Pachycephalosaurus

A thick-skulled plant-eater

Pachycephalosaurus was a plant-eater that walked on two legs and carried an unusually thick, dome-shaped skull roof, framed by small bony knobs and spikes around the back and sides. It grew to roughly 4.5 meters long. Its legs were long and strong, and its arms were short, with five-fingered hands.

Scientists have debated what the thick skull was for. Some think males rammed or shoved each other with it, similar to how some horned mammals fight today. Others argue the skull was not built to survive hard, head-on impacts, so the true use of the dome is still unresolved.

Its world

Pachycephalosaurus lived at the very end of the Cretaceous, between about 72 and 66 million years ago, shortly before the extinction event that ended the age of non-avian dinosaurs. Its fossils come from Montana, Wyoming, South Dakota, Colorado, and North Dakota in the United States, and from Alberta in Canada, preserved in rock layers such as the Hell Creek, Lance, Ferris, Denver, and Dinosaur Park formations. It lived alongside Tyrannosaurus, Triceratops, Troodon, and the small hunters Dromaeosaurus and Ornithomimus.

How it was found

Brown and Schlaikjer named Pachycephalosaurus in 1943, working from a skull recovered in the northern United States. The type species is Pachycephalosaurus wyomingensis. Since then, 31 separate finds have added new skulls and skeletal material, though no single Pachycephalosaurus skeleton found so far is complete, so parts of its body are still known only from related pachycephalosaurs such as Stegoceras, Homalocephale, and Prenocephale.