Yesterday in the open access journal PLoS ONE, paleontologists Jack Horner and Mark Goodwin published a long-awaited paper positing synonymy for a trio of iconic (and melodiously-named) dinosaurs. The bone-headed dinosaurs Pachycephalosaurus, Stygimoloch, and Dracorex are all one and the same animal, according to their work. The latter two are juvenile stages, whereas Pachycephalosaurus represents a full adult.
Skulls of Pachycephalosaurus (top), Stygimoloch (middle; the front of the skull is missing), and Dracorex (bottom; the skull is crushed from top to bottom). In particular, note the changes in skull size and similarities in spike placement. Modified after the original in Horner and Goodwin 2009.
How is this possible? The animals look so different, right? Pachycephalosaurus has this big bowling ball on top of its head, which the other two lack. Stygimoloch has a uniquely-shaped, narrow dome, and Dracorex has a completely flat head. Furthermore, Pachycephalosaurus lacks the elongated spikes that make the other two look so fearsome.
Well, it turns out that this can all be attributed to ontogenetic changes (i.e., change as the animals get older). Horner and Goodwin assemble multiple lines of evidence for this hypothesis.
First, the skulls of Dracorex, Stygimoloch, and Pachycephalosaurus form a size gradation from smallest to largest--exactly what one would expect for a growth series. By itself, this is not irrefutable proof, of course--it could just be that Dracorex had a small adult size compared to Pachycephalosaurus.
Second, many of the knobs and bumps on the skulls can be matched up one for one between individuals of the various specimens. Alternatively, one would also expect that closely related (but different) species might have similar patterns of bumps. As Horner and Goodwin admit, there is some variation between individuals of the different "species"--but, the authors also note that this sort of variation is entirely expected and occurs even within undisputed adult Pachycephalosaurus.
Third, specimens of Stygimoloch, both in CT scans and physically cut specimens, show an open suture between the two frontal bones of the dome. Pachycephalosaurus domes are completely fused up. Open sutures are often strong indications that an animal is still growing--and, it's particularly intriguing that a small "species" has them but a large "species" doesn't!
Finally, microscopic examination of the bones in two of the three "species" (Stygimoloch and Pachycephalosaurus; there weren't any Dracorex available for cutting up) shows that Stygimoloch was still growing (and thus not a full adult)--but Pachycephalosaurus specimens weren't growing much at all (and therefore were probably full adults).
Any one of these lines of evidence might be interesting, but not completely convincing. Taken together, however, they make a pretty compelling case that Dracorex and Stygimoloch are juvenile Pachycephalosaurus. Because Pachycephalosaurus was named first, the first two become junior synonyms. It's a shame, because they're such cool names!
As for duckbilled dinosaurs, horned dinosaurs, and even modern crested birds like the cassowary, the story in the pachycephalosaurs suggests that weird ornaments on the skull were something that happened only as the animals approached full size. The domes practically appeared overnight! The teenage years must have been a real headache for these dinosaurs.
Thanks to the wonders of open access, the article is freely available for all to read. Additionally, it is worth taking advantage of the rating and comment features at PLoS ONE [disclaimer: I am a section editor for that journal]. . .few other scientific publications allow the readers to annotate the papers directly!
Horner, J., & Goodwin, M. (2009). Extreme cranial ontogeny in the Upper Cretaceous dinosaur Pachycephalosaurus. PLoS ONE, 4 (10) DOI: 10.1371/journal.pone.0007626
I'm scared...this sort of makes sense.
This has pretty broad implications for diversity at the end of the Cretaceous.
Zach, this has pretty broad implications for diversity at the Hell Creek Formation, not the whole end of the Cretaceous.
Dammit. Yes, that one.
Why does this science keep changing it's model when new evidence shows the old model is wrong? Nice blog. Stop on by mine sometime.
I wonder why nobody uses the comments feature of PLOSONE. Are all, like me, of the opinion that their comments are ireelevant or too negative to publish this way? I often have a comment, but wait until I talk to the author(s) anyways (read: forever), or just send and email.
I think both of your reasons are likely ones, Heinrich. . .and part of it might be that because there is no tangible career reward for posting these comments (versus publishing a formal reply or including the comments in a larger publisher work), it isn't seen as worthwhile to both with them. See here for a discussion on comments at PLoS ONE, also.
I recently saw these hypotheses on the National Geographic feature, "Dinosaurs Decoded," and I can honsetly say that in paleontology, the farfetched often can be the irrefutable truth. Note I say "CAN be," NOT "IS!" Although, I'm torn on what "Jane" and the "Cleveland Skull" are. Has anyone looked for open sutures in a Nano specimen and compared them to a T. rex?
Even so, "Jane" may or not be full grown and a still growing animal doesn't necessarily mean it's the same taxa as another. N. lancensis may be similar in size and build to Albertosaurus. Is that a possibility?
OH NOOOO, please NOOOOOO!
Dracorex and Stigy are just soooo cool! :(
Great post. Congratulations on being selected as the PLoS ONE Pick of the Month.
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