Basically, the rundown is this: Dracorex, Stygimoloch, and Pachycephalosaurus are all postulated to be very closely related. As most of you already know, these are bone headed dinosaurs. The smallest (Dracorex) has no dome, and big squamosal spikes. Pachycephalosaurus (the largest) has a huge dome, with blunt spikes. And Stygimoloch , well, is sort of in the middle. Unfortunately, pachycephalosaurid fossils are extremely rare, and the record typically consists of fairly crappy material, including a lot of isolated, reworked fronto-parietal domes. Stygimoloch is the least known, originally named by Galton and Sues (1983) off of a squamosal.
Ontogeny of Pachycephalosaurus wyomingensis: juvenile, upper left ("Dracorex"), subadult, upper right ("Stygimoloch"), and adult (Pachycephalosaurus).
Histological analysis indicated that the smallest (Dracorex) was a 'juvenile', and that Stygimoloch and Pachycephalosaurus are 'subadult' and 'adult' (or, are at least in that relative ontogenetic order). The histology shows that in Stygimoloch and Pachycephalosaurus, the ornamental horns are undergoing resorption, and actually shrinking in size through ontogeny, and the dome is growing. Previously these features were intepreted to represent apomorphic conditions, should each fossil be mistaken (or assumed) for adults of different taxa.
Jack gave the talk version of this at the SVP meeting in 2007, and it wasn't very well received (which made it all the more entertaining). Unfortunately, there wasn't any time for questions. However, this year Jack gave a talk on metaplastic bone in dinosaurs, and he again described the histological evidence for the synonymy, and there was time for several questions. Inane questions like "why would these animals go through all the trouble of changing their head during growth - its expensive!", which received the answer "well, I don't know - but the histology shows that the dome was growing bigger, and the horns were getting littler." A brief comment here - besides the obvious option for intraspecific display (i.e. being able to tell another individuals age within a population), modern mammals do something far stranger - cervids (deer) grow out huge, heavy antlers every year, and then shed them. Just imagine a ten year old animal, and how much bone (by volume) that is, and how many calories that took to produce, in bulk.
I personally think this is a really neat paper; not everyone may agree with Horner on a lot of issues, but he constantly hounds graduate students at MSU to think in a transformational context, and not a typological context. Typologists either haven't trained themselves to think in terms of transformations (be they ontogenetic or phylogenetic), or are busy naming new taxa when they shouldn't be, and won't allow anyone to kill their 'baby'. Often in the world of dinosaur paleontology typology goes hand-in-hand with neo-nazi cladism. Fossil organisms change through time in two ways - ontogenetically in a single individual, and on evolutionary timescales - the full examination of both ranges of variation can lead to additional synonymization, and a more accurate (and hence, better) understanding of the fossil record and evolution. And a few pissed off, bitter dinosaur fanboys. You SVP-ers and vertpaleo list subscribers know who I'm talking about.
Anyway, I still need to read the entire paper; much of this post is based on my recollection of various presentations. I also apologize for the 1.5 week hiatus; I've got some good posts planned, and will come out shortly after this.
(1983) New data on pachycephalosaurid dinosaurs (Reptilia: Ornithischia from North America. Can J Earth Sci 20: 462–472.
Pachycephalosaurus. PLoS OneExtreme cranial ontogeny in the Upper Cretaceous Dinosaur
(2006) Major cranial changes during Triceratops ontogeny. Proc R Soc Lond Biol 273: 2757–2761.
(2007) Synonymy consequences of dinosaur cranial ontogeny. J Vert Paleont 27: 92A.