More Mysterious Palaeogene Eutherians

A few weeks ago, I wrote a post about some of the distinct groups of eutherian mammals that waddled through the world during the Palaeocene, the time period that followed directly after the end of the Cretaceous. At the time, many of the modern groups of mammals were either still fairly marginalised or yet to put in an appearance, and the relationships of most of those primordial eutherians such as pantodonts and taeniodonts remains a remarkable mystery. In this post, I thought I'd focus on one of those early groups that seems to get given an even shorter shrift than most (in fact, this post will be unillustrated because my attempts to find suitable free images online drew a complete blank) - the Tillodontia.

Tillodonts are known only from the Palaeocene and Eocene of North America and Eurasia. Most authors have recognised a single family, the Esthonychidae, though Lucas & Schoch (1998) positioned the genera Lofochaius and Basalina as a paraphyletic series outside that family*. They were medium to large herbivores (one of the later genera, Trogosus, may have weighed around 150 kg - Lucas & Schoch, 1998). Like most mammals of the time, these would not have been the most graceful of beasts - they would have probably been built more like a barrel on legs, perfect for the moist, densely-forested conditions of the time. One of the most distinct features of the tillodonts was the development of large, rodent-like incisors, which in one later clade became open-rooted and permanently-growing like those of rodents. The powerful dentition this gave tillodonts, together with the sturdy legs and claws found in those few species for which post-cranial material is known, would have allowed them to tackle some pretty resilient food-sources, and it is easy to imagine them gnawing bark off trees or digging up roots. A similar lifestyle appears to have also characterised another group of Palaeogene herbivores, the taeniodonts, which also developed rodent-like gnawing teeth. It was once suggested on this basis that taeniodonts and tillodonts were closely related to each other, but the gnawing teeth in taeniodonts were the canines, not the incisors, so the two groups could not have possibly shared a common gnawing ancestor.

*The authors of the late Palaeocene Chinese genus Yuesthonyx (Tong et al., 2003) established a new family for it, Yuesthonychidae. Not only would this family be redundant with its single genus, but Rose (2006) implies that Yuesthonyx is a more derived form not far from the origin of the Trogosinae (see below), making the recognition of a separate family for it all the more pointless.

The very earliest tillodonts such as Lofochaius and Meiostylinodon come from the Lower Palaeocene of China, and this would appear to represent the place of origin for the clade (Rose, 2006). The early Chinese genera were much smaller than the later trogosines, and had less exaggerated dentition. The first North American tillodonts make their appearance in the very end of the Palaeocene with the similarly generalised Azygonyx which survived into the beginning of the Eocene alongside Esthonyx, the most common genus of tillodonts. These forms all lacked permanently-growing incisors, the appearance of which marks the appearance of the clade Trogosinae in the Eocene. Trogosines are known from both North America (Tillodon and Trogosus) and China (Higotherium and Chungchienia), so their geographic origins are unclear. The Chinese Chungchienia had the most advanced dentition of any tillodont - not only were the second incisors a whopping 26 cm long(!), but the ever-growing rootless condition of the incisors was extended to the cheek-teeth (Chow et al., 1996), implying that it must have had an exceedingly tough diet.

While it is fairly well-established that tillodonts were not related to taeniodonts, it has been a decidedly more difficult prospect to establish exactly what they are related to. Van Valen (1963) suggested a close relationship to Arctocyonidae, a family of "condylarths", but this was based on comparisons with the relatively derived North American Esthonyx rather than the mostly then-undiscovered Asian genera. More recent authors have suggested a relationship with the pantodonts, with which tillodonts share dilambdodont cheek teeth. Basal tillodonts may also be difficult to distinguish from basal pantodonts (Rose, 2006). The Palaeocene North American Deltatherium may also be relevant to the origin of tillodonts. However, none of these groups has yet been subject to a proper cladistic analysis to determine whether their shared features indicate actual relationship or convergence. And even if these taxa do form a monophyletic clade, this still just takes a number of small problematic clades of unknown relationships to modern taxa and turns them into one big clade of unknown relationships to modern taxa!

REFERENCES

Chow, M., J. Wang & J. Meng. 1996. A new species of Chungchienia (Tillodontia, Mammalia) from the Eocene of Lushi, China. American Museum Novitates 3171: 1-10.

Lucas, S. G., & R. M. Schoch. 1998. Tillodontia. In Evolution of Tertiary Mammals of North America (C. M. Janis, K. M. Scott & L. L. Jacobs, eds.) pp. 268-273. Cambridge University Press.

Rose, K. D. 2006. The Beginning of the Age of Mammals. JHU Press.

Tong Y.-S., Wang J.-W. & Fu J.-J. 2003. Yuesthonyx, a new tillodont (Mammalia) from the Paleocene of Henan. Vertebrata PalAsiatica 41: 55-65.

Van Valen, L. 1963. The origin and status of the mammalian order Tillodontia. Journal of Mammalogy 44 (3): 364-373.