Eobasileus

Eobasileus ("dawn-king") is a genus of dinoceratan mammal in the family Uintatheriidae. One species is currently considered valid, E. cornutus, though it has been suggested that the closely related Tetheopsis may also belong to this genus. The first specimen of Eobasileus, consisting of a skull, several vertebrae, and bones associated with the limbs, was collected from Haystack Mountain, from strata belonging to the Washakie Formation. The species became the subject of various letters written by Edward Drinker Cope (its describer) and Othniel Charles Marsh, and their disagreements over its taxonomy and anatomy contributed to the beginning of the Bone Wars.

Eobasileus was the largest dinoceratan, measuring around at the shoulder. It looked much like the related genus Uintatherium, though with a far more elongated skull. Like Uintatherium, it had three pairs of blunt horns on its skull, possibly covered with skin like the ossicones of a giraffe. Eobasileus also had a pair of tusks shielded by bony protrusions of the lower jaw, which were likely larger in males than in females. These tusks were longer than those of any other dinoceratan, were recurved towards their tips, and were hollow for about half of their length. Being a dinoceratan, Eobasileus was likely a hindgut fermenter, having a single-chambered stomach wherein plant matter was broken down by microbial activity.