Yes, I know that this is old news to many of you, but it’s new to me, so I figured that there might be some other people out there who missed it too. In 2007, paleontologist John “Jack” Horner presented a talk stating that allometric growth changes need to be considered when distinguishing species. Allometric growth refers to physiological changes that take place as an organism changes in size. Studies using comparative morphology, computerized tomography, and osteohistology of the skull, all supported the hypothesis that Dracorex, Stygimoloch, and Pachycephalosaurus are all growth stages of a single species, Pachycephalosaurus. You can see the abstract of the 2007 talk at the Society of Vertebrate Paleontology website. You can read all the details from Jack Horner and Mark Goodwin in the online journal PLOS.