Skip to main content

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…

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.


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

Popular posts from this blog

R for beginners and intermediate users 3: plotting with colours

For my third post on my R tutorials for beginners and intermediate users, I shall finally touch on the subject matter that prompted me to start these tutorials - plotting with group structures in colour.

If you are familiar with R, then you may have noticed that assigning group structure is not all that straightforward. You can have a dataset that may have a column specifically for group structure such as this:

B0 B1 B2 Family
Acrocanthosaurus 0.308 -0.00329 3.28E-05 Allosauroidea
Allosaurus 0.302 -0.00285 2.04E-05 Allosauroidea
Archaeopteryx 0.142 -0.000871 2.98E-06 Aves
Bambiraptor 0.182 -0.00161 1.10E-05 Dromaeosauridae
Baryonychid 0.189 -0.00238 2.20E-05 Basal_Tetanurae
Carcharodontosaurus 0.369 -0.00502 5.82E-05 Allosauroidea
Carnotaurus 0.312 -0.00324 2.94E-05 Neoceratosauria
Ceratosaurus 0.377 -0.00522 6.07E-05 Neoceratosauria
Citipati 0.278 -0.00119 5.08E-06 Oviraptorosauria

The difference between Lion and Tiger skulls

A quick divergence from my usual dinosaurs, and I shall talk about big cats today. This is because to my greatest delight, I had discovered today a wonderful book. It is called The Felidæ of Rancho La Brea (Merriam and Stock 1932, Carnegie Institution of Washington publication, no. 422). As the title suggests it goes into details of felids from the Rancho La Brea, in particular Smilodon californicus (probably synonymous with S. fatalis), but also the American Cave Lion, Panthera atrox. The book is full of detailed descriptions, numerous measurements and beautiful figures. However, what really got me excited was, in their description and comparative anatomy of P. atrox, Merriam and Stock (1932) provide identification criteria for the Lion and Tiger, a translation of the one devised by the French palaeontologist Marcelin Boule in 1906. I have forever been looking for a set of rules for identifying lions and tigers and ultimately had to come up with a set of my own with a lot of help fro…

Top 10 scientifically important theropod dinosaurs of all time (off the top of my head)

I thought I'd do a fun post for once. And since list based articles are the norm for fun on the internet, I thought I'd do one on dinosaurs, but given that I know most about theropods, I've decided to restrict my list to theropods (...maybe in a future post, I'll do other clades).

My ranking is based mostly on scientific importance so it may not reflect awesomeness, and it is obviously subjective as to how I rank importance to science. For instance, interesting discoveries or unique palaeobiology are ranked relatively low compared to wealth of information and data or completely revolutionising our understanding of the evolution of theropods.

So here are my top 10 scientifically important theropod dinosaurs of all time (off the top of my head)

10. Megalosaurus

Being the first dinosaur to be named, Megalosaurus automatically deserves a spot on this list, but given the fragmentary nature of known fossil specimens, and being mostly useless as a meaningful source for biologi…