A head-butting dinosaur with a skull like a conker is helping scientists knock together a better picture of life 85 million years ago.

The plant-eater is the oldest bone-headed dinosaur in North America and possibly the world

The two-legged ‘bonehead’ dinosaur Acrotholus audeti had a two-inch thick skullcap which it may have used as a weapon.

Unlike most known dinosaurs it was small, weighing about 40 kilograms and about the same size as a dog.

Scientists believe the discovery hints at a greater diversity of small dinosaurs than had been suspected before.

Like other members of the bonehead family, Acrotholus left no clues to its existence other than its thickened skull.

The plant-eater is the oldest bone-headed dinosaur in North America and possibly the world. Yet its skull dome was unusually well-developed, suggesting that boneheads were highly diverse. Scientists suspect the same could be true of other small dinosaurs without bony heads whose scattered remains have so far eluded them.

“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,” said Michael Ryan, one of the study authors from The Cleveland Museum of Natural History in the US.

Two Acrotholus skull caps were found in southern Alberta, Canada.

A description of the finds is reported in the latest issue of the journal Nature Communications.

Co-author David Evans, from the Royal Ontario Museum in Canada, said: “The unique fossil record of these animals suggests that we are only beginning to understand the diversity of small-bodied plant-eating dinosaurs.”