Thursday, February 6, 2020

Marine Snouters of the Triassic II: Gunakadeit Rising

Alaska's own Gunakadeit joseeae by the amazing Gabriel Ugueto, or @SerpenIllus on the Twitters.

A very long time ago, I posted an essay about one of my favorite groups of Triassic marine reptiles: thalattosaurs. These lizard-shaped swimmers, while generally similar in body form, differ from one another quite strongly in terms of feeding adaptations. While Anshunsaurus and Askeptosaurus may seem, to our modern eyes, rather vanilla in their clearly faunivorous dentition, others strain credulity. Xinpusaurus, for example, possesses a bizarre notched upper jaw and, in some specimens, an elongate premaxillary spear which overshoots the the lower jaw by an impressive degree, calling to mind swordfish and swordfish-snouted ichthyosaurs. Thalattosaurus and its “claraziid” brethren have reduced, shell-crushing dentition and snouts which curve downward—Hescheleria takes this trend to a puzzling extreme. Thalattosaurs also occupied a wide range of body lengths (1-5 meters). While some were more terrestrially capable than others, the group never seemed to stray far from the nearshore niche.

In that post, I mention that thalattosaurs are found around the Northern Hemisphere, including one unpublished Alaskan taxon. I was lucky enough to see this fossil several years ago at the University of Fairbanks’ Museum of the North (many thanks, once again, to Dr. Patrick Druckenmiller for showing me and my wife around that wonderful place). It’s a beautiful specimen that I have a rather bad picture of on my phone, obscured by my own reflection on its glass enclosure.

At the time, I thought it looked a bit like Xinpusaurus or Endennasaurus, what with the needle-nosed rostrum, but the dentition was very different. Where Xinpusaurus has heterodont dentition of various sizes and shapes, and Edennasaurus lacks teeth altogether, this unnamed band member had sharp, needle-like teeth to match its needle-nose snout.

I eagerly awaited the little guy’s eventual publication, and that day came earlier this week.

Say hello to Gunakadeit joseeae (Druckenmiller et al. 2020)!

UAMES 23258
Continuing the proud tradition of naming Alaska’s prehistoric fauna after Native Alaskan creatures (Nanuqsaurus, Ugrunaaluk), Gunakadeit (pronounced “Goo-na’-ka-date”) was named after a Tlingit sea monster who brought good luck to those who saw it. The holotype, UAMES 23258, is nearly complete, lacking only the distal two-thirds of the tail and the hands. It was found in Southeast Alaska on one of the Keku Islands. It is of Norian age, so Gunakadeit is one of the youngest thalattosaurs on record along with Italy’s Endennasaurus.

With an estimated length of between 75-90 centimeters, that puts our boy at just under a yard long, toward the shorter end of the thalattosaur scale. Among other things, Gunakadeit is unique in having an extremely short post-orbital skull region, toothless anterior snout and needle nose, short neck, and upper temporal fenestrae—which were lost in other thalattosaurs (due to radically reducing the size of their squamosal and postorbital bones). The authors revised and added to the character list of Li et al. (2016), then ran Gunakadeit through the analysis. Surprisingly, it’s nowhere near Endennasaurus or Xinpusaurus, instead occupying an extremely basal position among thalattosaurs generally, but thalattosauroids specifically, branching off prior to the weird-snouted taxa. This is even stranger considering it’s one of the only surviving Norian members of the group, which opens up a 20 million year ghost lineage between it and the earliest known Ansian thalattosaurs.

A new phylogeny of thalattosaurs, from Druckenmiller et al. (2020).

The authors suggest that Gunakadeit went after soft-bodied prey either in open water or by probing in cavities and crevices with its forceps-like jaws. The little guy’s depositional environment—a volcanic island surrounded by coral reefs—indirectly supports this assessment. Aside from the lack of teeth, Edennasaurus differs in one other important way from Gunakadeit: it was probably capable of walking around on land when the mood struck it. This is probably not the case with Gunakadeit, who had poorly-ossified wrists and ankles, flattened forelimb bones, short limbs generally, and an awkwardly-shaped tail for landlubbing. It's also interesting that the youngest surviving thalattosaurs are superficial similarities in terms of their feeding apparatuses, and may indicate that the rest of their snouter cousins were outcompeted by other marine animals. And, indeed, thalattosaurs do not appear to have survived through the Norian.

Always exciting when a new Mesozoic taxon from Alaska is published, and this is no exception. But also, thalattosaurs are awesome.

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