fuzz up all the synapsids – Nix Illustration

Exaeretodon

Cynodonts were one of the few lineages of synapsids (“protomammals”) to survive through the Great Dying mass extinction into the Triassic. And while a major branch of cynodonts known as probainognathians would eventually go on to produce the ancestors of modern mammals, for much of the Triassic a separate branch called cynognathians were initially much more diverse and numerous.

Exaeretodon argentinus was a large traversodontid cynognathian, growing up to about 1.8m long (~6′), known from the Late Triassic (~234-227 million years ago) of what is now northwestern Argentina. It was a low-slung animal with short stocky limbs, sprawling at the front and semi-upright at the back, and had a large head with a fairly short narrow snout and wide flaring cheekbones accommodating massive jaw muscles.

Although it it had large fang-like canine teeth, further back in its jaws wide molar-like grinding teeth show it was a specialized herbivore – at least as an adult. Different skull proportions in juveniles suggest that young Exaeretodon may have actually started out life as omnivorous or carnivorous, with jaws better suited for crushing hard-shelled invertebrate prey.

One Exaeretodon specimen shows evidence of severe rib injuries that would have hindered its mobility and made it very difficult to forage for food or avoid predators. But in this case those injuries were healed, suggesting this species may have lived in social groups that helped to protect each other.

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.

Pachydectes

Modern mammals are the only living representatives of the synapsids, but back during the Permian there were numerous other evolutionary branches – first the pelycosaurs, and later their descendant the therapsids.

Some of the first non-mammalian therapsids were the biarmosuchians, mid-sized carnivores with a more upright posture than their pelycosaur ancestors. They had large canine teeth in their jaws and powerful bites, and some of them also developed elaborate ornamentation on their skulls, with various bony bumps and crests adorning their faces.

Pachydectes elsi was a 1.5m long (~5′) biarmosuchian living in what is now South Africa during the late Permian, about 265 million years ago. Bone texture indicates its head ornamentation was covered by either tough thickened skin or a keratinous sheath, and the large bulbous bosses on the sides of its snout had a particularly rich blood supply, suggesting these structures could have been continuously growing throughout its entire life.

But despite how well-protected it looked, Pachydectes’ skull was actually relatively fragile and wouldn’t have been able to withstand the impact forces of using its headgear for fighting or defense. Instead it may have been mostly used for visual display – and the blood supply to the snout bosses might even have given it the ability to “blush” them if they had a soft-tissue covering.

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.

Smilesaurus

Despite having a genus name that sounds more like it should belong to a cartoon dinosaur mascot for dental hygiene, Smilesaurus ferox was actually a real gorgonopsian, a predatory synapsid distantly related to modern mammals.

Living in South Africa during the Late Permian, around 259-254 million years ago, Smilesaurus was comparable to a medium-sized dog at around 1m long (3’3″). It had some of the longest sabre-like canine teeth of any known gorgonopsian, proportionally comparable to those of sabertoothed cats – and it may have hunted in a similar manner, using powerful grasping limbs to pin down struggling prey and then dispatching it with slashing bites.

…And it also turns out that when you don’t horribly shrink-wrap a gorgonopsian, you end up with something that looks rather like a bear-hippo.

(For some similarly chonky gorgonopsians, check out Tas’ @i-draws-dinosaurs reconstructions here. Bullet Man was definitely a bit of an inspiration in this.)

Diictodon

Even before Lystrosaurus briefly took over the world, the dicynodont synapsids were a highly successful group. These herbivorous beaked-and-tusked mammal-cousins were abundant all over the supercontinent of Pangaea during the Late Permian and occupied a wide variety of ecological niches, ranging from rat-sized to almost bear-sized. (And later during the Triassic some got even bigger.)

Towards the smaller end of that size range were species like Diictodon. Living around 259-254 million years ago in Southern Africa (but with fossils also found in northern China, suggesting a much larger geographic range) this dicynodont grew up to about 45cm long (1’6″) and was a gopher-like creature adapted for digging, with a tubular body and short muscular limbs.

It was a very common animal, making up around half of all vertebrate fossils in some locations. Numerous preserved spiral-shaped burrows have been found concentrated in small areas, going down as much as 1.5m (5′) into the ground.

Several different species have been named within the Diictodon genus, but currently they’ve all been lumped together under the single name of Diictodon feliceps. There’s a lot of anatomical variation between specimens, though, with some notably being smaller and lacking the distinctive tusks seen in others – which may be evidence of sexual dimorphism, with the tuskless individuals possibly being females. (Although differences in inner ear anatomy may also indicate they were a separate species entirely, in which case female D. feliceps might instead be represented by fossils showing smaller tusks.)

I’ve illustrated one of the tuskless forms here, since they don’t generally get as much attention as the tusked ones. It’s also speculatively fluffy and iridescent similar to modern golden moles.

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.

Tiarajudens

Synapsids just keep evolving saber teeth.

Both proto-mammals and true mammals have independently evolved oversized fangs quite a few different times in a lot of different lineages over the last few hundred million years (even in some modern ones), and one of the first to experiment with such a feature was Tiarajudens eccentricus.

Living in southern Brazil towards the end of the Permian period, about 265-260 million years ago, Tiarujudens was an early member of a group of known as anomodonts. These chunky herbivorous synapsids weren’t directly ancestral to modern mammals, but were instead evolutionary cousins, and their lineage eventually included tusked dicynodonts like the world-conquering Lystrosaurus.

Tiarajudens was around 1-1.2m long (3’3″-3’11”) and sported a pair of very long blade-like canine teeth in its upper jaw. Since the rest of its teeth were clearly adapted for eating plants – with one of the the earliest known examples of flat grinding molars that would have allowed it to chew up tough vegetation – these fangs probably served more of a display or defensive function.

The saber teeth may even have been a sexually dimorphic feature like in modern musk deer. Another anomodont from South Africa, Anomocephalus africanus, is incredibly similar to Tiarajudens except for a lack of fangs – and since South America and Africa were connected as part of Pangaea at the time, it’s possible that these two actually represent males and females of the same species.

Without finding a larger number of fossils we can’t know for certain, but it’s an interesting possibility at least.

Diademodon

Modern mammals are the only surviving members of a much larger evolutionary group known as the synapsids – which back in the Permian period were the dominant land vertebrates.

But much like all other life on Earth at the time, the synapsids were absolutely devastated in the “Great Dying” mass extinction at the end of the Permian, 252 million years ago. Only three lineages survived into the Triassic: the dicynodonts (who briefly took over the world), the therocephalians (who went extinct not long afterwards), and the cynodonts (who eventually gave rise to early mammals).

Diademodon tetragonus here lived right in the wake of the extinction during the Early and Middle Triassic, about 251-242 million years ago. Around 2m long (6′6″), it was one of the largest known cynodonts, and it must have been a fairly successful species since it ranged across a large chunk of Pangaea, known from modern southern Africa, South America and Antarctica.

It had pig-like cheekbones and enormous jaw muscles, along with sharp incisors and canine teeth at the front of its jaws and grinding molars at the back. This arrangement suggests that much like modern pigs it may have been an opportunistic omnivore, occasionally snacking on smaller animals and carrion – although an isotope analysis of its teeth indicates the vast majority of its diet was probably still vegetation in shady damp environments.