Wednesday, May 2, 2012
Dinosaurs with ouchies on their noggins
A pair of Pachycephalosaurus put their heads together at Drayton Manor Theme Park. Photo by Matthew Wells, via Flickr.
Since I was a kid, one image has been stuck in my head when I think of the pachycephalosaurs, those rugged ornithischians with the outrageously thick frontoparietal bone: two males ramming into each other headlong like the two dudes in the photo above. I'm pretty sure I'm not alone in this. Even when I read a piece expressing skepticism about the behavior, I think to myself, "okay, but we're going to stick with the rule of coolness here." It's the same attitude that annoys me when people choose to not believe that many theropods wore feathery integument of some fashion. Again, I remind myself that science isn't about accepting things because they seem to make sense or they're pleasant to believe. So it's good that he head-butting hypothesis has recently had a spate of research devoted to it. The latest is a new paper in PLoS ONE, Cranial Pathologies in a Specimen of Pachycephalosaurus.
The rock has offered us scant remains to work from, which is probably a good reason why the pachycephalosaurs - who Tom Holtz refers to lovingly as "buttheads" in his Dinosaurs encyclopedia- have long been the subject of debate. Why were those heads so thickened? How many taxa might simply represent different growth stages of one species? Could they sustain direct head-to-head impacts or was it more likely that they smacked each other sidelong? Because of the patchiness of their fossil record, it's been difficult to put many of these ideas to the test.
In this new study, University of Wisconsin paleontologist Joseph Peterson, with the help of Christopher Vittore from the Department of Radiology at Rockford Memorial Hospital in Rockford, IL employ CT scanning of a Pachycephalosaurus frontoparietal dome at the Burpee Museum of Natural History and compare its apparent signs of injury to lesions in modern animals. Are these spots best interpreted as sites of bone resorption, as the rigors of decay, fossilization, and erosion, or are they evidence of traumatic impact?
Peterson's study concludes that it ain't the first two. Bone resorption, studied extensively in chasmosaurine fossils, leaves a smooth surface, as opposed to the roughness of the lesions in the studied Pachycephalosaurus dome. Signs of likely taphonomic suspects - such as scavenging, insect traces, and water wear - are also absent. But the lesions do match the marks left on the the crania of modern birds which have sustained impact and lived to tell the tale. Moreover, Peterson writes that the marks "are clustered over the thickest region of the dome; this distribution corresponds well to the location of expected traumatic pathologies resulting from agonistic behavior proposed for pachycephalosaurids." That would be head-butting, though they could have been smacking into boulders instead, I suppose. It's less absurd than some recent dinosaur stories in the popular press.
This follows another recent study by Eric Snively and Jessica Theodor, who compared the skull of Stegoceras to modern "buttheads" and found that "some pachycephalosaurs were as competent at head-to-head impacts as extant analogs displaying such combat," including musk oxen and duikers. (check out Eric's guest post at Archosaur Musings for more on this). On Facebook, Andy Farke joked that the "P" in PLoS ONE may as well stand for "pachycephalosaur." That wouldn't be so bad, would it?
Also be sure to follow Dr. Peterson on Twitter and at his blog (which has been dutifully added to the LITC blogroll).