English: Achelousaurus horneri skull on display at the Museum of the Rockies in Bozeman, Montana.

This fossil was collected in 1987 in the Two Medicine Formation in Glacier County, Montana.

Achelousaurus lived about 74 to 72 million years ago. Only skulls have been found, so it is difficult to say how long or heavy they were. Adult skulls were about 5 feet in length.

Achelousaurus is a Centrosaurinae. Ceratopsids are a group of heavy, four-legged, frilled, beaked herbivores. The most famous of these is Triceratops. The Ceratopsinae, a family of the Ceratopsids, had long, triangular frills and eyebrow horns. The Centrosaurinae, the other family, had shorter and more rectangular frills, elaborate spines on the back of the frill, no eyebrow horns, and better-developed nasal horns.