Acrotholus audeti is believed to have lived in Alberta about 85 million years ago. Now, it's on display at the ROM.
A new dinosaur species is now on display at the Royal Ontario Museum, and researchers say it’s shedding light on the origins of all small dinosaurs. The new specimen, believed to have lived in southern Alberta, was discovered on an expedition led by David Evans, the ROM’s curator of vertebrate paleontology, and Michael Ryan of the Cleveland Museum of Natural History. It’s just one of many discoveries being made as part of the Southern Alberta Dinosaur Project.
The new dino, dubbed Acrotholus audeti, belongs to a class of dinosaurs with the nickname “bone heads.” They’re called that not because they were dumb (although they are believed to have had peanut-sized brains), but because the group, properly referred to as pachycephalosaurs, is characterized by thick, bony forehead domes. Acrotholus is believed to have been about six feet long and knee-high. Researchers believe it weighed 40 kilograms, and that it had a barrel-shaped body and strong hind legs. Evans thinks it would have looked like a T. rex, scaled down. This dinosaur, however, was an herbivore and lived 85 million years ago, whereas the T. rex was a carnivore that lived 66 million years ago.
The decisive fossil, a thick cranial plate, was discovered in 2008 by a University of Toronto PhD student, Caleb Brown. It took years of testing to verify that the specimen was, in fact, a new species. Sitting in the ROM’s staff offices, Brown recalled the day he made the find.
“We were working in the Milk River formation, which is an area with not a lot of fossils,” he said. “It’s not like Dinosaur Park, where you can walk and find a lot of fossils everywhere, so anything you can find is fairly significant. I was with one of my colleagues, Derek Larson. He was running geological sections to try to figure out where things are stratigraphically. I was mainly around just to sort of scout around and see if I could find anything, but also to have two people in the field for safety reasons.”
“I saw the dome sticking out of the side of a hill. Instantly, I knew it was something important because I could recognize the shape as a pachycephalosaur.”
“I also knew that we didn’t know of any dome-headed dinosaurs from that formation yet,” Brown added. “So instantly there was a good chance that this was a new species. It’s the best thing I’ve ever found.”
It’s an important find, because there aren’t many small-dinosaur fossils in existence. The scientific community is divided over why this might be, but there are essentially two different opinions on the matter. Some say there simply weren’t that many small dinosaurs to begin with, so the fossil record has very few of them. Others contend that the smaller dinosaur bones were more susceptible to the effects of weathering, and didn’t fossilize. Evans agrees with the latter.
“I think we still have a lot to learn about the shape of dinosaur diversity,” Evans said. “When you look at the distribution of species and their body sizes in modern ecosystems there are a lot more small ones than there are big ones. When you look at the fossil record of dinosaurs, you get the exact opposite—you get a lot more big ones than small ones. That’s where these pachycephalosaurs come in, because these characteristic domes are essentially like solid rocks that don’t get destroyed easily before preservation. They fossilize well. We can find them pretty easily, and they are the hallmark of a particular species. So they give us an interesting insight into the diversity of this particular small-body group.”
The purpose of the thick dome plating is uncertain, but some researchers have speculated that the pint-sized dinos used them for head-butting competitions, similar to the way elk lock horns during mating season. Each species of pachycephalosaur has its own distinctive dome shape.
Acrotholus audeti is just the first in a series of new discoveries that Evans’ team expects to release to the public. In the rocks of Alberta, they’ve discovered seven new specimens in the last nine years. ROM visitors can expect to see the results on display.
“The most exciting new dinosaurs on the horizon are new horn dinosaurs, relatives of triceratops,” Evans explained. “The two that we’re digging up right now, we’ve already got a number of their bones out of the ground. They really show a dramatic range of different hooks and spikes coming out of their skulls. It’s really exciting to see such a strange animal. These are some of the exciting discoveries you’ll hear about in a year or two. Still a lot of work to go on them, but they are very flashy, bizarre dinosaurs.” Evans hinted at one particular species with ten arm-sized spikes growing out of its head.
A paper on the discovery of Acrotholus audeti was published in the May 7, 2013 edition of Nature Communications, a science journal.
This post original misstated the name of one of Caleb Brown’s colleagues. He’s Derek Larson, not Derek Morrison.