Archaeotherium

Archaeotherium (, meaning "ancient beast") is an extinct genus of entelodont artiodactyl endemic to North America during the Eocene and Oligocene epochs (35–28 mya). Archaeotherium fossils are most common in the White River Formation of the Great Plains, but they have also been found in the John Day Basin of Oregon and the Trans-Pecos area of Texas. Archaeotherium's fossils come from North America, between the Priabonian and Rupelian stages of the Eocene and Oligocene (35–28 million years ago). Up to fifteen species of Archaeotherium have been identified, which are divided into three subgenera. One contains the type species, A. mortoni, among others; another contains very large taxa formerly named Megachoerus and Pelonax; and the last contains A. calkinsi.

Archaeotherium was distinguished from most entelodonts by having an unusually long snout and large jugal flanges, extensions of the zygomatic arches that are characteristic of the group. The latter may be the result of sexual dimorphism or species differentiation. Healed bite marks on several Archaeotherium specimens suggest that it engaged in facial biting behaviours, similar to dromedary camels, and their faces consequently may have had thickened skin for protection. Archaeotherium had powerful neck muscles, which were supported by a bony hump comprising the first six thoracic vertebrae. Its digits were unfused, capable of spreading, and may have supported a fleshy pad, enabling it to move more effectively on soft terrain. Like other entelodonts, it had large incisors and canines, triangular premolars, and small, bunodont molars. Archaeotherium species varied in size, with A. mortoni having a skull length of , and A. zygomaticus having a skull length of .