Pachycephalosaurids were a group of two legged plant-eaters characterised by a thick domed skull and bony bumps or spikes behind each eye. Some genera had very large domes, while others had flatter domes. Pachycephalosaurids lived in the Early to Late Cretaceous periods. We do not know the exact purpose of the domes, but as males had thicker domes than females they may have been used in fighting in the same way that modern deer and antelope use their antlers and horns to establish dominance. It is thought that pachycephalosaurids were descended from hypsilophodontids. Example genera are Gravitholus, Pachycephalosaurus, Stegoceras, Tylocephale and Yaverlandia. They ranged in size from 3-15ft, and their remains have been excavated in North America, England, Madagascar, Mongolia, and China.

Pachycephalosaurus means 'thick-headed lizard'. This dinosaur was a pachycephalosaurid of the Late Cretaceous period, characterised by a 9 inch thickening of bone on the top of its skull and 5 inch long spikes around the dome. Pachycephalosaurus was the largest so-called bone-headed dinosaur, and was a 15ft long herbivore. Its remains have been found in Alberta, Canada and Wyoming, USA.

Prenocephale or 'sloping head' gets its name from the distinctive shape of the dome on its head. A 7-8ft long pachycephalosaur of the Late Cretaceous period, Prenocephale was an herbivore closely related to Stegoceras. The dome of this dinosaur was decorated with bony knobs around the back of the skull, and its front teeth were sharp, probably for cropping the leaves of herbs and trees. Prenocephale remains have been found in Mongolia.