Pachycephalosaurus

Pachycephalosaurus is a genus of dome-headed dinosaurs, the best-known members of the pachycephalosaur family. They lived during the Late Cretaceous period, roughly 70 to 66 million years ago, in what is now western North America.

The genus name means "thick-headed lizard" and the type species is Pachycephalosaurus wyomingensis. Fossils are primarily skull domes and other skull elements, with relatively few postcranial bones recovered, making complete reconstructions challenging. It is among the larger and more recognizable members of its group.

Anatomy features include a thickened skull roof composed of dense bone, sometimes with a rounded dome and small bumps. The rest of the skeleton suggests a stout, bipedal animal with short forelimbs and powerful hind limbs. The dentition and beak indicate an herbivorous or omnivorous diet.

Pachycephalosaurus is known from rocks of western North America formed during the Late Cretaceous. The dome-like skulls have prompted debates about behavior: the dome may have been used in intra-specific combat or display for attracting mates or signaling social status. The exact function remains uncertain, but the dome is a hallmark of pachycephalosaurs.

As a member of Pachycephalosauridae within Marginocephalia, Pachycephalosaurus is related to other dome-headed dinosaurs and ceratopsians, illustrating the diversity of cranial adaptations in this group.