Pachycephalosaurus is a genus of pachycephalosaurid dinosaur from the late cretaceous period in what is now North America. It was once thought that Dracorex, Stygimoloch, and Pachycephalosaurus were separate genera. This was put into question because both Stygimoloch and Dracorex were only known from juvenile specimens and all three genera lived in the same place at the same time. Scientists eventually concluded that Stygimoloch and Dracorex were just juvenile Pachycephalosaurus.