Oligocene Hyracodon
Hyracodon nebrascensis
Brule Formation
White River Group
Middle Oligocene
Shannon County, South Dakota
This specimen is a good example of a maxilla section from the ancestral rhinoceros Hyracodon. It contains two complete molars from the right side of the skull. It is from an adult individual of young age base on the amount of ware seen. This sample is about 6 cm long.

Hyracodontids are best known in North America in the form of Hyracodon nebrascensis. The family may be found from the late Uinta (Late Eocene) to the Arikareean (Middle Oligocene). All were cursorial, sheep-sized mammals reaching 5 ft in length with a light chest and long legs for running. Hyracodons had no horns, thus giving them a horse-like appearance, hence the common name "running rhino".

Distantly related to the gigantic Paraceratherium of Asia, hyracodons were primitive browsers possessing simple dentitions that roamed in localized herds within the open forest and savannah. Migrating freely between North America and Asia during the Eocene, all genera but one, Hyracodon, disappeared by the end of the Chadronian. Hyracodon is distinguished from earlier genera by its longer limbs, and its ecological niche is sometimes compared to that of African equids or bovids. All species of this Family disappeared in the early Arikareean, probably because of the increased aridity that affected many other persistent White River faunas at the end of the Whitneyan and the appearance of modern grasses.