Oligocene Oreodont Merycoidodon
Merycoidodon sp.
Brule Formation
Middle Oligocene
Niobrara County, Wyoming
This is very good example of a Merycoidodon lower jaw section from the Middle Oligocene Brule Formation. This example is from the right side of the skull and contains 2 complete molars. The specimen is from an adult as indicated by the ware seen on the typical selenodont teeth. The maxilla section is about 5 cm long.

Merycoidodon (ruminating teeth) is an extinct genus of terrestrial herbivore of the family Merycoidodontidae, subfamily Merycoidodontinae (oreodont), more popularly known by the name Oreodon (hillock teeth) endemic to North America during the Late Eocene-Early Miocene sub-epochs (38—16.3 mya) existing for approximately 21.7 million years. Merycoidodon was named by Leidy (1848). The oreodonts are a group of artiodactyls related to camels that originated in North America. Its ancestors date back to the Eocene and its last descendants are known from the Pliocene, so that oreodonts, broadly speaking, lived throughout the whole of the Tertiary era.

Merycoidodon would have somewhat resembled a pig in appearance, but had a longer body, at about 1.4 metres (4.6 ft), and short limbs. The fore limbs had five toes (although the first one was vestigial), while the hind limbs had four. Given the shape of the limbs, it is unlikely that the animals would have been able to run fast. Unlike modern ruminants, they had a full set of teeth, although the molars were adapted for grinding up tough vegetation. Notably, they had strong, and very striking, canines.

The skulls of Merycoidodon have a pit in front of the eyes. Similar pits are found in the skulls of modern deer, where they contain a scent gland used for marking territory. Although Merycoidodon was not directly related to deer, it seems likely that it possessed a similar gland, which may imply that it, too, was territorial. Oreodonts lived in large herds and moved about from place to place. They seem to have had a predilection for well-watered regions, where food was plentiful and succulent. The number of fossils found implies that, at one time, oreodonts were as plentiful in South Dakota as zebras are today on the Serengeti Plains.