Merychychus, Agriochoerus antiquus and Rostramynodon and Rostramynodon

Merychyus elegans (Leidy 1858) was a North American Miocene herbivore with a retracted naris, tusk-like canine teeth, heavy body, long tail, short feet, and four-toed hooves. In the large reptile tree (LRT, 2340 taxa) Merychyus nests basal to hippos + desmostylians + mysticetes, close to Agriochoerus antiquus.

Agriochoerus antiquus (Leidy 1850, North America, Oligocene) was also basal to hippos, and desmostylians, derived from oreodonts.

Rostriamynodon grangeri (Wall and Manning 1986; AMNH 107635; Eocene) was originally considered an amynodontid rhino. In the LRT it nests with oreodonts like Agriochoerus antiquus basal to desmostylians and mysticetes. Note the shape of the nasals and the anterior extension of the frontals along with the wide molars and the molarized premolars.

Leidy J 1850. Descriptions of Rhinoceros nebrascensis, Agriochoerus antiquus, Palaeotherium proutii, and P. bairdii." Proc. Acad. Nat. Sci. Philadelphia 5: 121-22.

Leidy J 1858. Notice of Remains of Extinct Vertebrata, from the Valley of the Niobrara River, Collected during the Exploring Expedition of 1857, in Nebraska, under the Command of Lieut. G. K. Warren, U. S. Top. Eng., by Dr. F. V. Hayden, Geologist to the Expedition. Proceedings of the Academy of Natural Sciences of Philadelphia 10:15-89.

Wall WP and Manning E 1986. Rostriamynodon grangeri n. gen., n. sp. of amynodontid (Perissodactyla, Rhinocerotoidea) with comments on the phylogenetic history of Eocene Amynodontidae. Journal of Paleontology 60(4):911–919.

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