PALEOECOLOGY OF OREODONTS (MERYCOIDODONTOIDEA) WITHIN THE WHITE RIVER FORMATION; PRAIRIE DOGS OF THE OLIGOCENE

Oreodonts (Merycoidodontoidea) are the most commonly collected fossil from the White River Formation of central North America. Locally comprising 25% of the fauna, they were previously thought of as sheep-like ruminants. Migrating in large herds, they were presumably protected by sheer numbers and musk ox like defenses from sabertooth cats, Hyaenodons, and Archaeotherium predators. The White River Fm. was composed primarily of soft, yet firm, silty and ash-rich mudstones, a perfect digging and denning medium.

Observations from 36 multiple oreodont specimens (2-10 oreodont individuals per specimen) indicate they were burrowing denning mammals, using their distinctive large canines for defensive purposes, fending off predators at the mouth of their burrows. Most multiple specimens with unborn fetuses had 3 to 5 fetuses. Eight multiple specimens were composed of subadults (siblings) with 3 to 5 individuals in calcareous enriched nodules (dens) that vary from .5m x .6m x .2m to 2m x 1m x .3m in size. Two thirds of the subadult specimens were disarticulated (predated) with mostly skulls, feet and broken skeletal elements in den size nodules. These multiple oreodont specimens, both predated and nonpredated, are the primary physical evidence of burrowing, denning, and social behavior of oreodonts.

Multiple fetuses (3-5), digging feet with claws, large heads, short stocky bodies, large genetic diversity, and even the diagnostic large canines are additional evidence of a burrowing lifestyle. Two spectacular specimens with 6 and 10 individuals of different ontogenetic ages prove the social nature of oreodonts. The largest specimen contained 1 adult, 4 subadults, and five juveniles in a single large burrow.

Oreodonts dug burrows into soft consolidated sediments, raised litters of offspring, occupied dens all year, and defended themselves from predators at the burrow entrances. Predators successfully penetrating dens feasted on juveniles and subadults, leaving leftover skulls, feet, and broken bones within the den, which collapsed and form the core of nodules we find them in today. Oreodonts occupied an ecological niche similar to modern prairie dogs. The prolific oreodont was a keystone species within a burrowing community of organisms during the Oligocene.