CANADA: The discovery of a new thick-skulled dinosaur the size of a large dog may challenge our image of a pre-historic Earth dominated by supersized lizards, a study said Tuesday.

The planet may, in fact, have been inhabited by many more types of small dinosaur than widely thought, a group of researchers wrote in the journal Nature Communications.

“It would have been a world filled with a diversity of dinosaur life, both large and small,” study co-author David Evans of the Royal Ontario Museum’s natural history department said of the results.

Today, Earth is dominated by small-bodied animals, including mammals and reptiles.

But dinosaur fossil finds have painted a picture of a very different world during the Mesozoic era, from about 250 to 65 million years ago, in which monster-sized creatures prevailed. Scientists disagree on whether this meant the bigger animals were simply more numerous, or that their remains have been better preserved. Now, evidence for the latter theory has been found in fossilized skull fragments discovered in the Milk River Formation of southern Alberta, Canada. The remains are from a small, plant-eating dinosaur that strode the Earth hunched on two muscled hind legs some 85 million years ago.

About six feet from nose to tail and weighing in at 40 kg, the animal had a ridge of solid bone more than 10 cms thick on the top of the skull — possibly used in head-butting contests. The feature gave rise to its name: Acrotholus audeti after the Greek for “high dome.”

Acrotholus is the oldest species from a group of thick-skulled dinosaurs known as pachycephalosaurs in North America, and possibly the world, the researchers wrote.

From studying the new species’ place in the pachycephalosaur family tree, the team concluded there was a lot yet to be discovered about diversity in this and other groups of small dinosaur — classified as animals weighing less than 100 kgs each. “When we look back at the Age of Dinosaurs, it’s easy to focus on the big animals like T. rex,” said Evans.