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The German shepherd-sized Stegoceras, displayed at the Royal Tyrrell Museum in Alberta, lived about 72 million years ago in North America. (Sebastian Bergmann/Wikimedia Commons)

A small, plant-eating dinosaur that became extinct 65 million years ago was the king of the head butt, new research from the University of Calgary suggests.

Eric Snively and Jessica Theodor surveyed the heads of a large number of modern animals such as bighorn sheep and muskox, as well as the skull of a German shepherd-sized Stegoceras specimen from the University of Alberta. They found that the bony anatomy of the dino's dome would have been better at protecting the brain than anything in a modern head-butter.

Their research results were published in the peer-reviewed journal PLoS One this week.

The dome on Stegoceras's skull was about 10 centimetres thick, said Snively, a University of Calgary alumnus and post-doctoral researcher in biomedical engineering at Ohio University. 

"What surprised us was the internal structure of the skull. There's sort of alternating layers of stiff and compliant bone in the domes of these dinosaurs," he added. "It's almost as if they are wearing a double motorcycle helmet."

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A skull of Stegoceras validus from the American Museum of Natural History shows its unusual dome. (Ideonexus/Wikimedia Commons)

Theodor speculates that Stegoceras might have used its head the same way that bighorn sheep or muskox do today.

Head-butting is a way for the males of many modern hoofed animals to compete for breeding females, said Theodor, an associate professor in the biological sciences department at the University of Calgary.

There is some argument about whether the same sort of behaviour was followed by dinosaurs, but it's probable that was the case.

"I'm not going to go out and say, 'Yes, they absolutely were head-butting,' because half of my colleagues will jump down my throat," Theodor said.   

"But without going back in a time machine, I would say in this case it's almost certainly sex that's going on. These kind of behaviours, at least in mammals today, are pretty much all about males fighting each other for access to females."

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University of Calgary researcher Jessica Theodor (above) worked with Eric Snively to compare the dinosaur's skull with those of modern animals such as bighorn sheep and muskox. (Courtesy of University of Calgary)

Snively added that dinosaurs had to use what they were born with when it came to battling others. There's evidence, for instance, that Triceratops would square off with its horns and joust.

Others like T. Rex would take a more face-to-face and violent approach.

"It depends on what kind of equipment you have and what you're likely to do with it. We do know carnivorous dinosaurs were really like aggressive dogs on a bad acid trip or (with) 'roid rage' kind of going at each other and biting each other on the face."

Snively said Stegoceras was "very successful, very good" at what it did, but suffered the same fate as its bigger and buffer brothers when it died out about 65 million years ago.