Pachycephalosaurus (PACK-ee-SEFF-a-low-sore-us) Plant eater, 15 feet long, Late Cretaceous Period. 65 million years ago located in western North America. Pachycephalosaurus was the largest of the dome-headed dinosaurs. They probably used their thick skulls to butt each other as rams do today with their horns. This dinosaur’s name means “thick-headed” reptile. Pretty accurate for an animal with forehead as thick as a bowling ball!