Aumelasia, Sespia, Leptauchenia, Brachycrus and Merycoidodon

Aumelasia gabineaudi (Sudre 1979 (1980) Middle Eocene, 51–43mya, 42cm long (juvenile), skull 9.2cm long) was orginally described as a basal artiodactyl (Dichobunidae) based on its dentition. Here it nests as a basal oreodont, close to Merycoidodon.

The tail is quite long with 24 vertebrae. The mandible was massive. The incisors were spatulate. The lower canine was small. Five fingers and four toes were present with longer metatarsals than metacarpals. The hind limb was longer than the fore limb. Three complete skeletons from the Messel Pit are known, plus other scraps from other localities. Aumelasia ate leaves, seeds and fruit.

Brachycrus laticeps (originally Merycochoerus Douglass 1900; CM796; Miocene, 14mya; 1m long) was considered an North American oreodont with a tapir-like trunk and deep jaws, but here nests with South American Astrapotherium.Both are derived from North American Meniscotherium.

Sespia ultima (Schultz and Falkenbach 1968, Late Oligocene, 25 mya) is a small oreodont, nesting basal to the others in the LRT. Note the large antorbital fenestra (aof) here and in other species. Arrow points to the very loose jaws joint apparently not so loose in two other species. Note the very large tympanic bulla (dark yellow) and raised auditory meatus, as in extant horses by convergence.

Leptochaunia dorsa (Leidy 1856, Prothero and Sanchez 2008, Oligocene) was once considered an aquatic herbivore based on its high-placed orbit and ear. Prothero and Sanchez exchanged water for sand burrowing since this oreodont is found in former sand dunes. Perhaps the high-placed sensory organs migrated there to accommodate larger jaw muscles for grazing.

Merycoidodon culbertsoni (Leidy 1848; Late Eocene to Late Oligocene, 38–16 mya; 1.4 m in length)has been a traditional enigma, but nests here with Phenacodus at the base of the Mesonyx to mysticete (baleen whale) clade. Those two also nest at the base of the rest of the oreodonts at the base of the Homalodotherium clade + artiodactly clades. It is noteworthy that hippos do not nest with artiodactyls in the LRT, breaking a traditional paradigm. Merycoidon was a sheep-sized, short-legged herbivore living in herds that preferred well-watered areas. The forelimbs had five toes. The hindlimbs had four. The molars of these oreodonts were adapted for grinding, but the canines remained long as in mesonychids and hippos.

Oodectes herpestoides (Wortman 1901; late Paleocene to Middle Eocene) is a tiny sister to Merycoidodon in the LRT. Thse gave rise to ungulates. Note the elongate palatine. The premaxillary teeth remain in contact with each other. The premaxilla is transverse with procumbent dentition. The postorbital bar was likely complete. Hippos hone their canines and this taxon shows the genesis of the two teeth interacting.

Cope ED 1884. Synopsis of the species of Oreodontidae. Paleontological Bulletin 38:503-572.

Douglass E 1900. New species of Merycochoerus in Montana. Part I. American Journal of Science 10(60):428-438.

Leidy J 1848. On a new fossil genus and species of ruminantoid Pachydermata: Merycoidodon culbertsonii. Proceedings of the Academy of Natural Sciences of Philadelphia 4: 47–51.

Leidy J 1856. Notices of extinct Vertebrata discovered by Dr. F. V. Hayden, during the
expedition to the Sioux country under the command of Lieut. G. K. Warren. Ibid., vol. 8, pp. 311-312.

Prothero DR and Sanchez F 2008. Systematics of the Leptaucheniine oreodonts (Mammalia: Artiodactylia) from the Oligocene and earliest Mioceneof North America. In: Lucas et al., eds., 2008, Neogene Mammals. New Mexico Museum of Natural History and Science Bulletin 44.

Schultz CB and Falkenbach CH 1968. The phylogeny of the oreodonts, Parts 1 and 2. Bulletin of the American Museum of Natural History 139:498pp.

Sudre J 1980. Aumelasia gabineaudi n.g. n.sp. nouveau Dichobunidae (Artiodactyla, Mammalia) du gisement d'Aumelas (Hérault) d'âge Lutétien terminal. Palaeovertebrata, Mémoire Jubilaire R. Lavocat 9:197–211.

Thorpe MR 1921. Two new forms of Agriochoerus. American Journal of Science (8): 111–126.

Wortman JL 1901. Studies of Eocene Mammalia in the Marsh Collection, Peabody Museum. The American Journal of Science, series 4 12(70):281-296.

wiki/Aumelasia- in German
wiki/Oodectes
wiki/Merycoidodon
wiki/Sespia
wiki/Leptauchenia
wiki/Brachycrus