Ryan Steiskal

With domed heads and thick, bony skull protuberances, pachycephalosaurids are well known by seven-year-olds and palaeontologists alike. The dinosaurs are thought to have used their thick domes to headbutt each other, perhaps as part of courtship behaviour. But whereas children recreating these vicious displays simply ram plastic models of the animals together in a straight line, a study now suggests that pachycephalosaurs may have bashed one another in a number of different ways.

The work, presented this week at the Society of Vertebrate Paleontology's annual meeting in Raleigh, North Carolina, started with just one pachycephalosaur fossil. “We were looking at a dome and noticed these lesions that looked like they were from injuries,” explains Joseph Peterson, a palaeontologist at the University of Wisconsin in Oshkosh. He wondered whether the fossilized injury was one of a kind — or whether such lesions were common. To find out, Peterson and his colleague Collin Dischler started examining more domes.

Together, they were able to look at 102 domes from fossil collections around the world. Of these, 23 had lesions. Fascinated by the presence of so many injuries, Peterson and Dischler produced a three-dimensional computer model of a pachycephalosaur skull and mapped injuries onto it.

They noted that the shape of the dome differed from fossil to fossil, and that the placement of the injuries depended on the shape of the skull. Skulls with low domes tended to have injuries at the front, whereas skulls with higher domes had lesions evenly distributed between the front and back.

Living proof

The finding hinted that the pachycephalosaurs with differently shaped skulls were bumping heads in different ways. To find out what those ways were, the researchers considered the butting of sheep, goats and bison.

“The lesions we were seeing were strikingly similar to those that we often see on the skulls of modern mammals that ram heads,” says Peterson.

The team found that bighorn sheep (Ovis canadensis) charge each other head on; that mountain goats (Oreamos americanus) bump one another in the flanks; and that bison (Bison bison) wrestle with their horns. On the basis of these examples, Peterson and Dischler speculate that the high-domed pachycephalosaurs with parietal injuries were side-bumping like mountain goats, and that the frontal injuries are indicative of bison-like wrestling.

“It could be that we are seeing two different species bashing in different ways, but it could also be a single dinosaur species where juveniles and adults exhibit different bashing behaviours,” says Andrew Farke, a palaeontologist at the Raymond Alf Museum of Palaeontology in Claremont, California.

Bone to pick

Some palaeontologists disagree that pachycephalosaurs rammed one another at all, and there is much scepticism over whether the lesions really are ‘butt prints’.

“It is an intriguing study but after examining one of these ‘injuries’ I found it hard to rule out the possibility that this might have simply been the skull being chipped after death,” says Bruce Rothschild, a palaeo-osteopathologist at the University of Kansas in Lawrence.

Farke is confident that the debate will soon be resolved. “A look at skull bone microstructure should determine if the lesion sites actually suffered trauma,” he says.

Whatever the outcome, seven-year-olds will undoubtedly keep on bashing.