Pachycephalosaurus

Pachycephalosaurus () is a genus of pachycephalosauridornithischiandinosaur. The type species, P. wyomingensis, is the only known definitive species. It was among the last species of non-avian dinosaurs on Earth before the Cretaceous–Paleogene extinction event, which lived during the Maastrichtianage of the Late Cretaceousperiod in what is now western North America. Remains have been excavated in Montana, South Dakota, Wyoming, and Alberta. Mainly known from a single skull and a few extremely thick skull roofs (at 22 cm (8.7 in) thick), Pachycephalosaurus is estimated to have reached 4.5 m (15 ft) long and weighed 370–450 kg (820–990 lb). More complete fossils would come to be found in the following years.

Like other pachycephalosaurids, Pachycephalosaurus was a bipedalherbivore, possessing long, strong legs and somewhat small arms with five-fingered hands. Pachycephalosaurus is the largest-known pachycephalosaur, known for having an extremely thick, slightly domed skull roof; visually, the structure of the skull suggests a "battering ram" function in life, evolved for use as a defensive mechanism or intra-species combat, similar to what is seen with today's bighorn sheep or muskoxen (with male animals routinely charging and head-butting each other for dominance). This hypothesis has been disputed in recent years.

The genus Tylosteus is synonymous with Pachycephalosaurus. The proposed synonymy for Stygimoloch with Pachycephalosaurus, however, has been disputed in recent literature due to temporal difference in the rock layers where specimens attributed to each genera have been recovered. The genus Dracorex has been considered synonymous with either of the two genera.