therocephalia – Nix Illustration

Tetracynodon

Therocephalians were a group of synapsids very closely related to – or possibly even ancestral to – the lineage leading to modern mammals. They were a diverse and successful group of carnivores during the latter half of the Permian, but suffered massively during the “Great Dying” mass extinction, with only a handful of representatives making it a few million years into the Triassic.

Tetracynodon darti was one of these rare Triassic therocephalian survivors, living in what is now South Africa around 251 million years ago. Only about 25cm long (~10″), it had slender limbs and strong claws that suggest it was a scratch-digger. Its long snout was lined with pointed teeth, and it was probably an active predator hunting by snapping its jaws at fast-moving prey like insects and smaller vertebrates.

Its combination of small size, burrow-digging habits, and unspecialized diet may be the reason it scraped through the Great Dying when most of its relatives didn’t – but unfortunately it seems to have been a “dead clade walking”, disappearing only a short way into early Triassic deposits.

Ericiolacerta

The synapsids were an incredibly successful and diverse group during the Permian period, but after the devastating “Great Dying” mass extinction event 252 million years ago only three lineages survived into the Triassic – the cynodonts (close relatives and ancestors of modern mammals), the dicynodonts (beaked tusked weirdos who briefly took over the world), and the therocephalians.

Therocephalians were close relatives of cynodonts, and convergently evolved several very mammal-like anatomical features in their skulls, teeth, and limbs. But unlike their cousins this lineage never fully recovered in the Triassic, and they ultimately disappeared completely around 242 million years ago.

Ericiolacerta parva was one of these short-lived Mesozoic therocephalians, known from the early Triassic (~252-247 million years ago) of South Africa and Antarctica, in regions that were connected at the time as part of the supercontinent of Pangaea. It was a fairly small animal, about 20cm long (~8″), with small sharp teeth that indicate it mainly fed on insects, and semi-opposable thumbs and inner toes that suggest it was also a capable climber.

Holes in the bones of its snout would have carried numerous nerves and blood vessels, which may be evidence of sensitive fleshy lips and possibly whiskers. And while there’s no direct evidence of fur in therocephalians, they do appear to have been active warm-blooded animals – and possible fossilized synapsid hair from the Permian period suggests fuzziness might have been ancestral to all of the “protomammal” lineages that survived into the Triassic.

Perplexisaurus

If there’s any equivalent to carcinization in mammals, it’s turning into an otter-beaver-like semi-aquatic form.

Because it just keeps happening.

Modern examples alone include otters, beavers, muskrats, giant otter shrews, desmans, aquatic genets, yapoks, lutrine opossums, and platypuses – and in the fossil record there were early pinnipeds, remingtonocetids, pantolestids, stagodontids, and Liaoconodon going as far back as the early Cretaceous. Even outside of the true mammals there were also Castorocauda, Haldanodon, and Kayentatherium during the Jurassic, and much further back in the late Permian there was the early cynodont Procynosuchus.

So a non-cynodont synapsid doing the exact same thing really isn’t all that surprising.

Perplexisaurus foveatus was a member of the therocephalians, a group of synapsids that were close evolutionary “cousins” of the cynodonts-and-true-mammals lineage. Similar in size to a modern rat, about 20cm long (8″), it lived in Western Russia during the Late Permian about 268-265 million years ago.

At the time this region was a river plain with a tropical climate, experiencing seasonal floods that turned the whole area into what’s known as “viesses” (a name based on the abbreviation “V.S.S.” standing for “very shallow sea”), vast shallow lake-seas that persisted for weeks or months at a time.

So this little animal has been interpreted as being semi-aquatic, swimming around and feeding on aquatic invertebrates and tiny fish and amphibians. Its skull had numerous pits around the front of its face, suggesting that it had a highly sensitive snout – probably whiskery, allowing it to hunt entirely by touch in dark murky water, but it’s also been proposed to have possibly had an electroreceptive sense similar to modern platypuses.

Weird Heads Month #30: Lumpy-Faced Synapsids

Among the synapsids (“proto-mammals”), head ornamentation evolved multiple times in the therapsids, from basal members of the group like Tetraceratops, burnetiamorphs, and dinocephalians to later lineages like dicynodonts and gorgonopsids.

But these sorts of structures don’t seem to have really ever developed in one of the lineages most closely related to the ancestors of modern mammals, a group known as therocephalians.

…With the exception of Choerosaurus dejageri.

Living in South Africa during the late Permian, around 259-254 million years ago, this small synapsid was only about 35cm long (1’2″) but sported some large bulging bony bosses on the sides of both its snout and lower jaw.

The bosses would have been covered by tough skin in life, similar to modern giraffid ossicones.

A study of Choerosaurus‘ skull found that its head was rather delicately built, and the bosses were relatively fragile and lacked the sort of reinforcement needed to resist impacts, suggesting that these structures weren’t used as weapons for fighting each other but were probably more for display – so they may even have been brightly colored.

The upper jaw bosses were also well-supplied with nerves and blood vessels, and would have been quite sensitive to touch.