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.