Enigmatic oreodont, Merycoidodon, joins the LRT

Something of an enigma.
Wikipedia reports, “Merycoidodon is an extinct genus of terrestrial herbivore.” That’s rather vague for a common sheep-sized fossil from the USA.

Figure 1. Merycoidodon reconstruction traced by an unknown artist from an AMNH mount photo.

Figure 1. Merycoidodon reconstruction traced by an unknown artist from an AMNH mount photo.

In Late Eocene
to Late Oligocene (38–16mya) deposits, Merycoidodon (Leidy 1848) lived in large herds, principally in South Dakota. but also found from Alberta to Florida, typically preferring well-watered areas. Leidy considered it a member of the ruminantoid Pachydermata‘.

Figure 2. Merycoidodon skull. Colors added.

Figure 2. Merycoidodon skull. Colors added.

Merycoidodontoidea
Wikipedia reports, “Merycoidodontoidea, sometimes called “oreodonts,” or “ruminating hogs”, is an extinct superfamily of prehistoric cud-chewing artiodactyls with short faces and fang-like canine teeth. As their name implies, some of the better known forms were generally hog-like, and the group has traditionally been placed within the Suina (pigs, peccaries and their ancestors), though some recent work suggests they may have been more closely related to camels.” Evidently the phylogenetic nesting of Merycoidodon is not clear to the Wikipedia writers. That may be due to its generalize appearance.

Spaulding et al. 2009
nested Merycoidodon ancestral to Camelus + Lama, derived from Hyracotherium and Cainotherium, among tested taxa. The Spaulding et al. cladogram separated hippos from mesonychids, nesting hippos with Diacodexis (largely incomplete) and Indohyus, an omitted tenrec in the LRT.

Figure 3. the Merycoidodon cladogram includes hippos, whales and a number of extinct taxa.

Figure 3. the Merycoidodon cladogram includes hippos, whales and a number of extinct taxa.

In the large reptile tree
(LRT, 1376 taxa) Merycoidodon nests firmly as the proximal outgroup at the base of the Mesonyx to mysticete (baleen whale) clade (subset Fig. 3). Merycoidodon also nests between the Phenacodus clade and the Homalodotherium clade + artiodactyl clades.

Figure 1. Ocepeia: before and after. The original reconstruction is here compared to a tracing of CT scan, duplicated left to right.

Figure 4. Ocepeia: before and after. The original reconstruction is here compared to a tracing of CT scan, duplicated left to right.

It is worth noting again
that hippos do not nest with artiodactyls in the LRT, breaking a traditional paradigm.

Figure 1. Mesonyx, the first known mesonychid was a sister to Hippopotamus in the large reptile tree. So maybe it was a plant eater.

Figure 5 Mesonyx nests between oredonts, like Merycoidodon, and hippos, like Hippopotamus.

I’ve been curious about oreodonts for decades.
What were they? Happy to finally test it and nest it where it belongs, basal to hippos, and transitional to modern hoofed ruminants. The generalized appearance of Merycoidodon is appropriate to its basal and transitional nesting. Based on its nesting basal to Ocepeia (middle Paleocene), the genesis of Merycoidodon must extend to the early Paleocene, if not before.

Figure 3. Hippopotamus. This stout, wide-faced, fanged mammal does not nest with deer.

Figure 6. Hippopotamus. This stout, wide-faced, fanged mammal does not nest with deer,but with Mesonyx.

References
Leidy 1848. On a new fossil genus and species of ruminantoid Pachydermata: Merycoidodon culbertsonii. Proceedings of the Academy of Natural Sciences of Philedelphia Vol IV, 47-51.

Merycoidodontidae (Thorpe 1923)
Mesonychidae (Cope 1880)

wiki/Merycoidodontoidea
wiki/Merycoidodon

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