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 fossilised 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 (1.8 metres) from nose to tail and weighing in at 40 kilogrammes (88 pounds), the animal had a ridge of solid bone more than 10 centimetres (four inches) 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 kilogrammes (220 pounds) each.
"When we look back at the Age of Dinosaurs, it's easy to focus on the big animals like T. rex," said Evans.
"But there is a growing body of evidence that the landscape would have been filled with small dinosaurs as well."
More is known about pachycephalosaurs than many other small dinosaur groups, mainly because their thick skulls were better able to resist the ravages of the elements and time.
The rest of their skeletons, like those of most small dinosaurs, were much more easily weathered or chewed up by predators before they could be turned into fossils.
"We can predict that many new small dinosaurs 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," said Michael Ryan of the Cleveland Museum of Natural History.