Achelousaurus ( /əˌkiːloʊˈsɔːrəs, ˌækɪˌloʊəˈsɔːrəs/[1]) is a genus of centrosaurine ceratopsid dinosaur that lived during the Late Cretaceous Period of what is now North America, about 74.2 million years ago. The first fossils of Achelousaurus were collected in Montana in 1987, by a team led by Jack Horner, with more finds made in 1989. In 1994, Achelousaurus horneri was described and named by Scott D. Sampson; the generic name means "Achelous lizard", in reference to the Greek deity Achelous, and the specific name refers to Horner. The genus is known from a few specimens consisting mainly of skull material from individuals, ranging from juveniles to adults.
A large centrosaurine, Achelousaurus supposedly was about 6 m (20 ft) long, with a weight of about 3 t (3.3 short tons). As a ceratopsian, it walked on all fours, had a short tail and a large head with a hooked beak. It had a bony neck-frill at the rear of the skull, which sported a pair of long spikes, which curved towards the outside. Adult Achelousaurus had rough bosses (roundish protuberances) above the eyes and on the snout where other centrosaurines often had horns in the same positions. These bosses were covered by a thick layer of keratin, but their exact shape in life is uncertain. Some researchers hypothesize that the bosses were used in fights, with the animals butting each other's heads, as well as for display.
Within the Ceratopsia, Achelousaurus lies within the clade Pachyrostra (or "thick-snouts"). It has been suggested that it was the direct descendant of the similar genus Einiosaurus (which had spikes but no bosses) and the direct ancestor of Pachyrhinosaurus (which had larger bosses). The first two genera would be transitional forms, evolving through anagenesis from Styracosaurus. There has been debate about this theory, with later discoveries showing that Achelousaurus is closely related to Pachyrhinosaurus in the group Pachyrhinosaurini. Achelousaurus is known from the Two Medicine Formation and lived in the island continent of Laramidia. As a ceratopsian, Achelousaurus would have been a herbivore and it appears to have had a high metabolic rate, though lower than that of modern mammals and birds.
