Pachycephalosaurus

A dome of solid bone up to 25 centimeters thick capped the skull of Pachycephalosaurus wyomingensis, the largest and best-known of the thick-headed dinosaurs. This herbivore reached about 4.5 meters and near 450 kilograms and lived during the Late Cretaceous, roughly 100 to 66 million years ago. It has long prompted debate over whether the dome was used for head-to-head combat, blows to a rival's flanks, or mainly display. Barnum Brown and Erich Schlaikjer described it in 1943 from Montana. Its reinforced skull, often the only part to survive fossilization, is one of the most distinctive structures known in any dinosaur.