Like modern mountain goats, it seems pachycephalosaurs, or "thick headed reptiles," liked to bonk each other on the heads.

All the dinosaurs of this group had incredibly thick dome-shaped skulls, some with bony knobs and spikes on the sides of their heads and along their snouts. Some scientists believe that male pachycephalosaurs butted heads for the right to rule the herd.

These rare two-legged plant eaters had five-fingers and three toes and probably ate fruit, leaves, seeds and perhaps insects. They lived mostly in North America, but also in Europe, Central Asia, and even Madagascar.