Archaeotherium (, meaning "ancient beast") is an extinct genus of entelodont artiodactyl endemic to North America during the Eocene and Oligocene epochs (35—28 mya), existing for approximately . Archaeotherium fossils are most common in the White River Formation of the Great Plains, but it has also been found in the John Day Basin of Oregon and the Trans-Pecos area of Texas. Taxonomy Archaeotherium was named by Joseph Leidy (1850). Its type is Archaeotherium mortoni. It was synonymized subjectively with Entelodon by Leidy (1853) and synonymized subjectively with Elotherium by Leidy (1857). It was assigned to Entelodontidae by Leidy (1850), Peterson (1909), Scott (1940), Galbreath (1953), Russell (1980), Carroll (1988) and Effinger (1998). Archaeotherium, along with all other Entelodontidae, is an artiodactyl whose exact taxonomic position has been disputed, but taxonomists agree the group lies between the Suina (pigs and peccaries), and the anthracotheres, hippopotamuses, and whales. Description Archaeotherium was about tall at the shoulder and around long. Adults weighed 150–250 kg (330-530 lbs). The largest specimens, though much less common, which had been described under the name Megachoerus had skulls up to long, stood about tall at the shoulder and weighed up to . There is no sign of sexual dimorphism in the canine teeth; females and males may have been similar in size. Though Archaeotherium is one of the best preserved of all entelodonts, skulls of entelodonts are far more commonly preserved than full skeletons, and the large size of the head in proportion to the body makes it difficult to accurately estimate the animal's living size and weight. In life, Archaeotherium probably somewhat resembled a cow-sized peccary with a longer face, humped shoulders, wide cheekbones, and bosses on the face similar to male warthogs, but without a pig-like nasal disk. Paleobiology It lived in forested and riverbank environments before the evolution of grasslands. Like all entelodonts, the genus had typical artiodactyl legs but lacked specializations for fast running; though it supported its weight on cloven hooves, the foot bones remained unfused, and the toes could spread as camelid feet do. This structure, unique to entelodonts, may have helped the living animal move on soft ground. The head was unusually large, and the high spines on the vertebrae above the shoulders supported strong neck muscles and tendons to handle the weight of the head. The brain was tiny, but had relatively large olfactory lobes, suggesting that the animal had