Achelousaurus (Achelous’s lizard) is a genus of dinosaur that lived in what is now North America during the Late Cretaceous Period. The genus was named by paleontologist Scott Sampson in 1995. The type species is A. horneri. The specific name, horneri, honors Jack Horner, an American paleontologist famous for his Montana dinosaur discoveries.
Achelousaurus was a quadrapedal (walked on all fours) herbivore with a parrot-like beak and a rough bony area on the snout and two more behind the eyes. It also had two horns on the end of its long bony neck frill. The total body length of this specimen was about 20 feet. It was considered a medium-sized ceratopsian. The Achelousaurus skulls found have bony areas in the same places where other ceratopsians had horns, giving it the appearance of having its horns ripped off.
Achelousaurus was discovered in the U.S. state of Montana, in the Two Medicine Formation (a geologic formation deposited between 83.5 and 70.5 Mya). Achelousaurus lived during the Late Cretaceous Period and its fossils were found on the highest levels of the formation, so it is estimated that this specimen lived near the end of that timeframe. The skull of the adult Achelousaurus measures over five feet long. The fossils found of this specimen are housed at the Museum of the Rockies in Bozeman, Montana.