Pachycephalosaurus

Pachycephalosaurus (/ˌpækɪˌsɛfələˈsɔːrəs/; meaning "thick-headed lizard", from Greek pachys-/παχύς- "thickness", kephalon/κεφαλή "head" and sauros/σαῦρος "lizard") is a genus of pachycephalosaurid ornithischian dinosaur. The type species, P. wyomingensis, is the only known species, but some researchers argue that the genus Stygimoloch might be a second species, P. spinifer or a juvenile specimen of P. wyomingensis. It lived during the Maastrichtian age of the Late Cretaceous period in what is now western North America. Remains have been excavated in Montana, South Dakota, Wyoming, and Alberta. The species is known mainly from a single skull, plus a few extremely thick skull roofs (at 22 cm/9” thick). More complete fossils would come to be found in the following years.

Pachycephalosaurus was among the last species of non-avian dinosaurs on Earth 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, 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 actually been highly disputed in recent years.