Pachycephalosaurus is a genus of pachycephalosaurid dinosaur. It lived during the Late Cretaceous Period (Maastrichtian stage) of what is now North America. Remains have been excavated in Montana, South Dakota, and Wyoming. It was an herbivorous or omnivorous creature which is only known from a single skull and a few extremely thick skull roofs. This dinosaur is monotypic, meaning the type species, P. wyomingensis, is the only known species. Pachycephalosaurus was one of the last non-avian dinosaurs before the Cretaceous_Paleogene extinction event. Another dinosaur, Tylosteus of western North America, has been synonymized with Pachycephalosaurus.