New taxon: Taoheodon baizhijuni

After lamenting the dearth of dicynodont research this year when I covered the publication of stable isotope analysis on Endothiodon, two dicynodont-focused papers have dropped in the interim! If that's how it's going to be, I should whinge about wanting papers more often.

I'm planning on covering each of these papers when I can, and first up is the description of a new genus and species published back in July by Liu (2020). Welcome Taoheodon baizhijuni, the first (and as of writing, the only) new dicynodont of 2020!

Taoheodon baizhijuni

The skull of Taoheodon baizhijuni (IVPP V 25335), from Liu (2020).

Etymology: Named after the Tao He stream, a nearby river that eroded the valley the fossil was discovered in. The species is named after the fossil hunter who discovered it, Bai Zhijun. The '-odon' (tooth) should be self explanatory.

Described by: Jun Liu, 2020

From: The Shanxi Province in northern China, in the lower Sunjiagou Formation.

Age: The Late Permian, probably around the latest Wuchiapingian to the Changhsingian approx. 254-253 mya.

Known from: A slightly squashed and eroded skull plus a partial lower jaw. 

As far as dicynodonts go, Taoheodon is fairly nondescript in appearance. It's not particularly large or small, and it's got your basic Dicynodon-esque shape—short snout, two tusks, a toothless beak, plus a slight boss on the snout—the standard affair for an animal in the Dicynodon area of the family tree. Taoheodon is uniquely set apart from the rest of the Dicynodon-nexus by a few minor details around the postorbital bones, braincase and the pterygoids. The fossils have been eroded a bit, so the skull's missing the tip of the snout and most of the zygomatic arches, while the jaw is missing the tip and everything behind the reflected lamina, but there's enough of them to get the gist of the shape.

IVPP V 25335 in dorsal view, from Liu (2020).

Taoheodon is the latest member of the ever growing collection of Dicynodon-like genera floating around in Dicynodontoidea. Most of these animals used to be stuffed into a severely over-bloated Dicynodon until it was finally blown apart by synapsid expert Christian Kammerer in 2011, resurrecting many old names and coining several new ones for the plethora of species crammed in there in the process. Permian dicynodontoids are one of the most unstable parts of the dicynodont family tree, so sorting out who's related to who is still an unresolved challenge. By itself, Taoheodon doesn't fix this conundrum, but it's still another piece of the puzzle and, perhaps, it might just hold some key biogeographical information.

Taoheodon was found by Liu to be most closely related to two genera of Late Permian or Early Triassic* dicynodonts from Laos described just last year, Repelinosaurus and Counillonia. These two were originally thought to be unrelated, as the earliest kannemeyeriiform and another member of the Dicynodon-nexus (or 'Dicynodon-grade' as it's actually called), respectively. The addition of Taoheodon into the mix appears to have linked them instead as each other's closest relatives, dragging Repelinosaurus out of Kannemeyeriiformes in the process (admittedly not a shocking outcome, purported basal kannemeyeriiforms are known to jump around like that).

This makes some biogeographical sense, the two Laotian species are each other's closest relatives, and they are in turn closest to a species from northern China, very neat. But what's yet more interesting is that these three then grouped together with the two Russian genera Vivaxosaurus and Delectosaurus as successive outgroups, before all together forming a clade with Dicynodon itself at the base.

Liu identified this group as the 'core-Dicynodon' clade, and noted that they form a neat little series starting in South Africa (Dicynodon), up through Russia (Vivaxosaurus and Delectosaurus), North China (Taoheodon) and finally Laos (Counillonia and Repelinosaurus). It's tempting to see this as recording the geographic dispersal of a clade starting with Dicynodon in southern Gondwana, migrating and speciating northwards into Laurasia and finally ending up in Laos. Laos was mostly isolated from mainland Pangaea at the time on its own tectonic block, so the presence of these two dicynodonts there could mean that a land connection between the South China block and the Indochina block existed by the latest Permian.

Under this arrangement Taoheodon seems to be a sort of 'missing link' between the European and Laotian dicynodonts, both cladistically and geographically. It's a very neat and tidy—and thus, appealing—idea, but at the same time it's entirely possible that this is just a quirk of the existing data used in the phylogenetic analysis, and that the addition of more data in the future will shoot it down as nothing more than a red herring (surely Permian dicynodontoids wouldn't make it that easy for us). Whichever way it turns out, Taoheodon demonstrates that even an unassuming new species can potentially be key parts in making evolutionary relationships click.

My attempted decompression and life reconstruction of Taoheodon.

(*P.S. The Laotian dicynodonts were intriguingly suggested to date from the earliest Triassic based on radiometric dating of the rocks they were found in, which would make them the only Dicynodon-esque dicynodonts to survive into the Triassic (for Counillonia, at least). However, there's been some quibbles over the precision of this dating, and Liu further considered it unlikely that Laos was even habitable to dicynodonts in the earliest Triassic, and so combined with the close affinity to known Permian genera like Taoheodon, he suggested that the Laotian dicynodonts were more likely to be from the Late Permian.

However, another paper (Romano et al. 2020) has been published since then that reviewed the global distribution of Early Triassic tetrapods. From their observations, they concluded that the Early Triassic equatorial "death belt" was less extensive than previously thought, restricting it down from reaching 30° north to just 15°. The exact position of Laos during the Early Triassic is, to my knowledge, not entirely pinned down yet, so maybe, maybe, the Laotitan dicynodonts were just north enough to skirt the "death belt"...assuming they're even Triassic in age after all.)

References

Liu, Jun 2020. Taoheodon baizhijuni, gen. et sp. nov. (Anomodontia, Dicynodontoidea), from the upper Permian Sunjiagou Formation of China and its implications. Journal of Vertebrate Paleontology. In press: e1762088.

Olivier, C.; Battail, B.; Bourquin, S.; Rossignol, S.; Steyer, J.-S.; Jalil, N.-E. 2019. New dicynodonts (Therapsida, Anomodontia) from near the Permo-Triassic boundary of Laos: implications for dicynodont survivorship across the Permo-Triassic mass extinction and the paleobiogeography of Southeast Asian blocks. Journal of Vertebrate Paleontology. 39(2): e1584745.

Romano, M., Bernardi, M., Petti, F.M., Rubidge, B., Hancox, J. and Benton, M.J. 2020. Early Triassic terrestrial tetrapod fauna: a review. Earth-Science Reviews, In press, p.103331.