This happy little bonehead is Dracorex hogwartsi, a Late Cretaceous pachycephalosaur that is contemporaneous with Pachycephalosaurus, both locally and temporally. While the original fossil was crushed dorsoventrally, Bob Bakker provided a wonderful restoration in the description. Notice the two distinct "bands" of small horns on the snout, the decorated jugal, and the large horn rosettes erupting from the squamosals. Most importantly, though, check out those supratemporal fenestrae--they're unique among known pachycephalosaurs, which seem to have closed their supratemporals in favor of developing thickened bony domes.
This is the beautifully-preserved skull of an adult Pachycephalosaurus. It is a wonderful skull, and when it was discovered, it was the only other complete skull known from a pachycephalosaur, the other being Stegoceras (who still holds the record for "best known postcranial skeleton among boneheads"). Pachycephalosaurus also has a double-band of small horns on the snout (though they are compressed because of the large dome) and a decorated jugal. The large squamosal horns of Dracorex are not seen in Pachycephalosaurus, although the latter certainly has squamosal horns--they just aren't as prominant. Finally, of course, the supratemporal fenestrae have been completely closed off, and a huge 8-inch-thick dome has overtaken the top of the skull.
This is Stygimoloch, a genus known only from extremely fragmentary remains. All of the named specimens are pieces of dome with some squamosal thrown in. The genus is diagnosed by its three or four large squamosal horns, which resemble those of Dracorex. Despite its rarity, however, Stygimoloch most certainly had a dome. In his 2006 treatment of the Pachycephalosauridae, Robert Sullivan suggest that, based on squamosal morphology, Dracorex, Sygimoloch, and Pachyrhinosaurus form a natural group (the Pachycephalosaurini).
Now, the question is this: Can all three taxa be folded into Pachycephalosaurus based on ontogeny? That's a difficult question, especially given the extremely fragmentary remains of Stygimoloch. Ignoring that thorny devil for now, though, let's focus on the dragon king and the thick-headed lizard.
If Pachycephalosaurus is an adult Dracorex, then we must account for the differing squamosal morphology. In Dracorex, the squamosal horns are large, pointy, and obvious. In Pachycephalosaurus, they are large, rounded, and short. Sullivan does not believe that the "nodes" of Pachycephalosaurus have been "worn down," a contention that I agree with. How would Pachycephalosaurus have been using its squamosal spikes that would result in wearing them down? We must also account for the domelessness in Dracorex. The lack of unquestionable juvenile remains of pachycephalosaurs is troubling, here. Sullivan suggests that the flat-headed condition of some Asian taxa (i.e. Homocephale) is a derived feature, and that the dome is basal for Pachycephalosauridae. This is not to say that the dome was not present right out of the egg, but rather the dome is a defining ancestral feature of the family. Pachycephalosaurs did not diverge along two lines from a domeless form, nor did domeless forms predate the domed animals. Rather, the first pachycephalosaur had a dome, and domeless adult species secondarily lost it.
This has important implications for Dracorex. First, if we consider Dracorex to be a member of the Pachycephalosaurini (which I believe it is), its domelessness must be considered either a juvenile trait or a derived feature that runs counter to the other members of its immediate clade and instead converges on a few Asian taxa. This would be like Citipati regrowing a long tail while its sister taxa (Nomengia and Oviraptor) retain a pseudo-pygostyle. A reduced tail is a primitive trait for the Oviraptoridae. That's not very parsimonious, and I don't think it flies. If Dracorex is of the Pachycephalosaurini, I believe it is a juvenile.
Did juvenile pachycephalosaur lack a dome? For that answer, we must turn to the Pachycephalosauridae's sister group, the Ceratopsia. Peter Dodson has repeatedly shown that all of species-specific features of the horned dinosaurs are related, primarily, to sexual maturity. Centrosaurus and Pachyrhinosaurus looked essentially the same as juveniles and teenagers (Brachyceratops and Monoclonius are now considered juvenile and teenage ceratopsians of general affinity, respectively). It was only when those pubescent hormones started kicking in that ceratopsians began to grow their specific horns and frills. Notice that Monoclonius has a large nasal horn. Well, that horn was reabsored and reworked into a roughened bony boss in Achuelosaurus. The small orbital horns of Monoclonius were reabsorbed in Styracosaurus. In Triceratops, it's possible that the solid frill so characteristic of that taxon were simply closed off in adults, while juveniles still had frill "windows."
The point here is that all of the visual aides produced by the Ceratopsia seem to be linked with sexual maturity. The same could very well be true of Pachycephalosauridae. This would mean that the characteristic dome of Pachycephalosaurus, Stegoceras, Prenocephale, Tylocephale, etc. would all be secondary sexual characteristics that "arose" (HA!) after sexual maturity. This would also mean that dome-less adults, like Homocephale, are paedomorphic features, retained into adulthood (there is a ceratopsian analogue here--Avaceratops may be a hornless adult, although that verdict is far from certain).
So, we can establish with some certainty that Dracorex may be a juvenile pachycephalosaurine. It is certainly not immediately related to the Asian flat-headed pachycephalosaurs, so it is not an Asian export living alongside North American boneheads. But how do you explain the complex spikes and scutes of Dracorex's head? Are the spike bands, decorated jugals, and large squamosal horns not sexual characters? If those features are purely for protection (and they certainly would provide it), I would expect to see them in juveniles. And it's entirely possible that the impressive squamosal horns of Dracorex were reabsorbed as it grew, so that they appear to be blunt in Pachycephalosaurus. As it stands, the jury is out on this one.
It's a shame we can't say too much about Stygimoloch, but a few things are clear. First, Stygimoloch is probably of the Pachycephalosaurini, based solely on its squamosal morphology. Second, because Stygimoloch has a dome, the holotype is probably an adult. Third, because the adult seems to have large pointy squamosal spikes, it seems clear that you can have both impressive squamosal spikes AND a dome.
So what does THAT mean for Dracorex? It probably means that Dracorex is a juvenile Stygimoloch, although more complete remains of the latter would undoubtedly clear up the situation. As for Pachycephalosaurus, I think that it is a unique taxon. I would expect to see similar squamosal horn morphology in a juvenile animal, given that Dracorex has horned squamosals. Pachycephalosaurus is also a larger animal than either Dracorex or Stygimoloch, and why would an animal grow significantly larger after attaining sexual maturity?
So my conclusions are as follows:
1) Dracorex is of the Pachycephalosaurini, and it represents a juvenile animal.
2) Juvenile pachycephalosaurs were most likely domeless.
3) Domes in pachycephalosaurs are related to sexual maturity.
4) Domeless adult pachycephalosaurs (Homocephale) have secondarily lost their domes through paedomophosis.
5) Stygimoloch is of the Pachycephalosaurini.
6) Stygimoloch retains the large squamosal horns of Dracorex, but also has a dome.
7) Dracorex is a juvenile Stygimoloch, and Stygimoloch is a sister taxon to Pachycephalosaurus.
8) In the future, I predict that if any juvenile Pachycephalosaurus are found, it will have no dome, but also rounded squamosal spines.