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Ontogeny and taxonomy of pachycephalosaurs

There were two interesting talks on pachycephalosaurs at the SVP annual meeting in Austin, Texas. I don’t know if they were coincidental but both talks dealt with the possible synonymy of Dracorex, Stygimoloch and Pachycephalosaurus.

The first talk was by John Horner. Horner (2007) used comparative cranial morphology, computer tomography and osteohistology to hypothesize that Dracorex, Stygimoloch and Pachycephalosaurus all represent different stages in an ontogenetic series of a single taxon Pachycephalosaurus.

Aside from having a flat head, Dracorex differs from Stygimoloch and Pachycephalosaurus in having large supratemporal fenestrae. Dracorex also has extensive ornamentation along the squamosals. Stygimoloch has closed off its supratemporal fenestrae and have a well-developed frontoparietal dome incorporating the rostral part of the frontal and postorbital but not the lateral and caudal elements of the skull. Stygimoloch also has extensive cranial ornamentations along the squamosal. Pachycephalosaurus has an extreme doming of the frontoparietal with the incorporation of the prefrontal, squamosal, and postorbital into the dome. The squamosal ornamentations are not extensive as in Dracorex or Stygimoloch. However, all three taxa show very similar or near identical ornamentation patterns on the snout.

Bone histology and CT scans reveal the internal architecture of the domes in these pachycephalosaurs. The bones comprising the dome are highly spongy in both Dracorex and Stygimoloch but are completely solid in Pachycephalosaurus. Horner suggests that the bone was still growing in Dracorex and Stygimoloch while it had already stopped growing completely in Pachycephalosaurus.

Horner’s talk was followed by Robert Sullivan’s. Sullivan (2007) points out that while flat-headedness have been inferred to be the ancestral condition in pachycephalosaurs, small fully domed pachycephalosaurs occur much earlier in the fossil record with the flat-headed morphology occurring more frequently in younger strata. Because of this stratigraphic incongruence, Sullivan proposes the possibility that flat-headed morphology is an early ontogenetic stage that is delayed in the later larger mature individuals. If we are to accept this hypothesis and if doming and closing of the supratemporal fenestrae occurred later in ontogeny, then the taxonomic validity of many of the flat-headed pachycephalosaurs would be in doubt. This is particularly true for Dracorex and Stygimoloch as they are from the same formation as Pachycephalosaurus.

Sullivan also suggested that the squamosal ornamentations may have been rubbed off in the extremely old Pachycephalosaurus.

However, Robert Bakker, who was out in the hall at SVP with cast replicas of the skull or skull elements of Dracorex (whole skull), Stygimoloch (partial skull) and Pachycephalosaurus (can’t remember what parts of the skull he had), argued that there are substantial differences in the three skulls to distinguish them as separate genera. Dracorex in particular apparently has unique and diagnostic features in the snout. Bakker presented an analogy from the modern Serengeti where extremely similar but distinct species of antelope (? …or something) coexist. Good point. I wouldn’t be surprised if there were several species of pachycephalosaur living at the same place considering how relatively abundant pachycephalosaurs are at the Hell Creek Formation.

So who knows. I’ll just wait till these works get peer-reviewed and published before I make my mind up…

References:
Horner, J. 2007. Synonomy consequences of dinosaur cranial ontology. J. Vert. Paleontol. 27: 92A.

Sullivan, R. 2007. Doming, heterochrony, and paedomorphosis in the Pachycephalosauridae (Ornithischia: Dinosauria): taxonomic and phylogenetic implications. J. Vert. Paleontol. 27: 154A.

Comments

Zach Miller said…
Definately interesting. Given how ceratopsians don't acquire their species-specific characters until sexual maturity, I would not be at all surprised to learn that young pachycephalosaurs looked a lot different than their parents. I'm a bit more willing to accept the Dracorex = Pachycephalosaurus than the Stygimoloch = either, though. As far as I know, Sygimoloch and Dracorex are the same size, and both are much smaller than Pachycephalosaurus. It's interesting that this is brought up at SVP, because in Thomas Holtz, Jr.'s new dinosaur book (which is awesome, btw) he mentions the possibility that Dracorex and Pachycephalosaurus are cospecific.

I guess if there's some credence to the "Stygimoloch as teenage Pachycephalosaurus" theory, it's that the only known skull of the latter has really dulled, rounded squamosal horns, which could certainly have to do with age and skirmishes. But Bakker does make a good point, that antelopes are all basically the same, but with slightly different ornimentation.
dracoman said…
I found Dracorex in May of 2003. Bob Bakker, Phil Currie, Eva Koppelhus, Peter and Neil Larson were all present when the skull was first revealed. Bob and Phil were convinced this was a new species and adult dino. They talked of the suture development as one of the factors in stating that the new species was an adult. I am not a paleontologist, however there are many significant differences in the three species. This appears to be yet another media grabbing event and bashing of Dr. Bakker and Dr. Sullivan by Horner.

Brian Buckmeier

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