drepanosauromorpha – Nix Illustration

Mirasaura

Back in 1939, fossil collector Louis Grauvogel discovered a couple of reptile fossils in Middle Triassic-aged deposits (~247 million years old) in eastern France. A large preserved structure was noted above the animal’s back, but for many years it was interpreted as an unrelated fish fin, insect wing, or plant frond.

It was only when the State Museum of Natural History Stuttgart acquired the specimens in 2019 that they were recognized as representing something very special: a long-sought-after relative of the bizarre and enigmatic Longisquama!

Mirasaura grauvogeli grew to around 30cm long (~1′) and was, if anything, even stranger than its relative. It had humped shoulders, grasping limbs, and a bird-like head with large forward-facing eyes and a long pointed snout that was toothless at the front, probably used to probe for small invertebrates in cracks and crevices.

But most strikingly it had up to 20 tall structures overlapping along its back to form a sail-like crest. Although they were superficially feather-like in shape with preserved melanosomes that resemble those of birds, structurally they weren’t feathers at all – but they also weren’t modified scales. Instead these appear to have been an entirely novel type of skin appendage, made up of continuous sheets with a midline shaft and a corrugated texture.

The crest was probably used for visual display, and 80 additional fossils of isolated crest structures suggest they were regularly shed and regrown.

Along with Longisquama, Mirasaura appears to have been an early member of the drepanosaur lineage – a group of wonderfully weird tamandua-like reptiles whose evolutionary relationships are still disputed, with different studies currently recovering them as either a unique early offshoot of the diapsids or as archosauromorphs.

(Interestingly, a specimen of Drepanosaurus reportedly preserves some soft tissue on its back that may also be one of these strange new crest structures. Drepanosaurs just keep on getting weirder and weirder and I love them.)

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.

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.

Drepanosaurus

One of my favorite palaeontological discoveries of 2016 also involved one of my favorite extinct animals: the odd little “monkey lizard” Drepanosaurus.

Living during the Late Triassic of Europe and North America (~218-212 mya), this reptile measured about 50cm long (20″). It was already known to have some particularly weird anatomy, featuring a humped back, grasping feet, a prehensile tail ending in vertebrae modified into a pseudo-claw, enormous claws on the second finger on each hand, and a bizarre arrangement of its forearm bones.

In 2016, new fossils gave us a better look at those arm bones, and they turned out to be even stranger than previously thought. As well as the radius and ulna being unusual, several wrist bones were also heavily modified and elongated, creating an arm setup unique among all known tetrapods. This may have been a specialization for “hook-and-pull” digging, ripping up tree bark to get at burrowing insects – but now the real mystery is why no other tetrapods have ever managed to modify their limb bones to this sort of extreme.