Rostriamynodon enters the LRT with amynodontid ‘rhinos’

Revised March 10, 2022
with a new tracing of Rostriamynodon (Fig 1) and a new nesting.

Holbrook 1999
added Rostriamynodon (AMNH 107635, Fig. 1) to his study on perissodactyls (Fig. 3) following work by Wall and Manning 1986 who thought Rostriamynodon was a basal rhino close to Amynodon, In the large reptile tree (LRT, 1763 taxa then, 2061 taxa on 3.10.22; subset Fig. 2) that is confirmed.

Figure 1. Rostriamynodon skull in three views, colors added.

Figure 1. Rostriamynodon skull in three views, colors added.

Figure 3. Cladogram from Holbrook 1999 with LRT colors added. Taxon exclusion mars this cladogram.

Figure 3. Cladogram from Holbrook 1999 with LRT colors added. Taxon exclusion mars this cladogram.

Taxa in this blogpost:
Amynodon advenus (Marsh 1877; 1m in length; Oligocene-Eocene, 40-23 mya) was originally considered an aquatic rhino. Here it nests with Mesohippus. The long neck and other traits are more horse-like than rhino-like. Manual digit 5 was retained. The skull was deeper as in basal forms like Hyracotherium.

Rostriamynodon grangeri (Wall and Manning 1986; AMNH 107635; Eocene) was originally considered amynodontid rhino. Note the splitting of the nasals and the anterior extension of the frontals along with the wide molars and the molarized premolars.

References
Hollbrook LT 1999. The phylogeny and classification of Tapiromorph perissodactyls (Mammalia). Cladistics 15:331–350.
Holbrook LT, Lucas SG and Emry RJ 2004. Skulls of the Eocene perissodactyls (Mammalia) Homolgalax and Isectolophus. Journal of Vertebrate Paleontology 24(4):951–956.
Scott WB and Osborn HF 1887. Preliminary Report on the Vertebrate Fossils of the Uinta Formation, Collected by the Princeton Expedition of 1886. Proceedings of the American Philosophical Society 24(126):255-264.
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.

wiki/Notostylops
wiki/Rostriamynodon not yet posted
wiki/Protapirus
wiki/Isectolophus not yet posted

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