allokotosauria – 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.

Spinosuchus

Allokotosaurs were a group of mostly-herbivorous archosauromorph reptiles, distantly related to the ancestors of crocodiles, pterosaurs, and dinosaurs. They lived across Eurasia, Africa, and North America during the mid-to-late Triassic period, and their lineage included some weird and diverse forms – such as the bull-horned Shringasaurus, the long-beaked Teraterpeton, and possibly also the gliding kuehneosaurids.

Spinosuchus caseanus here was yet another one of these Triassic allokotosaurian weirdos, part of the trilophosaurid family and closely related to Trilophosaurus and Teraterpeton.

Living about 221-212 million years ago in what is now northwest Texas, USA, Spinosuchus was around 2.2m long (~7’2″) and had distinctive elongated neural spines along the vertebrae of its back and the base of its tail, forming a “high back” or short “sail”. Since it’s only known from a partial spinal column the rest of its anatomy isn’t known for certain, but it probably had body proportions similar to its close relative Trilophosaurus, with sprawling limbs and a short-snouted beaked head adapted for herbivory.

Like many other fossil “sailbacked” animals the exact function of Spinosuchus’ elongated vertebrae is unclear, but the structure may have been used for visual display. I’ve depicted it here with a speculative frill of colorful elongated scales, along with a flashy dewlap.

Weird Heads Month #07: The Wonderful Creeping Thing

The Triassic was an incredibly weird time, full of evolutionary experiments in the wake of the worst mass extinction in Earth’s history.

Teraterpeton hrynewichorum here was part of group known as allokotosaurs, a lineage of mostly-herbivorous archosauromorphs that also included the long-necked bull-horned Shringasaurus.

Living in Nova Scotia during the Late Triassic, around 235-221 million years ago, Teraterpeton (meaning “wonderful creeping thing”) was first named in the early 2000s based on a skull and partial skeleton, with some additional skeletal material being described recently in 2019.

Its head had a confusing mix of anatomical features, with a long beak-like toothless snout at the front of its jaws, small sharp interlocking cheek teeth further back, a huge nasal opening, and a closed-up fenestra at the back of its skull making it look more like the skulls of marine reptiles.

It also had a lizard-like body, perhaps up to 1.8m long (~6′), with rather long slender limbs and large blade-like claws, and more anatomical weirdness in the pelvic region convergently resembling those of distantly related groups like rhynchosaurs and tanystropheids. It had a sprawling posture, but its hind limb musculature suggests it might have been capable of getting up into a more erect stance when walking, somewhat similar to modern crocodilians’ “high walk” gait.

It was clearly quite an ecologically specialized animal, but quite what it was specialized for is still uncertain. It was presumably a herbivore like its close relatives, but it must have been eating a very different diet with its long beak, and its deep claws could have been used for scratch digging to get at roots and tubers.

Another possibility it that it could have been an insectivore with a diet similar to modern aardvarks or armadillos, probing with its beak and digging with its claws for insects, grubs, and other invertebrates. Since termite-like social insect nests do seem to have existed around the same time, it might even have been one the earliest known animals to specialize in myrmecophagy.

Shringasaurus

Shringasaurus indicus, an archosauromorph reptile from the Middle Triassic of India (~247-242 mya). About 3-4m long (9′10″-13′1″), it was part of a group known as the allokotosaurs, specialized herbivores with lizard-like bodies, and was related to the strange long-beaked Teraterpeton.

Partial remains of about seven different Shringasaurus individuals of varying ages have been found, with several of them having large curving horns over their eyes convergently similar to those of the later ceratopsids. A couple of adult specimens lack horns entirely, suggesting the feature was sexually dimorphic with only the males developing ornamentation – a very rare arrangement among archosauromorphs, but similar to some horned mammals.