Pachycephalosaurus was always introduced to me as the ancient equivalent of a Bighorn Sheep (Ovis canadensis), always looking for some excuse to bang their heads together. As I grew older I didn’t really buy the head-to-head butting hypothesis, especially since the heads of these dinosaurs were domed and did not provide a wide, flat space to distribute the stress of impact, but the inferred behavior nevertheless remains a somewhat entrenched (see R.T. Bakker’s illustration of the head-butting Dracorex, a flatter-headed pachycephalosaur, at the HMNH). Indeed, Pachycephalosarus seems to be the genus that defines how this group is seen, but there is a wider variety of pachycephalosaurs with flatter skulls like Dracorex and Stegoceras in North America and Homalocephale in Asia. A recent article in Science, however, outlines Jack Horner’s hypothesis that Dracorex and Stygimoloch should really be called Pachycephalosaurus.
One of the recurring problems in paleontology is the proper identification of species that are sexually dimorphic or of juvenile individuals of a particular species. Since we can’t directly observe these animals during the course of their lives, sometimes fossils that have been juveniles have been deemed to be a separate species or genera, only later being discovered to be immature individuals. (It’s not always possible to refer a juvenile to a specific genus or species, however; some lambeosaurine hadrosaur juveniles remain enigmatic, making it difficult to tell whether they grew up to be Lambeosaurus or Corythosaurus.) It is possible to go the other way as well, though, paleontologist David Evans showing that the hadrosaurs that Peter Dodson once considered to be sexually dimorphic members of the same species were actually different species of crested hadrosaur. While hadrosaurs are not pachycephalosaurs they do present a cautionary tale about ornithischians; many ornithischian dinosaurs have similar body plans with different heads, differences seeming smaller or greater depending on how you weight the characters
Unfortunately I was not present at SVP meeting this year to hear about the proposed hypothesis of Horner, Woodward, or Goodwin, but the Science piece provides a few details. While researching ontogeny in the pachycephalosaur Stegoceras, the researchers noticed that adults lacked radial canals in the bones of the skull that are more abundant in juveniles. Using this as a model, they looked at the skulls of Dracorex and Stygimoloch and said that Stygimoloch has abundant radial canals its skull and that Dracorex seems to be consistent with the proposed growth series (the skull couldn’t be cut open as there is only one). The article also notes that that horns of Stygimoloch appear to be in the process of being absorbed, which would probably be a necessary requirement if it really is a juvenile (or at least immature) Pachycephalosaurus. Dracorex seems to fit as even in the paper that describes it some juvenile features are mentioned, seeming fitting the new idea.
There are a few problems with the new hypothesis, however. First is that as far as I know (and please correct me if I’m wrong) there is only one complete skull of Pachycephalosaurus (see picture above), and while there are other parts of the skull and dome that are known the amount of data we have for Pachycephalosaurus is still relatively meager. Likewise, Stygimoloch and Dracorex may show some juvenile characters or reabsorption of bone (presumably, under the hypothesis, to be redistributed in a more prominent dome), but this does not mean that they are juvenile Pachycephalosaurus. What will have to be done (if it hasn’t already) is a similar study to that which David Evans did in which the time and location of these animals will have to be compared to see if it makes sense for them all to be folded into one genus. On top of that, the records for these animals is relatively poor, thus making comparisons difficult. What if the Stygimoloch material studied does represent a juvenile but the adult has not yet been discovered? The same goes for Dracorex, and there could be immature Pachycephalosaurus out there as well (although as this article illustrates some think we’ve already found them).
There is another issue at stake here that may very well shape some of the arguments; did dinosaurs experience high diversity in the end Cretaceous or was their diversity dwindling prior to extinction? Horner favors an overall reduction in the variety of dinosaurs present in North America before the extinction, the lumping of the pachycephalosaurs being consistent with his views. Likewise, those who favor diversity might be more likely to favor the animals as separate, and even though such broader trends should be supported or refuted by the available evidence I won’t be as naive to think that it plays some role in this issue.
My problem here is that I’m suffering from a lack of information; I didn’t hear the presentation, I’m not an ornithischian expert, and the fossil record for these dinosaurs is relatively scant. Put it all together and the most I can really say is “More fossils, please,” although if I’m to be honest I’m somewhat skeptical of the new hypothesis. Hopefully someone will do the work to match up the histology with the stratigraphy and biogeography, all of which are factors that are going to be important, but I imagine we’ll have to wait a bit for things to be confirmed or refuted (and even then it’s still provisional, awaiting future finds and analysis).
[Hat-tip to The World's Fair]
Bakker, R.T.; Sullivan, R.M.; Porter, V.; Larson, P.; Saulsbury, S.J. (2006) “Dracorex hogwartsia, N. Gen., N. sp., a Spiked, Flat-Headed Pachycephalosaurid Dinosaur From the Upper Cretaceous Hell Creek Formation of South Dakota.” New Mexico Museum of Natural History and Science Bulletin 35, p. 331-345.
Brown, B. and Schlaikjer, E.M. (1943) “A study of the troödont dinosaurs, with the description of a new genus and four new species.” Bulletin of the AMNH ; v. 82, article 5
Stokstad, E. (2006) “SOCIETY OF VERTEBRATE PALEONTOLOGY MEETING: Crestfallen: Sexually Dimorphic No More” Science Vol. 314. no. 5801, p. 921
Stokstad, E. (2007) “SOCIETY OF VERTEBRATE PALEONTOLOGY MEETING: Did Horny Young Dinosaurs Cause Illusion of Separate Species?” Science Vol. 318. no. 5854, p. 1236