synapsid – Nix Illustration

Dimacrodon

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-20^th 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.

Spectember 2025 #03: Dicyny World

Sophie Shepherd requested: “Predator and prey from an alternate Mesozoic where archosaurs and cynodonts never displace dicynodonts as the dominant terrestrial vertebrates. Lystrosaurus-descendants conquer the world!”

Well into an alternate Jurassic, long after the Great Dying, the descendants of the opportunistic survivor Lystrosaurus are thriving. Cynodonts and rauisuchian-like archosaurs still saw decent success as large predators during this world’s Triassic, but the extinction at the end of that period wiped most of them out, and now the lystrarch dicynodonts are the dominant land vertebrates.

A digital sketch of a speculative descendant of the protomammal Lystrosaurus. It's a hairy deer-like animal with a turtle-like beak, lumpy bosses on the top of its snout, short pointed "devil horns" above its eyes, and antler-like horns growing out from its cheekbones.

The most common herbivorous lystrarchs are big bulky Lisowicia-like forms, but in open savanna-like habitats smaller fast-running deer-like forms like Diablocervops shepherdi are starting to diversify.

Standing around 1m tall at the shoulder (~3’3″), Diablocervops has particularly elaborate facial ornamentation, with bosses on its snout, pointed brow horns, and long pronged horns flaring out from its cheekbones. These structures only develop towards maturity but aren’t sexually dimorphic, used for visual displays and in head-pushing contests to establish dominance hierarchies within herds.

It lacks teeth, and grinds mouthfuls of food against its tough palate before swallowing, also using a gizzard-like stomach chamber with gastroliths to further process tough vegetation.

A digital sketch of a speculative carnivorous descendant of the protomammal Lystrosaurus. It resembles a Komodo dragon with a hooked eagle-like beak, a pair of short tusks, thick bony ridges on its brows and cheeks, and thick lumpy leathery skin covering its body.

Meanwhile, following the loss of the former archosaur and cynodont predators, Dakorhynchus gorgoides is part of a new lineage of predatory lystrarchs descended from small pig-like omnivorous burrowers that had remained quite similar to their Lystrosaurus ancestors for much of the Triassic.

Around 4m long (~13′), it’s built rather like a large monitor lizard or crocodilian, with a low-slung semi-erect posture and hairless pebbly leathery skin. It’s an ambush predator that lunges with short bursts of speed, grappling with its well-muscled forelimbs and using slashing strikes from its hooked beak and short pointed tusks to subdue prey.

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.

Angelosaurus

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.

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.

Crystal Palace Field Trip Part 1: Walking With Victorian Monsters

This past week I’ve been out of town and unable to work on much art, but instead here’s something a little different. I finally got the chance to go visit some familiar old faces out in the wilds of south London, so let’s go on a little tour of these iconic Victorian-era retrosaurs…

A photograph of an informational sign in London's Crystal Palace Park. The text on it reads, "The Crystal Palace Dinosaurs 1854, a journey through time and science". Three of the iconic Victorian dinosaur statures are also pictured below the title, showing the Iguanodon, Hylaeosaurus, and Megalosaurus.

The Crystal Palace Dinosaurs take their name from the original Crystal Palace, a glass-paned exhibition building originally constructed for a World’s Fair in Hyde Park in 1851.

In 1854 the structure was relocated 14km (~9 miles) south to the newly-created Crystal Palace Park, and a collection of over 30 life-sized statues of prehistoric animals were commissioned to accompany the reopening – creating a sort of Victorian dinosaur theme park – sculpted by Benjamin Waterhouse Hawkins with consultation from paleontologist Sir Richard Owen.

The Palace building itself burned down completely in 1936, and today only the ruins of its terraces remain in the northeast of the park grounds.

Two images of Victorian London's Crystal Palace building. On the left an old black-and-white photograph from around 1854 shows the original structure, a grand glass-paned building with ornate terraced gardens in front of it. On the right a more modern photo from 2011 shows what little remains today – just the ruins of the terraces and stairs. — *The Crystal Palace building then and now*
Left image circa 1854 (public domain)
*Right image circa 2011 by Mark Ahsmann (CC BY-SA 3.0)*

Six sphinx statues based on the Great Sphinx of Tanis also survive up among the Palace ruins, flanking some of the terrace staircases. They fell into serious disrepair during the latter half of the 20^th century, but in 2017 they all finally got some much-needed preservation work, repairing them and restoring their original Victorian red paint jobs.

A photograph of one of the surviving sphinx statues in the Crystal Palace ruins, reclining on a plinth beside some stone steps. It's recently renovated with a coat of terracotta red paint to match its original Victorian-era appearance. In the background the huge Arqiva Crystal Palace telecom tower can be seen.

…But let’s get to what we’re really here for. Dinosaurs! (…And assorted other prehistoric beasties!)

Rechnisaurus

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.

Cabarzia

Cabarzia trostheidei here lived during the early Permian in what is now Germany, about 295 million years ago.

Despite its very lizard-like appearance it was actually part of the varanopid lineage, a group of scaly amniotes traditionally classified as early synapsids (distant relatives of modern mammals), but which more recently have been proposed to instead be sauropsid reptiles closer related to early diapsids.

It was around 50cm long (1’8″), and its short arms, long legs, slender body, and long tail suggest it was capable of shifting into a bipedal posture when running at high speeds, similarly to some modern lizards – probably mainly to escape from larger predators, but possibly also used to pursue fast-moving prey like flying insects.

And whether varanopids were actually synapsids or sauropsids, this makes Cabarzia the earliest known example of an animal running on two legs.

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.