Main Group: Marginocephalian Dinosaurs
Time: Late Cretaceous
Size: 16 1/2 ft (5 m) long
Fossil Finds: North America
Of the group of dinosaurs known as pachycephalosaurs ("Bone Headed Dinosaurs"), Pachycephalosaurus was the largest. Two other pachycephalosaurian species, Dracorex and Stygimoloch are now considered different growth stages of Pachycephalosaurus.