage, but unlike its mostly-scaleless modern relatives its body was covered in small shark-like placoid scales.

It also showed a large degree of sexual dimorphism, with males and females almost looking like different species entirely. Males are identified by the presence of claspers and were up to 15cm long (6″), with four pairs of spiny “horns” on their heads, larger more pointed dorsal fins, and rows of spines along their tails. Females were less than half the size of males at just 7cm long (2.75″), with only one pair of smaller “horns” and none of the additional spines.

The rounded bodies and relatively small paddle-like tail fins of both sexes suggest they weren’t very strong swimmers, probably relying on their large dorsal fin spines to defend themselves – which may have been venomous much like those of modern chimaeras.

Stegouros

While some ankylosaurs are famous for their specialized tail clubs, Stegouros elengassen here had something else entirely going on with its rear end.

Known from the late Cretaceous of southern Chile, about 75-72 million years ago, this small ankylosaur was around 1.5m long (~5′), roughly the size of a large dog. It had a proportionally larger head and more slender limbs than most other ankylosaurs, and a pelvis more resembling a stegosaur, but its most distinctive feature was its tail – it had a completely unique never-before-seen type of tail weapon, with a flat “frond-like” structure formed from several pairs of large fused osteoderms making a shape resembling a macuahuitl.

It seems to have been part of a previously unrecognized very early-branching lineage of Gondwanan ankylosaurs – the parankylosaurians – with its closest relatives Antarctopelta and Kunbarrasaurus also included in this new group. And since the tail regions of both of those other species are poorly known, this means they may also have possessed macuahuitls.

Gordodon

Some of the earliest large terrestrial herbivores on Earth were the edaphosaurids – a very early-branching group of synapsids, the evolutionary lineage whose only modern surviving members are mammals. Like their more famous cousin Dimetrodon these animals sported huge elaborate sails on their backs formed from highly elongated vertebral spines, but despite the similarity in appearance they actually seem to have evolved these structures completely independently.

Known from a single partial skeleton discovered in southern New Mexico, USA, the edaphosaurid Gordodon kraineri dates to around the very end of the Carboniferous or the very earliest Permian, about 299 million years ago.

It was fairly small for an edaphosaurid at about 1.5m long (~5′), and seems to have had transitional anatomy between earlier and later members of the group. Its sail spines were thicker than those of earlier species but still less heavyset than those of later forms, and while each spine had numerous side projections these structures were small, thorn-like, and randomly distributed, unlike the more organized thick crossbars seen in Edaphosaurus.

Its head was proportionally small compared to its body, but still relatively large for an edaphosaurid, and it had an unusually long neck for an early synapsid. But its most distinctive features were its jaws and teeth – it had a narrow snout with a pair of large incisor-like teeth at the front of both its upper and lower jaws, followed by a large toothless gap (a diastema) and then a short row of small peg-like teeth. Like Edaphosaurus it also would have had batteries of grinding tooth plates inside its upper and lower jaws, but probably not as extensively.

Overall its tooth arrangment looked more like a modern herbivorous mammal than an early synapsid, much more highly specialized than anything else known to be alive at the time – the next synapsid known to convergently evolve similar teeth lived around 90 million years later!

It probably had a very different diet to its relatives, with its specialized teeth and fairly slender body suggesting it may have been a selective feeder, cropping the softer more nutritious parts of plants like the fleshy seeds and cones of gymnosperm plants.

Its discovery also hints that herbivorous edaphosaurids in general were much more diverse than we previously thought, and there may be even more surprising forms out there still to be discovered.