You don’t get to headbutt your way through life without picking up a few scars along the way. If you repeatedly ram your skull against your peers, you’ll pick up injuries, especially on the parts that suffer the most impacts. And when you die, your skeleton will preserve a record of your violent past.

Joseph Peterson from the University of Wisconsin has used this principle to study the lifestyle of a group of dinosaurs called pachycephalosaurs. The name comes from the Greek for “thick-headed lizards” and refers to the group’s most distinctive feature—a thick dome atop their skulls, usually fringed with small spikes. What was it for?

The most popular answer is that these dinosaurs used their skulls as battering rams, charging each other head-on to fight over mates, territories or both. The domes would have protected their brain during such collisions. But other dinosaur specialists, like Mark Goodwin and Jack Horner, have said that the dome was too brittle to be used as a ram. Instead, they think it was a billboard. The domes helped pachycephalosaurs to identify members of their own species; maybe they were even brightly coloured to patterned to attract the opposite sex.

The billboard idea, in turn, has its problems. Critics point out that the domes are much the same across different pachycephalosaur species, and change dramatically in shape as the animals grow up. They’re hardly not the best badges of identity.

View Images Pachycephalosaurus skeletons. Credit: Kabacchi

Peterson approached this debate from a new angle. He was inspired by a specimen of Pachycephalosaurus at the Burpee Museum of Natural History in Rockford, Illinois, which has large dents on the top of the dome. Perhaps these were the result of a collision with another individual? “No one paid much attention to them until we started thinking about how common those types of features really were,” says Peterson.

Together with Collin Dischler and Nicholas Longrich, he spent five years studying the domes of 109 pachycephalosaurs from over 14 species, searching for irregularities. He found a lot, including fractures that had since healed, and thick, irregular surfaces that indicated past infection and inflammation. And to his surprise, 22 percent of the domes across 9 species showed signs of damage. “That’s much higher than we expected,” says Peterson.

View Images Skull injuries on pachycephalosaur skull domes. Credit: Peterson et al. 2013. PLOS

Other palaeontologists had noticed these features before. “A lot of us, me included, had always assumed they were the result of erosion after the death of the animal,” says Andy Farke from the Raymond M. Alf Museum of Palaeontology. He means that the skulls might have been pitted by pebbles in a stream, or damaged by bone-eating worms, for example. “The new study pretty convincingly shows that this isn’t the case—these features are something that happened during the lifetime of the animal.” (Farke himself has used the pattern of bone dents in Triceratops skulls to show that these iconic dinosaurs locked horns in combat.)

If the pachycephalosaur skulls were eroded naturally, they would have holes everywhere. Instead, two-thirds of the injuries are on the frontal bone on the roof of the skull—the area that would suffer the most impacts during head-on collisions. This strongly suggests that the animals were indeed ramming each other. “We can equivocate over how solid the case for head-butting is, but I’m pretty confident that the skulls weren’t just for looks,” says Farke.

View Images Possible pachycephalosaur combat moves. Up Up Down Down Left Right Left Right A B to pull off a finisher. Credit: Ryan Steiskal

Many pachycephalosaur species had these injuries, despite existing at different times and having differently shaped domes, which suggests that the entire group was a dynasty of head-butters. And only the fully-domed specimens showed signs of damage. Skulls with flatter domes, which Peterson thinks belonged to females or youngsters, were free of injuries. This implies that, as in modern goats or cattle, only the males charged each other.

In fact, Peterson found more support for the head-butting idea by looking at modern mammals. Goats, which usually hit each other in the sides, tend to have rib and spinal injuries. Bison, which lock heads and push, suffer a mix of head and spinal damage. But sheep, which do ram each other head-on, mainly suffer from head injuries much like those of pachycephalosaurs.

But there’s a slight flaw in this comparison—we don’t have many complete pachycephalosaur skeletons. Their skulls are easily found, but we cannot tell if they also sustained the same sort of rib and spinal injuries as bison or goats. “I’d really, really like to take a look at injuries in the rest of the body for pachycephalosaurs,” says Farke. “I’m not entirely convinced that head-to-head ramming definitely occurred in these animals, and wonder if they might have been primarily flank-butters.”

Horner is even less convinced. Based on his own analysis, he says that the dome’s internal structure and bone tissues were completely different to that of modern head-butting animals. “This suggests that pachycephalosaurs could only head butt once, and that trauma would have likely killed them,” he says.

Peterson acknowledges that the bone in the domes is unlike anything in living animals. “Simply put, the domes are just weird!” he says. However, he points to work from Eric Snively at Ohio University, who scanned several domes and put them through the same virtual crash-tests that engineers use on cars. The result: “these domes could withstand quite a wallop”.

He says that he and Horner have agreed to disagree. “At the end of the day, we will never know what pachycephalosaurs used their bizarre domes for because we obviously cannot directly observe their behaviour,” he says.

If the domes were truly rams, they could have been billboards too. After all, prominent weapons like antlers or horns also make inherently good ads for an individual’s strength and vigour. “I think that the idea of the domes being used as a display structure is very likely,” says Peterson. “A display structure is great for communicating about how tough animals are… but sometimes they have to back that up.”