Thomashuxleya: another former notoungulate, joins the Condylarthra

Yet another
former notoungulate, dog-bear-like Thomashuxleya rostrata (Ameghino 1901; Eocene, 48-40 mya; 0.33m skull length, 1.3 m overall length; Fig. 1). Named after Thomas Huxley, Darwin’s ‘bulldog’ (his strong supporter), this peccary-mimic had a generalized 11 teeth (per quadrant), four toes (per foot, but look at the odd distribution! and meanwhile several descendants retain five toes so this is a red flag on the data). Since all the data for this taxon currently comes from this drawing, based on several specimens and still incomplete, the manus data might be a little untrustworthy… to say the least. The pedal data is conjectural from the start.

Figure 1. Thomashuxleya using the only available data. Here the dental formula is 3.1.4.3.

Figure 1. Thomashuxleya using the only available data. Here the dental formula is 3.1.4.3. Skeleton modified from the chimaera created by Simpson 1967. Based on this data is not a notoungulate nor a toxodont. That diverging thumb suggests a climbing ability, lost in all later taxa. Plantigrade pes and digitigrade manus.

Tred carefully here
|the reconstruction offered (Fig. 1) is based on an old line drawing and that is based on a chimaera of specimens. Back in the day, as we’ve seen with other museum mounts, that was common practice. Thomashuxleya has been referred to the Isotemnidae family of Notoungulata. Evidently other taxa in this family are known from isolated teeth or jaw fragments, but not enough to warrant their own Wikipedia page. Dr. Darin Croft includes Toxodon as a sister to this clade.

According to Croft 2016
Isotemnids have low-crowned teeth that gradually decrease in size from back to front without a diastema. That also sounds like Arsinoitherium, doesn’t it? So, it’s not a unique trait.

Several new basal condylarths
have been added to the LRT. We’ll look at them soon.

References
Ameghino F 1901. Notices préliminaires sur des ongulés nouveaux des terrains crétacés de Patagonie [Preliminary notes on new ungulates from the Cretaceous terrains of Patagonia]. Boletin de la Academia Nacional de Ciencias de Córdoba 16:349-429
Croft D 2016.
Horned armadillos and rafting monkeys: the fascinating fossil mammals of South America. Indiana University Press 320 pp.
Simpson GG 1967. The beginning of the age of mammals in South America. Part II. Bulletin of the American Museum of Natural History 137, 1-260.
Simpson GG 1980Splendid Isolation, the Curious History of South American Mammals. Yale University Press, New Haven, CT.

tetrapod-zoology/isotemnid-toxodonts-2012/
wiki/Thomashuxleya

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