Pachycephalosaurus was an herbivore that roamed North America during the Late Cretaceous period. The skull is adorned with a very thick dome and sports multiple bony projections (horns) covering much of the snout and the back of the skull. Despite the unique and highly derived skull, the postcranial skeleton remained quite primitive. The Pachycephalosaurus has been nicknamed ‘the Butthead dinosaur’ by Dr. Robert Bakker. There is also speculation that this animal ‘used its head’ in romantic displays similar to big horn sheep and a handful of other modern-day mammals.