Nesting the tapir-mimic Astrapotherium with mesonychids and hippos

Astrapotherium magnum (Burmeister 1879; Paleocene-Miocene, 59-12 mya; 3m long; Fig. 2 ) was considered a South American ungulate and a member of the order, Astrapotheria, by the authors of Wikipedia. They report, “The history of this order is enigmatic.”

Here
(Fig. 2)  Astrapotherium nests
with the basal mesonychid, Mesonyx, at the base of the clade that ultimately produced Hippopotamus. This LRT relationship has not been recovered by prior workers. It is a derived representative of a basal clade. Perhaps that is what has made it historically difficult to nest.

Like a hippo,
the large and ever-growing curved canines of Astrapotherium scraped against each other during life producing sharp tips. Uniquely, the rostrum was much shorter than the mandibles. The feet and toes were all small. The narial opening was elevated to the top of the skull. Astrapotherium likely had a tapir-like (Fig. 3) trunk.

Hatcher 1901 reports,
“In my figure of the fore foot… there is an error for which I am quite unable to account in the substitution of a phalanx for the first metacarpal.” (I can only take Hatcher at his word, Fig. 1).

The large canines of Astrapotherium
distinguish it from Tapirus (Fig. 3) and ally it with Mesonyx, Harpogolestes and Hippopotamus.

The large reptile tree, with its wide gamut of taxa, provides more than 800 opportunities for all new additions to nest within. And subsequent additions might nest between two current sisters, separating them. This provides further evidence that Hippopotamus is not like the other ungulates, which nest elsewhere. Rather it is a living representative of the Mesonychidae, a clade traditionally considered extinct.

References
Burmeister 1879. Description physique de al République Agentine, T. III 1879:517.
Hatcher JB 1901. Report of the Princeton University Expeditions to Patagonia 1869-1899. Mammalia of the Santa Cruz Beds. IV. Astrapotheria. Scott WB ed. Vol. 6, Paleontology 3. Princeton, NJ Stuttgart 1909-1928.

wiki/Astrapotherium