English: Three Pachycephalosaurus skulls, showing the development from juvenile to adult, at the Museum of the Rockies in Bozeman, Montana. In the lower right is the mis-named Dracorex -- once thought to be a genus of the Pachycephalosaurs but now understood to be the juvenile form. Although the skull protrusions are spikes rather than knobs, and the smooth dome over the skull unformed, all protrusions are in EXACTLY the same location as they are in the adult Pachycephalosaurus -- proving this is a juvenile form. (It is unclear is this is a fossil or cast, or where it is from.)

At the bottom center is the mis-named Stygimoloch. Once considered to be a genus of Pachycephalosaur, it is now recognized to be a subadult. (It is unclear is this is a fossil or cast, or where it is from.)

At the upper left is the adult Pachycephalosaurus, discovered in Carter County, Montana.