Pachycephalosaurus is a genus of dinosaur belonging to the group Pachycephalosauria. It lived during the Late Cretaceous period, approximately 70 to 65 million years ago. The first fossils of Pachycephalosaurus were discovered in the late 19th century in Montana, USA. Since then, additional findings have provided paleontologists with valuable insights into this unique dinosaur.

Pachycephalosaurus was a medium to large-sized dinosaur, measuring around 15 to 26 feet (4.5 to 8 meters) in length. Its most distinctive feature was its thickened skull roof, which formed a bony dome at the top of its head. The dome could be up to 8 inches (20 centimeters) thick in some individuals, providing protection for the brain during head-butting behaviors. Pachycephalosaurus is known from its skull, there is no other fossils that have been discovered yet. Studies done of its close relatives such as Stygimoloch and Prenocephale indicate that these dinosaurs probably used their skulls in vigorous head-butting competions during courtship battles, in the same way modern day sheep and goats do.

The central part of the head consisted of very thick bone that would have acted like a helmet, protecting the dinosaur´s small brain by carrying the shock waves away from the area of impact.

Pachycephalosaurus´s skull was almost 2 feet or 60cm in length and the thickest part of the skull was nearly 8 inches or 20cm. The skull had thick spiky nodules of bone around the rim and smaller portruding horns on the snout. It had triangular teeth with coarse serrations along the edges for shredding tough plant matter.