| Taoheodon | |
|---|---|
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Therapsida |
| Suborder: | †Anomodontia |
| Clade: | †Dicynodontia |
| Infraorder: | †Dicynodontoidea |
| Genus: | †Taoheodon Liu,2020 |
| Species: | †T. baizhijuni |
| Binomial name | |
| †Taoheodon baizhijuni Liu, 2020 | |
Taoheodon is anextinctgenus ofdicynodonttherapsid from theSunjiagou Formation in theShanxi province ofChina, dated to theWuchiapingianage of theLate Permian. Itstype and only known species isT. baizhijuni.Taoheodon was a close relative of the well knownDicynodon, and may represent abiogeographical link between theSouth AfricanDicynodon and similar dicynodonts found inLaos.
Taoheodon was a medium-sized dicynodont (basalskull length over 10 centimetres (3.9 in) long) currently only known from an incomplete skull and lower jaw. Based on the anatomy of other closely related dicynodonts such asDicynodon,Taoheodon was probably a squat, sprawling quadruped with a short tail and a proportionally large head. Like other dicynodonts,Taoheodon was almost entirely toothless, sporting only a pair oftusks and atortoise-likebeak.[1][2]
Like other dicynodonts,Taoheodon had a short skull with largetemporal fenestra at the back, largeorbits and a short snout, which inTaoheodon was proportionately short even for a dicynodont. Its skull is slightly longer than wide, with elongated temporal fenestra. The externalnostril is rounded and not especially large for a dicynodont, but the area of bone behind and beneath it is hollowed out and concave compared to the rest of the snout. Thenasal bones along the roof of the snout are relatively flat, but are nonetheless rugosely textured and bore a single weakly developed boss of tough skin orkeratin on top of the snout. Likewise, thelacrimal andprefrontal bone form a distinct boss that bulges out to the side in front of each eye. By comparison, thepostorbital bar behind the eyes is smooth and unornamented. The caniniform process housing the tusk is directed downwards from the snout, and sits entirely in front of the eyes. Thepineal foramen (or "third eye") on the roof of the skull is large and positioned relatively far back.[1]
Themandible ofTaoheodon is mostly known from part of thedentary, with portions of theangular,surangular andsplenial bones. The dentary is large and robust, with a rough, pitted surface texture at its front and along the top surface, corresponding to the horny beak typical of dicynodonts. The tip of the lower jaw is missing, so the exact shape of the beak is unknown; however, a low and wide curved ridge defines a clear edge between the side and front faces of the beak. Like other dicynodonts, the angular supports a prominent reflected lamina, which may have supported theeardrum in non-mammaliantherapsids. InTaoheodon, the reflected lamina is large and rounded, facing down and back from themandibular fenestra.[1][3]
Theholotype and only known specimen ofTaoheodon, IVPP V 25335, was discovered in the valley of a tributary of the Tao He river, running through the lower part of the Sunjiagou Formation. The Sunjiagou Formation has been dated to the Late Permian (Lopingian epoch), although the exact age of the lower beds has been debated; either representing the late Wuchiapingian age or earlyChanghsingian. The lower Sunjiagou Formation is composed of grey to greenish-greymudstones and fine grainedsandstones, although the fossil ofTaoheodon itself was found contained within an eroded rock nodule. This erosion resulted in the loss of thezygomatic arches and the tip of the snout from the specimen which had been exposed prior to collection, and the specimen has also been slightly compressed from top to bottom during fossilisation. The specimen was described in 2020 by Jun Liu as a new genus and species,Taoheodon baizhijuni.Taoheodon was named for the nearby Tao He river where it was discovered, combined with theAncient Greek-odous for 'tooth', a common suffix in dicynodont generic names. Thespecific name is in honour of thefossil collector Bai Zhijun who discovered the specimen.[1][4]
Taoheodon was a member of the dicynodontinfraorderDicynodontoidea, and is distinguished from all other dicynodonts by three uniqueautapomorphies: the top of postorbital bars behind the eyes have a shallow depression (fossa) where they meet the rest of the skull, thebasisphenoid (a part on the underside of thebraincase) slopes anterodorsally at a shallow angle in the basisphenoid-basioccipital tubera, and unlike other dicynodonts it lacks a keel on thepterygoid bones of the palate.Taoheodon was included in an updatedphylogenetic analysis of dicynodonts using the combined datasets of Olivieret al. (2019)[5] and Kammerer (2019).[6] Within Dicynodontoidea,Taoheodon was found to group within aclade containingDicynodon and very similartaxa that Liu identified as the "core-Dicynodon" clade.
Thecladogram produced by Liu (2020), simplified and focused on the relationships of dicynodontoids, is shown below:[1]
"Core-Dicynodon" clade |
The results of the analysis are almost identical to the cladograms produced from the previous studies, however, the position of the two Laotian dicynodontsCounillonia andRepelinosaurus differs from their original descriptions. The two Laotian genera were found to clade together withTaoheodon in the "core-Dicynodon" clade, contrasting with the analysis of Olivier and colleagues which originally foundRepelinosaurus to be the basalmostkannemeyeriiform. Liu foundTaoheodon and the Laotian dicynodonts to share a number of features, including notably short snouts, pineal foramens placed further back on the roof of the skull, anteriorly inclinedocciputs, a fairly straight suture between the nasals andfrontals, lacking thepostfrontal bones, fairly flatpostorbitals in the temporal area, and a large fossa on the ventral surface of the intertemporal bar.[1][5]
The close relationship ofTaoheodon to the Laotian dicynodonts suggests that there was a direct link between theNorth China Block,South China Block andIndochina Block that created a corridor of land for dicynodonts in Northern China to disperse into Laos on the Indochina Block and speciate. Thispaleobiogeographic inference has implications for the timing of the collisions between these landmasses, which although uncertainly dated, have typically been inferred to have occurred later during theTriassic period. The presence of a clade of closely related dicynodonts between these landmasses suggests that they were connected in some way by the end of the Permian. Furthermore, their position within a "core-Dicynodon" clade indicates thatTaoheodon was part of a lineage of dicynodonts that could freely migrate from North China throughRussia to South Africa.[1]
In the lower part of the Sunjiagou Formation,Taoheodon coexisted with the diminutivepareiasaurianparareptileSanchuansaurus (a possible a relative of the better knownElginia), a probable largecryptodont dicynodont, and the late surviving semi-aquatic predatoryembolomereSeroherpeton.[4][7][8] The lower part of the Sunjiagou Formation is largely composed offluvial andlacustrine sediments with infrequentcoal seams that indicate a wet, swampy environment with abundant lake-shore habitats that preserve vertical burrows and rich, intensive bioturbation by aquatic organisms.[9][10] Plant remains includeNingxiaites specialis, a partiallydeciduousconifer with anevergreen canopy that would shed only some of its leaves annually.[11]
