‘Bone-headed’ dino find hints at more small dinosaurs than we thought

There's a new dinosaur making its debut at the Royal Ontario Museum today, thanks to a team of Canadian and U.S. scientists, and the accommodating Alberta rancher that the new dino is named after.

Acrotholus audeti was a species of pachycephalosaur, or 'bone-headed' dinosaur, that lived about 85 million years ago, in what is now Alberta, Canada. They were likely plant-eaters, walked on two legs, and could reach up to about 2 metres in length and around 40 kg, fully grown. They were characterized by the thick dome of bone on top of their skulls (Acrotholus means 'high dome'), that could reach a thickness of over 10 centimetres, which was likely used for defense and in head-butting contests to secure a mate.

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The specimens on display in the museum now were identified by a research team with members from the ROM, the University of Toronto, and the Cleveland Museum of Natural History, and were found on the Albert ranch of Roy Audet. One of these specimens was discovered over 50 years ago, but it wasn't until the second, better specimen was found in 2008 that the scientists could identify this new species.

"Acrotholus provides a wealth of new information on the evolution of bone-headed dinosaurs. Although it is one of the earliest known members this group, its thickened skull dome is surprisingly well-developed for its geological age," said lead study author Dr. David Evans, a curator in the ROM's Vertebrate Palaeontology collection, according to a ROM press statement. "More importantly, the unique fossil record of these animals suggest that we are only beginning to understand the diversity of small-bodied plan-eating dinosaurs."

Identifying this new species adds to the growing evidence that smaller dinosaurs were much more common and diverse than scientists previously thought. The fossil record contains many more larger dinosaur specimens, but this may be due to the fact that larger dinosaur bones are more likely to survive to be fossilized and are more easily found. Smaller bones may be destroyed before they have a chance to be preserved, or they may simply go unnoticed.

"The only reason we know that pachycephalosaurs were so diverse is because of these domes which seem to be preferentially preserved in the fossil record," Dr. Evans said in an interview with The Canadian Press. "And so if other dinosaur groups are anything like pachycephalosaurs, they were probably a lot more diverse too. This has implications for how you reconstruct the diversity of particular dinosaur communities through time."

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"We can predict that many new small dinosaur species like Acrotholus are waiting to be discovered by researchers willing to sort through the many small bones that they pick up in the field," co-author Dr. Michael Ryan, a curator at the Cleveland Museum of Natural History, said in the ROM statement. "This discovery also highlights the importance of landowners, like Roy Audet, who grant access to their land and allow scientifically important finds to be made."

The study on Acrotholus audeti was conducted by Drs Evans and Ryan, along with U of T graduate students Caleb Brown, Derek Larson and Ryan Schott. The research paper, titled 'The oldest North American pachycephalosaurid and the hidden diversity of small-bodied ornithischian dinosaurs', was published May 7th in the journal Nature Communications, and the specimens of Acrotholus audeti are now on display at the Royal Ontario Museum.

(Images courtesy: Julius Csotonyi, Derek Larson/ROM)

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