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, but some researchers argue that there might be a second species, P. spinifer. 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 22 centimetres (9 inches) thick. More complete fossils have been found in recent years. Pachycephalosaurus was among the last non-avian dinosaurs before the Cretaceous–Paleogene extinction event. The genus Tylosteus has been synonymized with Pachycephalosaurus, as have the genera Stygimoloch and Dracorex in recent studies. Like other pachycephalosaurids, Pachycephalosaurus was a bipedal herbivore with an extremely thick skull roof. It possessed long hindlimbs and small forelimbs. Pachycephalosaurus is the largest-known pachycephalosaur. The thick skull domes of Pachycephalosaurus and related genera gave rise to the hypothesis that pachycephalosaurs used their skulls in intra-species combat. This hypothesis has been disputed in recent years. History of discovery Remains attributable to Pachycephalosaurus may have been found as early as the 1850s. As determined by Donald Baird, in 1859 or 1860 Ferdinand Vandeveer Hayden, an early fossil collector in the North American West, collected a bone fragment in the vicinity of the head of the Missouri River, from what is now known to be the Lance Formation in southeastern Montana. This specimen, now ANSP 8568, was described by Joseph Leidy in 1872 as belonging to the dermal armor of a reptile or an armadillo-like animal. It became known as Tylosteus. Its actual nature was not found until Baird studied it again over a century later and identified it as a squamosal (bone from the back of the skull) of Pachycephalosaurus, including a set of bony knobs corresponding to those found on other specimens of Pachycephalosaurus. Because the name Tylosteus predates Pachycephalosaurus, according to the International Code of Zoological Nomenclature Tylosteus would normally be preferred. In 1985, Baird successfully petitioned to have Pachycephalosaurus used instead of Tylosteus because the latter name had not been used for over fifty years, was based on undiagnostic materials, and had poor geographic and stratigraphic information. This may not be the end of the story; Robert Sullivan suggested in 2006 that ANSP