Pachycephalosaurus wyomingensis, also known as the thick-headed lizard, was a genus of pachycephalosaurid dinosaur that lived in North America during the Late Cretaceous period, between about 76.5 and 65.5 million years ago. It was one of the last surviving members of its family before their extinction at the end of the Cretaceous. The type species is P. wyomingensis, which was first described in 1942 by James Lull and Edwin Colbert from partial remains found in Wyoming, United States.
Pachycephalosaurus was a medium to large sized dinosaur, reaching lengths of up to 9 m (30 ft). It had a small skull with large eyes and a beak-like mouth. Its most distinctive feature was its thickened skull roof bones, which may have been used for head-butting or mating rituals. The function of these bones is still debated by scientists today. The back part of its skull had two large openings (fenestrae), which were surrounded by bony ridges (sutures). These features are thought to have helped reduce the weight of its skull.
Pachycephalosaurus is thought to have been an herbivore that fed on low-lying plants such as ferns and cycads. It is believed to have moved in herds and may have migrated seasonally in search of food sources. Although it has often been portrayed as a fierce predator in popular culture, there is no evidence that Pachycephalosaurus was anything other than a peaceful plant-eater.