Most of the more complete fossil material of these animals comes from the mid-Jurassic of China, but one species from elsewhere is also known from a partial skeleton.
Haldanodon exspectatus here lived in central Portugal during the Late Jurassic, about 155 million years ago. Around 15-20cm long (6-8″), it had small eyes and short chunky well-muscled limbs with the front paws adapted for digging. Since it inhabited a very swampy environment it probably wasn’t a pure mole-like burrower – extensive tunnels would have constantly flooded – but it may have instead been a similar sort of semi-aquatic animal to modern platypuses and desmans, foraging for invertebrates in the water and excavating burrows in the banks.
Roughened areas of bone on its snout may also have supported a patch of tough keratinous skin, which would have helped protect its face while digging.
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.
Living during the Late Carboniferous in Nova Scotia, Canada, this 60-70cm long (2′-2’4″) distant cousin to modern mammals was previously known only from the fossilized remains of juveniles – with all known specimens showing slightly elongated spines on their vertebrae that gave it a sort of high-backed “proto-sail” appearance.
A single vertebrae identified as belonging to Echinerpeton shows a much much longer spine than anything we’ve ever seen before, and confirms that this species actually had a large elaborate true sailback – making it the earliest known tetrapod to experiment with this type of anatomy.
This individual seems to have been older than the other known specimens, but still not fully grown, leaving the possibility that fully mature Echinerpeton may have had even larger sails than this.
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.
And Estemmenosuchus mirabilis here was particularly elaborately ornamented, earning it a name meaning “wondrous crowned crocodile”. It lived in the Perm region of Russia during the mid Permian, about 268-265 million years ago, and was one of the largest dinocephalians, reaching at least 3m long (9’10”).
It had two big antler-like structures on its head, two wide cheek flanges, and a small nose horn, almost looking like the synapsid version of a ceratopsid dinosaur – and with its big bulky body, fairly erect-legged posture, and herbivorous-or-omnivorous diet it may have been a fairly close ecological equivalent to them, too.
But it’s also possible it was semi-aquatic, and it certainly does have a very hippo-like appearance when reconstructed with a decent amount of soft tissue.
One specimen of Estemmenosuchus even preserved skin impressions around its face, which were described in Russian in the early 1980s. They show scaleless glandular skin with a slightly bumpy texture, similar to that of hairless mammals or some amphibians. Since it occupied a point in the synapsid family tree close to where hair may have originated (somewhere in the Permian therapsids), it’s not clear if it was entirely hairless or if it had just secondarily lost some of it.
Sometimes the really weird thing about a head isn’t any sort of ridiculous ornamentation.
Sometimes it’s just the wrong size.
That’s what was going on with Cotylorhynchus romeri from the early Permian of North America, living about 280-272 million years ago. Despite looking like a big fat lizard this creature was actually a very early synapsid, closer related to modern mammals than to reptiles, and it was a distant cousin of other stem-mammals like the famous Dimetrodon.
Around 3.5m long (11’6″), it was one of the largest herbivores of the early Permian, with a very wide barrel-shaped body, chunky limbs, and a comically small head. Such a tiny head isn’t necessarily unique – another synapsid Edaphosaurus also had a fairly small skull compared to its body, and dinosaurs like stegosaurs, sauropods, and moa had heads even more disproportional. But something about Cotylorhynchus in particular just looks… incredibly odd.
It also had some surprisingly sizeable nostril openings in that little skull, and it had may have had a very good sense of smell or perhaps some sort of specialized breathing system like the modern saiga’s “air conditioning” nose.
Although usually depicted as a fully terrestrial animal, the structure of Cotylorhynchus‘ bones and its flattened paddle-like hands and feet have recently been used to suggest that it may have been semi-aquatic, more of a Permian hippo than a cow. But such a lifestyle would have required it to have a much more efficient method of breathing than previously thought – suggesting it had a mammal-like diaphragm, and possibly also explaining that weird nose.
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.
Although their fossils are known from both North America and Europe, their European remains are very rare and fragmentary. Currently the best-known specimen is made up of a recently-discovered partial spinal column and a few hand and tail bones.
(Interestingly the paper that names Remigiomontanus also makes a brief mention that the protruding cross-bars on edaphosaurid sails may have anchored larger keratinous coverings, which could have made them look even more spectacularly spiky and suggests their sails may have served an anti-predator function. Hopefully if this is true we’ll see further information get officially published about it sometime!)
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.
Fossils of a surprisingly large dicynodont were first reported in 2008, but it wasn’t until just recently (in late 2018) that this giant creature was finally given an official name – Lisowicia bojani.
Close in size to a modern elephant, at around 2.6m tall (8′6″) and 4.5m long (14′9″), it was by far the largest known example of its kind to have ever lived. And while most other dicynodonts had upright hindlimbs and sprawling forelimbs, Lisowicia seems to have developed a fully upright posture much more similar to that of quadrupedal dinosaurs and modern mammals.