Pachycephalosaurus (; meaning "thick-headed lizard," from Greek "pachys-/" "thick", "kephale/" "head" and "sauros/" "lizard") is a genus of pachycephalosaurid dinosaurs. The type species, "P. wyomingensis", is the only known species. It lived during the Late Cretaceous Period (Maastrichtian stage) of what is now North America. Remains have been excavated in Montana, South Dakota, Wyoming and Alberta. It was a herbivorous creature which is primarily known from a single skull and a few extremely thick skull roofs, at 9 inches thick. More complete fossils have been found in recent years. "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", as have the genera "Stygimoloch" and "Dracorex" in recent studies.