Pachycephalosaurus: The Cretaceous dinosaur with a domed skull

Pachycephalosaurus is easiest to remember for the thick dome of bone on top of its head — rather like a plump tomato balanced on its forehead. It was a plant-eating dinosaur that lived during the Late Cretaceous. Let's meet this most unusual skull.

Its body

The signature feature is the thick, rounded dome at the top of the head, where the bone is thickened into a smooth, hard cap. Scientists have debated its purpose for a long time: it may have been used for display and recognition between individuals, for head-butting between rivals, or simply to dissipate heat or make the head look more imposing. So far, there is no single agreed answer.

The dinosaur was not especially large and walked on two hind legs. It had small teeth that suited cropping plants, small fruits or seeds. The front limbs were shorter than the hind limbs and were probably used for grasping and balance.

Where it lived and what it ate

Pachycephalosaurus is usually viewed as a plant-eater, perhaps eating leaves, fruit and seeds, though some research suggests it might also have eaten small animals; the full range of its diet is still debated. It lived in the Late Cretaceous, on the same continent as Tyrannosaurus rex and Triceratops.

Where its fossils were found

Most Pachycephalosaurus fossils have come from North America, in rocks of the United States and Canada. Recently researchers have found exceptionally complete skulls, which let a team study the structure of the dome in much greater detail.

What the fossils tell us — and what they do not

The skull and some of the skeleton reveal the animal's general build and the shape of the dome, but how the dome was actually used, what colour the animal was, and how it moved each day have to be inferred from evidence. So the colours and motions on the stage are informed reconstructions, not confirmed facts.

A note for grown-ups

You could guide a child this way: "If it really used its head to bump rivals, why would it need such thick bone?" Encouraging a child to connect a feature to a possible function is exactly how palaeontologists think.