In the late 19th century, rare fossil remains of a small chalicothere were recovered from early Miocene rocks of the John Day Formation of north-central Oregon. Totaling eight isolated teeth and four foot bones, and gathered by various collectors from several localities, these specimens were not at first recognized as chalicotheres, and were originally assigned to a dental species, Lophiodon? oregonensis, and a postcranial species, Moropus distans. All were assigned to the chalicothere genus Moropus by Holland and Peterson in 1914. The lack of precise geographic and stratigraphic data did not permit any definitive assessment of age, origin, or evolutionary stage.

We report here newly discovered teeth and foot bones of M. oregonensis found from 1994–1998 at precisely located levels in the John Day Formation. These new discoveries indicate that M. oregonensis occurs in upper John Day units but is absent from the lower part of the formation. One tooth occurs in proximity to a tuff dated at 22.6 ± 0.13 Ma, hence establishes a minimum age for the appearance of the species in North America. New dental and foot elements allow us to synonymize Moropus distans with M. oregonensis, now the type species of the genus Moropus. Additionally, a small chalicothere, probably M. oregonensis, occurs at four Arikareean Gulf Coast sites in north Florida and east Texas. It is suggested that these animals, along with the Oregon chalicotheres, represent relatively primitive populations inhabiting mesic coastal environments of the Pacific margin and southern United States in the early Miocene.