Pachycephalosaurus is a genus of pachycephalosaurid dinosaur that lived at the late cretaceous period, 66 million years ago. Pachycephalosaurus means thick-headed lizard, because it had a bony dome on its skull. It used its skull for fighting donimants, hurting predators, and mating.
Pachycephalosaurus is the largest member of the Pachycephalosaurid family and its cousins are Dracorex, Homalocephale, Microcephale, Prenocephale, Stegoceras, Stygimoloch, and Wannanosaurus. All species had dome-shaped heads and walked on two legs.