Judith River/White River Fossils
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Mammals from White River formation

Rhinoceros 

The rhinoceros is thought to have originated in North America.  There were three families of Rhinoceros during Oligocene times: the Hyracodontidae or cursorial rhinos, the Amynodontidae or aquatic rhinos, and Rhinocerotidae or true rhinos.  Living rhinos are from the latter group only; the first two groups are extinct. 

The Hyracodontidae or Hyracodonts were small, fast, hornless animals, resembling the horses of the day.  There has been speculation that they were out-competed by the horses, leading to the former's extinction.

Rhinos are Perrisodactyls, odd toed ungulates.

Subhyracodon was a member of the "true" (i.e., surviving to this day) rhino family.  Note the large diastema, or gap, between the canines and molars.

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