A head-butting dinosaur with a skull like a conker is helping scientists get a better picture of life 85 million years ago.
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.
The plant-eater's 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.
Dr Michael Ryan, one of the authors of the study, said:
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.
Dr David Evans, who co-wrote the article about the findings in the Nature Communications journal, said:
The unique fossil record of these animals suggests that we are only beginning to understand the diversity of small-bodied plant-eating dinosaurs.
Two Acrotholus skull caps were found in southern Alberta, Canada.
Like other members of the bonehead family, Acrotholus left no clues to its existence other than its thickened skull.