Monday, May 14, 2018

One More Strange Reptile


Remember my post from last year about azendohsaurs and trilophosaurs? Well just the other day, paleoartist extraordinaire Gabriel Ugueto posted a sketch of something called Teraterpeton. I had no idea what this zany-looking reptile was, so I looked it up and was flabbergasted to find that it's a trilophosaur that is quite unlike Trilophosaurus. You'll notice that I did mention Teraterpeton in passing in that post, which must mean I didn't think it was a trilophosaur or at least was not unambiguously a trilophosaur. Turns out I'm incorrect--Teraterpeton is a perfectly good trilophosaur, and therefore an allokotosaur!


A drawing of the header image from Sues (2003).
Hans-Dieter Sues named and described Teraterpeton hrynewichorum based on a wonderful skull, a partial skeleton, and a referred right dentary. Now, even for a Late Triassic hellasaur, Teraterpeton is pretty trippy: the skull is long and low, characterized by a short maxilla, where the teeth are concentrated, followed by an extremely long, toothless snout that may have ended in a beak. The maxillary teeth occlude with matching teeth at the back of the dentary. Like Trilophosaurus, the lateral temporal fenestrae is closed off. You may think that Teraterpeton has an extremely large antorbital fenestrae and that the naris is quite small.

In fact, that's probably not the case. Trilophosaurus lacks an antorbital fenestrae, as do the closely-related azendohsaurs. Sues initially believed the large opening to be an antorbital fenestra but, given what bones it's bordered by, "the opening most likely represents a greatly enlarged external narial fenestrae." Indeed, the premaxillae are extremely long and the nasals have been reduced to oddly-shaped bones that have been shoved backwards toward the skull roof. Behind them, the prefrontals form small lateral flanges overhanging the orbits.

Dentary dentition
The mandibles match the structure of the skull. Each tooth has a single tall cusp and a smaller "heel." The teeth are wider than they are long, which is expected for a trilophosaur, but they are otherwise quite different from Trilophosaurus. Interestingly (to me, anyway), each tooth has a "collar" of thin bone below its bulbous base that covers up what would be the upper portion of the root. Teraterpeton also surprisingly large palatal teeth that run right up against the maxillary teeth and are of the same morphology.

The rest of the skeleton is mostly unremarkable although the incomplete right manus has distinctive claws: they are tall but narrow and "blade-like." This is also a major difference from Trilophosaurus, who has slender fingers and relatively small claws. Trilophosaurus is usually considered arboreal, but what was Teraterpeton doing?

The intimidating claws of Teraterpeton
Well, at SVP 2016, Pritchard & Sues reported on new Teraterpeton material, including a complete pelvic girdle and hind limbs, and the base of the tail. The pelvic girdle shares features with a bunch of unrelated groups like rhynchosaurs, tanystropheids, and lepidosaurs (but were likely homoplastic and not indicative of a close relationship) and the calcaneum is similar to Azendohsaurus. The pedal claws are just like the manual claws. Despite the marked differences between Trilophosaurus and Azendohsaurus, Pritchard & Sues still found Teraterpeton to be a trilophosaurid, and trilophosaurids are the sister group of Azendohsaurus (which we already knew thanks to Nesbitt et al. (2015)). However, Pritchard & Sues did  not--at least according to their abstract--comment on how Teraterpeton differed in terms of lifestyle from Trilophosaurus.

It was almost certainly not arboreal, but the large claws; long, toothless snout; and closely-packed occluding teeth remind me of burrowing insectivores like aardvarks and armadillos who slurp up bugs and grubs with a long tongue and crunch them up with strong molars. Note that this is not the same as specializing for eating eusocial insects like termites or ants: in those animals, dentition is typically reduced in size or lost entirely. For example, anteaters are completely toothless and aardwolves have extremely small molars.

I'm coming around to the idea that Teraterpeton was the Late Triassic equivalent of the aardvark. But this also speaks to the great morphological distance between Teraterpeton, a burrowing aardvark-like animal, and Trilophosaurus, an arboreal iguana-like animal. It makes me think that the Trilophosauridae is far more diverse than we currently know, which is an exciting thought!

An adorable aardvark emerging from its burrow, from Wikipedia.

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