For some time, there has been a debate about how many species of dome-headed (pachycephalosaur) dinosaurs existed. The largest and best known, Pachycephalosaurus, has always been a stand-alone, valid species, known from adult material. ‘Dracorex‘ was named in 2006, and represented a juvenile pachycephalosaur, while ‘Stygimoloch‘ was a subadult. All 3 species were found in the Hell Creek Formation of Montana, and several people have hypothesised that they represent ontogenetic morphs (or ontogimorphs) of Pachycephalosaurus .
Unfortunately it has been particularly difficult to determine exactly what is going on with the ontogimorphs and what species they are because most finds of small pachycephalosaur material lacks cranial ornamentation, because they are so young that they haven’t started the dome-development. However, some newly described material represents the smallest known Pachycephalosaurus specimens found. Goodwin and Evans  describe three very small bones also from the Hell Creek Formation of Montana from a juvenile Pachycephalosaurus, recognised by the cranial ornamentation pattern.
They found that the arrangement of cranial spikes on the posterolateral surface, jugal, and parietal of the new specimen matches what is seen in both ‘Dracorex’ and ‘Stygimoloch‘. More exciting, the new material better clarifies the development of the pattern of cranial ornamentation in Pachycephalosaurus. First of all, rather than being fused osteoderms, the spike-like nodes are actually represented by bony outgrowths of the cranial bones. Furthermore, the squamosal spikes don’t seem to develop later in ontogeny like previously assumed, but are actually present in early juveniles. We are finally getting a more clear idea of exactly how the cranial ornamentation developed in pachycephalosaurs.
Pachycephalosaurs have always been my favourite dinosaurs, partly because of the (possibly out-dated) idea that they engaged in head-butting. Or possibly just because they look so silly with their big dome-heads. I love stories like this where we discover ontogenetic sequences rather than just naming new species, but the Harry Potter fan in me is a bit sad that ‘Dracorex hogwartsia‘ is likely not a valid species…
- Horner and Goodwin (2009) Extreme cranial ontogeny in the Upper Cretaceous dinosaur Pachycephalosaurus. PLoS One 4: e7626.
- Goodwin and Evans (2016) The early expression of squamosal horns and parietal ornamentation confirmed by new end-stage juvenile Pachycephalosaurus fossils from the Upper Cretaceous Hell Creek Formation, Montana. Journal of Vertebrate Paleontology 36:2, e1078343.