Smithsonian.com

A pair of Pachycephalosaurus face off at the Museum of Ancient Life in Utah. Photo by the author.

Did dome-headed dinosaurs really butt heads? While not one of the most important subjects in paleontology, the question is one of the most fraught. The thick-skulled dinosaurs look as if they were perfectly suited to cracking heads, much like modern bighorn sheep do, but whether or not dinosaurs like Pachycephalosaurus really knocked noggins has depended on who you asked. While some studies have concluded that these dinosaurs were fully capable of bashing skulls, other analyses have disagreed and pointed out that rounded, dome-shaped heads were actually poor weapons in such contests.

The evidence from bone histology and the estimated defensive capabilities of pachycephalosaurs is ambiguous. But a conspicuous lack of skull pathologies seemed to support the idea that these dinosaurs were not butting heads, but instead rammed each other in the flanks or used their domes primarily as flashy ornaments. If pachycephalosaurs were regularly crashing headlong into one another, we would expect many of their skulls to show impact damage from such encounters.

For many years, no one had recorded the expected injuries. That changed this week thanks to a new PLoS One paper by Joseph Peterson and Christopher Vittore. The subject of their paper, titled “Cranial pathologies in a specimen of Pachycephalosaurus,” is a damaged portion of skull from the largest and most famous of all the dome-headed dinosaurs.

The dinosaur’s skull looks as if someone went at it with a hammer. Two large depressions—augmented by numerous smaller pits inside and along their margins—pock the top of the dome. Peterson and Vittore considered several possibilities, including damage caused to the bone after the animal’s death, bone resorption and trauma incurred during the dinosaur’s life. Injury followed by infection seems to be the explanation most consistent with the evidence. And this may not be the only skull of its kind. Towards the end of the paper, Peterson and Vittore point out that a skull of the pachycephalosaur Gravitholus and another belonging to Texacephale appear to have similar injuries to the top surfaces of their skulls.

Case closed, right? This would seem to be pretty good evidence that Pachycephalosaurus really did butt heads. But we should take care in how far we extend hypotheses from one skull. The injuries on the Pachycephalosaurus skull accord with the idea that these dinosaurs were butting heads, but we can’t actually know what happened to this particular dinosaur. The case for head-butting dinosaurs just got a boost, but it would be premature to say whether pachycephalosaurs definitely did or did not engage in the behavior on a regular basis. If the dinosaurs commonly crashed craniums, other damaged domes should be out there. There may be some waiting in the rock or sitting on museum shelves. One thing seems certain, though—Peterson and Vittore’s dinosaur probably had one hell of a headache.

For more on this research, see David Orr’s post at Love in the Time of Chasmosaurs.

Reference:

Peterson, J., & Vittore, C. (2012). Cranial Pathologies in a Specimen of Pachycephalosaurus PLoS ONE, 7 (4) DOI: 10.1371/journal.pone.0036227