You can go ahead and prune two names off the pachycephalosaur family tree--it turns out that
Dracorex hogwartsia and
Stygimoloch spinfer represent the juvenile and subadult (respectively) of
Pachycephalosaurus wyomingensis.
Horner and Goodwin came to this conclusion after realizing that all three genera had horns in the same places, and that the bone histeology of
Dracorex conforms to that of a juvenile animal, while
Styigmoloch is more or less a "teenager." Taken together with the fact that all three are known from the same formation and time (Hell Creek), and you've got a growth series!
So the short version: What were once three genera have been shrunk down to ONE.
That has implications for dinosaur diversity at the end of the Cretaceous in the Hell Creek formation.
Nanotyrannus is probably a juvenile
Tyrannosaurus, too, and at SVP this year, one presenter suggested that
Torosaurus represents an old growth stage of
Triceratops. If all this is true, then dinosaur diversity plummeted at the end of the Cretaceous in North America, well before the comet hit.
But more interestingly, it shows that pachycephalosaur skulls were extensively remodeled during growth, just like ceratopsians. As the two groups are usually united in a monophyletic Marginocephalia, this kind of transformative growth could be a synapomorphy of that group.
It's a brave new world. Download the paper FREE from here:
http://www.plosone.org/article/info:doi/10.1371/journal.pone.0007626