diapsida – Nix Illustration

Unguinychus

Drepanosaurs were a weird little group of tree-climbing Triassic reptiles with prehensile claw-tipped tails, chameleon-like bodies, humped backs, grasping feet, long necks, and somewhat bird-like skulls that may have been tipped with toothless beaks in some species.

Recently some of them have been recognized as also having adaptations for digging and ripping into insect nests, similar to modern anteaters, with highly specialized forelimb bones and a massively enlarged hoked claw on each hand.

And now we have another one of these digging drepanosaurs: Unguinychus onyx, whose name delightfully translates to “claw claw claw”!

Living in what is now New Mexico, USA during the late Triassic, around 215-208 million years ago, Unguinychus is only known from its enlarged hand claws but was probably similar in size to some of its close relatives, likely around 40cm long (~1’4″).

Based on skin impressions from the early drepanosaur Kyrgyzsaurus it also would have been covered in small scales, possibly with a skin crest and a chameleon-like throat sac.

Drepanosaurs’ evolutionary relationships are rather unclear, with various studies classifying them as an early branch of diapsid reptiles, as close relatives of the gliding kuehneosaurids, or as protorosaurian archosauromorphs. But recently another idea has been proposed, instead placing them slightly further up the archosauromorph evolutionary tree in the allokotosaur lineage close to trilophosaurids – and notably making them very closely related to fellow Triassic bird-headed weirdo Teraterpeton.

Hupehsuchus

Hupehsuchians were small marine reptiles closely related to ichthyosaurs, known only from the Early Triassic of southwestern China about 249-247 million years ago. They had toothless snouts, streamlined bodies, paddle-like limbs, and long flattened tails, along with a unique pattern of armor along their backs made up of overlapping layers of bony osteoderms.

Hupehsuchus nanchangensis was a mid-sized member of the group, about 1m long (3’3″). Newly-discovered fossils of its skull show that its long flattened snout had a distinctive gap between the bones (similar to the platypus-like snout seen in its relative Eretmorhipis) with an overall shape surprisingly convergent with that of modern baleen whales – suggesting that this hupehsuchian may have been a similar sort of filter-feeder.

A diagram comparing Hupehsuchus' skull to that of a modern baleen whale. — *Hupehsuchus skull compared to a modern minke whale*
From fig 2 & fig 3 of Fang et al (2023). First filter feeding in the Early Triassic: cranial morphological convergence between *Hupehsuchus* and baleen whales. *BMC Ecol Evo* 23, 36. https://doi.org/10.1186/s12862-023-02143-9

Grooves in the bones along the outer edges of its upper jaws may be evidence of filtering structures similar to baleen, although with no soft-tissue preservation we don’t know exactly what this would have looked like. Its slender flexible lower jaws probably also supported a large expandable throat pouch, allowing it to filter plankton out of larger volumes of water.

Araeoscelis

Araeoscelis gracilis was a superficially lizard-like animal that lived during the mid-Permian, around 275 million years ago, in what is now Texas, USA. About 60cm long (~2′), it had a slender body, proportionally long legs, and a solidly-built skull with strong teeth, suggesting that it was a fast runner that specialized in cracking open the carapaces of thick-shelled prey.

It was one of the last known members of a lineage known as araeoscelidians, which are usually considered to be very early members of the diapsid reptiles – but a recent study has proposed they might have even more ancient roots than that, possibly being a branch of stem–amniotes instead.

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.

Champsosaurus

Champsosaurus might look a lot like an unarmored crocodilian, but it was actually only very distantly related to them – this animal was part of a completely extinct reptile lineage known as choristoderes, and its very gharial-like appearance was the result of convergent evolution.

Found in freshwater habitats across North America and Europe, several different species of Champsosaurus are known from around the middle of the Late Cretaceous through to the end of the Paleocene, surviving through the devasting K-Pg mass extinction 66 million years ago.

Champsosaurus laramiensis here lived in western North America and ranged right across the time of the extinction event, dating to between about 70 and 62 million years ago. Around 1.5m long (~5′), it had a flattened skull that was very wide at the back, supporting powerful jaw muscles, with a long narrow toothy snout that could sweep rapidly through the water to snap at fish in a similar manner to modern gharials. Its nostrils were right at the tip of its snout, and it may have used it like a snorkel, only sticking the very end out of the water to breathe.

Skin impressions show it was covered in numerous tiny scales, most less than 0.5mm in size (0.01″), which wouldn’t have been particularly visible from a distance.

There also seems to have been some sexual dimorphism in this species, with females having much more well-developed limb bones – allowing them to occasionally haul themselves out onto the shore to lay eggs, while males were probably fully aquatic and unable to support themselves on land.

Ancistronychus

Drepanosaurs were already some extremely weird animals, even among all the other weirdos of the Triassic period.

These strange little tree-climbing reptiles had chameleon-like bodies, humped backs, long necks, and oddly bird-like skulls with toothless beaks – and then some of them also had bizarre forelimb anatomy with a single enormous claw on the second finger of each hand, along with a claw on the tip of their prehensile tail.

But new discoveries are showing that some members of this bizarre group were doing something different.

Ancistronychus paradoxus here lived during the late Triassic, about 227 million years ago, in what is now the southwestern United States. Measuring around 50cm long (1’8″), its enormous hand claws were unusual compared to its close relatives, with a distinctly wide and hooked shovel-like shape.

Along with another recently-discovered species, Skybalonyx skapter, and the weird burly arms of Drepanosaurus, this suggests that instead of tree-climbing some drepanosaurs were instead much more specialized for digging. They may have been Triassic equivalents to modern anteaters or pangolins, using their enlarged claws to excavate burrows and rip their way into insect nests.

