Apr 11
A remarkable new discovery is shedding light on one of the greatest survival stories in Earth's history, and answering a decades-old scienti
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A remarkable new discovery is shedding light on one of the greatest survival stories in Earth's history, and answering a decades-old scienti
Dimetrodon, the most famous of all extinct synapsids! It's a shame most of that fame seems to include it getting called a dinosaur. It's both closer to us and much older than the dinosaurs! They lived about 290 million years ago during the early Permian in the southwestern United States, which was a swampy river delta area at the time. It was a carnivore about the size of a crocodile and, of course, had a rather impressive sail on its back that only a glimpse of made it into this portrait. I decided to restore it with the scaly skin that Varanopsids, an even earlier more rootward branch of the the synpasid family tree, are known to have, although bare skin with neither scales nor fur as Estemmosuchus had is also a possibility.
Drawn while listening to Dimetrodon by The Doubleclicks.
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.
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Dimacrodon hottoni was a synapsid that lived during the mid-Permian, about 272 million years ago, in what is now Texas, USA.
Known only from incomplete skull material, it had a thin bony crest on its forehead and a long snout with unusually toothless jaw tips — which had a rough bone texture suggesting there was a small keratinous beak there.
Its full body proportions aren't known, but since its skull measured around 50cm (~1'8") it was probably at least 2.5-3m long (~8-10').
When its fossil remains were first discovered in the mid-20th century it was thought to be a dicynodont-like anomodont, but later examination in the 1990s suggested it was actually a more basal "pelycosaur-grade" synapsid, possibly a sphenacodont close to early therapsids. There hasn't been any further study on Dimacrodon since then, though, so its exact evolutionary relationships remain very murky.
Its ecology is equally unclear, but its beak-like jaws suggest it may have been somewhat herbivorous. It would have lived around a coastal river delta in a semi-arid climate, alongside herbivorous caseids like Cotylorhynchus and Angelosaurus, predatory sphenacodontids like Dimetrodon, small lizard-like parareptiles and captorhinids, and aquatic temnospondyl amphibians.
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Angelosaurus dolani was an early synapsid, part of the caseid family and closely related to the more well-known Cotylorhynchus.
Living in what is now Texas, USA during the mid-Permian, about 270 million years ago, it's only known from partial skeletal remains but was probably around 3m long (~10').
Like other large herbivorous caseids it would have had a tiny head with proportionally big nostrils, a short neck, a large barrel-shaped body accommodating a voluminous gut, a long tail, and strong sprawling limbs. But compared to its relatives Angelosaurus was particularly bulky, with shorter thicker heavily-muscled limbs and stubbier digits ending in broad hoof-like claws.
In closely related caseids the presence of teeth on the roof of the mouth and a well-developed hyoid apparatus suggests these animals had big tough tongues, which may have been used to mash mouthfuls of plant matter against the palatal teeth to partially break it up before swallowing.
Based on skin impressions from other early synapsids, Angelosaurus probably had crocodilian-like scutes on some parts of its body – likely on its underside and tail, and maybe also on the top of the head as indicated by the pitted bone texture of caseid skulls – but whether the rest of its skin was scaly or naked and glandular is currently unknown.
In recent years there have also been some proposals that large caseids may have had a semiaquatic hippo-like ecology, but this idea is controversial.
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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.
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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.
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Rechnisaurus cristarhynchus here was a member of the dicynodonts, a group of stocky herbivorous beaky-jawed synapsids that were distantly related to modern mammals. Living in what is now eastern India during the Middle Triassic, about 247-242 million years ago, it's only known from a single fossil skull – but based on the body proportions of better-known close relatives like Kannemeyeria it was probably somewhere around 1.2m long (~4').
It had a raised bony crest running down the middle of its snout, with deep bowl-like depressions on either side that probably served to make the crest seem visually larger it already was. (They probably didn't house any weird soft-tissue structures, however, since these type of dicynodonts tended to have very extensive keratinous coverings over their snouts.)
It also had raised bony areas around its parietal eye, and extensive bony flanges covering most of its tusks giving its face a sort of jowly appearance. All these features were probably for visual display and may have been brightly colored in life.
And, while I usually like to reconstruct dicynodonts as extensively fluffy… recently some fossil specimens of Lystrosaurus have been found showing bumpy leathery skin impressions. This doesn't necessarily mean that all dicynodonts were hairless (especially since there are still those Permian coprolites), but since kannemeyeriiformes like Rechnisaurus were quite closely related to Lystrosaurus, I've gone with no fuzz at all on this one.
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