Period: Late Cretaceous
Order, Suborder, Family: Ornithischia, Marginocephalia, Pachycephalosauridae
Location: North America (United States)
Length: Estimated 15 feet (4.5 meters)
The largest pachycephalosaur was Pachycephalosaurus. First found in rocks of Late Cretaceous age in Montana, Pachycephalosaurus was named and described by Barnum Brown and Eric Schlaikjer. Pachycephaiosaurus is known only from a number of large domed skull roofs and a nearly complete skull. As the name Pachycephalosaurus ("thick-headed reptile") indicates, the top of the skull was very thick, almost nine inches in some skulls. Unlike other pachycephalosaurs, the snout stuck out, giving it an almost piglike profile. The top of the dome was smooth, much like that of Prenocephale and Stegoceras.
The back rim of the dome of Pachycephalosaurus and the top of the snout were covered with spikes and small horns. The teeth were simple, triangular blades. It ate soft plants.
Like its close relatives Stegoceras, Prenocephale, and Stygimoloch, Pachycephalosaurus probably used its domed head for head-butting contests. The thick skull roof would have protected the small brain from damage. Males protected their territory and females by using this behavior.