Rostriamynodon: ancestor to elephants + manatees, not a perissodactyl

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, which nests basal to horses in the large reptile tree (LRT, 1763 taxa; subset Fig. 2).

So, a bit of a taxonomic mess here.
Taxon exclusion is once again the problem. In the LRT (subset Fig. 2) Rostriamynodon nests between the traditional ‘notoungulate’ Notostylops and the elephant + manatee clade.

Isectolophus was also added to the LRT,
(subset Fig. 2) and it nested uncontroversially basal to Protapirus in the tapir clade in both competing studies.

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.

Isectolophus scotti (Scott and Osborn 1887; Early Oligocene to Early Miocene) nests basal to Protapirus in the large reptile tree.

Notostylops murinis (Ameghino 1897, Riggs and Patterson 1935; 75cm in esitmated length; Eocene; FMNH-P13319; Fig. 4) is widely considered a ‘notoungulate’ a clade that has been split into several parts in the large reptile tree. Here it nests with the above specimen of Ectocion in the clade of elephants, rock hyraxes and sea cows. The jaws narrowed anteriorly. The anterior incisors were enlarged, like those of rodents. The mandible was more robust. No canines were present. The premolars were molarized

Rostriamynodon grangeri (Wall and Manning 1986; AMNH 107635; Eocene) was originally considered amynodontid rhino, but here nests between Notostylops and the elephant + siren clade. 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.

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