Guizhouichthyosaurus

In the mid-Triassic seas, covering what will one day be part of southwestern China, an ichthyosaur flails at the surface desperately trying to deal with an ambitiously large meal.

240 million years later human paleontologists will name their kind Guizhouichthyosaurus tangae, and initially assume that their narrow snout and small peg-like teeth are suited only for a diet of small soft-bodied fish and cephalopods.

In reality they eat a much wider range of prey – including other marine reptiles.

But for a 5m long (16’5″) Guizhouichthyosaurus, perhaps this particular catch is a little too much. The unlucky thalattosaur was a rather large example of a Xinpusaurus xingyiensis – nearly matching the ichthyosaur in length at around 4m long (13’2″), although much less bulky – and after biting off the head and tail the predator is still struggling to actually eat the sizeable carcass.

Even with a gravity assist from holding their prize vertically up above the water, swallowing is proving difficult and the Guizhouichthyosaurus can’t breathe around it.

They’re slowly suffocating.

They’ll eventually get it down their gullet, but by then it’ll be too late. Weak and dizzy from asphyxiation, they’ll soon sink to the sea floor and never resurface, their body settling not very far from where their prey’s severed tail fell.

Mixosaurus

Mixosaurus cornalianus here was a small early ichthyosaur, only about 0.8-1m long (2’7″-3’3″) and generally considered to be transitional between the eel-like swimming style of basal forms and the more dolphin-like later forms. Living during the Middle Triassic, about 242 million years ago, it inhabited a shallow tropical sea that covered what is now the modern border between Switzerland and Italy.

It was previously thought to be a slow swimmer with a low and poorly-developed tail fin, and whether it even had a dorsal fin or not was unclear. But now new specimens with soft tissue impressions have given us a big surprise.

Not only did it actually have a fairly well-developed semilunate tail fin, but it also had a dorsal fin positioned much further forward on its body than expected, giving it a shape similar to some small sharks and representing the current earliest known dorsal fin of any amniote.

Bundles of stiffening collagen fibers inside its fins were very similar to those known from later Jurassic ichthyosaur species, indicating that this adaptation evolved much earlier in the lineage than previously thought. Along with stomach contents showing it mainly ate both cephalopods and small fish – fairly fast-moving prey – this suggests it was a capable open-water swimmer. It wouldn’t have been quite as speedy as its much more specialized Jurassic relatives, but it may have still been about as efficient as the small modern sharks it resembled.

Weird Heads Month #24: Hook-Snouted Swimmers

Thalattosaurs were another group of weird Triassic animals, found in coastal marine environments all around the world. Their evolutionary relationships are unclear beyond “they were some sort of diapsid reptile”, and they were well adapted for aquatic life, with streamlined lizard-like bodies, short limbs with webbed feet, and long paddle-like tails.

Most of them had long narrow toothy snouts, but others had odd spear-shaped noses or downturned upper jaws.

Hescheleria rubeli here was one of the strangest, living in Europe during the mid Triassic, about 247-235 million years ago. It was one of the smaller known species of thalattosaurs, around 1m long (3’3″), and had a particularly bizarre-looking head.

A close-up of the head of the extinct marine reptile Hescheleria. The front of its snout is sharply downturned, forming a near-right-angled hooked shape, with small sharp teeth at the front of its jaws along with a pair of large conical bony projections in its lower jaw.

Its snout was so sharply curved downward that it formed a right-angled hook relative to the rest of its jaws, sort of resembling the initial interpretation of Atopodentatus but without the vertical split.

There were also small sharp teeth at the front of its mouth, along with a pair of large conical bony projections on its lower jaw.

This weird arrangement must have been highly specialized for something, but its actual function is still unknown. One suggestion is that the large jaw-spikes were used to crunch into hard-shelled prey, although there doesn’t seem to have been any reinforced surface in the upper jaw for them to crush against.

But I personally wonder if maybe these jaws were the equivalent of the hooked kypes seen in the males of some modern salmonid fish – structures associated with dominance fighting.

Hovasaurus

The Permian-Triassic extinction 252 million years ago was the most severe mass extinction in Earth’s history, so incredibly devastating that it’s been nicknamed the “Great Dying” – but there were still some animals that somehow just… didn’t seem to really notice it at all.

And one of these surprisingly resilient species was Hovasaurus boulei.

It was part of a group known as the tangasaurids, a fairly early evolutionary branch of diapsid reptiles from Madagascar and East Africa that originated sometime in the mid-Permian, just before the common ancestor of modern lizards and archosaurs.

Hovasaurus lived in Madagascar both just before and for some time after the Great Dying, dating to around 252-247 million years ago. Growing up to about 90cm long (~3′), it was one of the largest tangasaurids and was also highly specialized for aquatic life in freshwater rivers, with an eel-like tail twice the length of the rest of its body and heavy thickened ribs.

Hundreds of fossils have been found representing life stages from hatchling to adult, and juvenile Hovasaurus actually seem to have been almost fully aquatic – they had proportionally shorter limbs and may have behaved similarly to modern sea turtles, crawling into the water shortly after hatching and only returning to land as adults once they had longer better-developed legs.

Many fossils also preserve clusters of pebbles in their abdominal cavities, which are thought to have been used as extra ballast to help weigh them down in the water when hunting small fish and invertebrates.

It’s not entirely clear why these odd little aquatic reptiles were apparently unaffected by the Great Dying. Perhaps, much like the many freshwater species that survived though the later end-Cretaceous mass extinction, Hovasaurus was simply very good at dealing with sudden changes in its environment and food availability due to the variability of river habitats, and was able to weather though the worst of the extinction without much trouble.

Or maybe it was just one of the lucky ones.