Just like every other organism on earth, Pachycephalosaurus started out tiny. And just like most other dinosaurs known to the general public, it is associated with the adult specimens of the animal. It doesn’t help that remains of Pachycephalosaurus and its relatives are often fragmentary, complicating efforts to identify and classify the dinosaurs.
In 2009, paleontologists Jack Horner and Mark Goodwin suggested that specimens labeled Stygimoloch and Dracorex were actually the juvenile forms of Pachycephalosaurus. All three thick-headed dinosaurs originated from the Hell Creek Formation of North America and lived near the end of Cretaceous age.
Interestingly, only adult specimens of Pachycephalosaurus have been found, while Dracorex and Stygimoloch are only known from juveniles. These observations, in addition to the comparison of multiple specimens, lead the paleontologists to conclude that the animal had different shaped head ornaments in different stages of their lives. The subadult form Stygimoloch likely lost its prominent spikes and developed domes as they matured to be the specimens referred to as Pachycephalosaurus.