Dog-sized dinosaurs may have been more diverse than thought

The bones of tiny dinosaurs simply didn’t make it to the fossilization process, weathered more during it, and are harder to find and identify now, Evans and a team of colleagues argue in a paper published in Nature Communications Tuesday.

“We have to be really careful about what we can infer” from the fossil record, says Evans: What we dig up is not necessarily an accurate reflection of what was roaming the planet tens of millions of years ago.

Caleb Brown, a University of Toronto graduate student, discovered the thick, distinctive skull cap of a pachycephalosaur on an Albertan rancher’s land in 2008 while on a digging trip in organized by Evans and a colleague, Cleveland Museum of Natural History’s Michael Ryan.

The team was excited: it was obviously a new species. Later, they found it was a match with a partial skull cap discovered over 50 years ago in the same region, now sitting in the ROM’s vast storage rooms.

On the basis of those two skulls, the researchers were able to name the new animal: Acrotholus audeti, after Roy Audet, the rancher who gave the team access to his land and took an active interest in their work.

Acrotholus is 85 million years old, the oldest pachycephalosaur in North America and possibly the world. The skull is highly developed from an evolutionary standpoint given its geological age, adding more diversity to the pachycephalosaurid family tree.

And the dome-head dinosaur provided a neat natural experiment.

Unlike today’s array of mammals, birds and reptiles, which features many very small animals like squirrels and finches and far fewer large-bodied ones like emus and rhinos, scientists have argued that the age of dinosaurs was dominated by massive species with few ecological niches for ones 100 kilograms and smaller. Some have argued that babies of the biggest dinosaurs crowded out the smallest ones.

To test that theory, Evans and his team compiled a database of dome-headed dinosaur fossils and those of other small dinosaurs like leptoceratops and thescelosaurs.

The diversity shown in the fossil record of the biggest dinosaurs vastly outstrips that of small dinosaurs. Among its small-bodied brethren, pachycephalosaurs are the most diverse.

But more than 85 per cent of pachycephalosaur species are based on the discovery of their hardy, super-thick skull caps, which had the greatest chances of surviving for millions of years. Their thin-boned skeletons, which are similar to that of other small dinosaurs, are much rarer, with just four or five ever discovered, Evans showed.

“These domes drive our perception of the diversity of these dinosaurs,” says Evans. “It seems likely that these other small little dinos, which have the same delicate (body) bones, are probably a heck of a lot more diverse than we ever thought.”

Even though pachycephalosaurs skulls have survived well, Evans believes their diversity is still underestimated, based on missing diversity in their family tree.

But that puzzle is not likely to be solved soon.

“We may never find these things,” says Evans.