Pachycephalosaurus was a genus of ornithischian dinosaur which lived during the last five million years of the Upper Cretaceous. It lived in what is now North America. "Pachycephalosaurus" was a member of the Pachycephalosaurid family. This dinosaur is only known from a single skull and a few extremely thick skull roofs. From this it is estimated that its length was about 4.5 to 5 metres long (about 15 feet). It is the largest known pachycephalosaur.