How do we know some dinosaurs had beaks?

The ornithomimid dinosaur is depicted with a beak

The ornithomimid is shown with a beak. Click to zoom.

These hadrosaur and chicken skulls show similar alterations to the bone texture in the area where the beaks grew

Skulls of a hadrosaur,a type of dinosaur (left), and a modern chicken (right). Click to zoom.

Beaks are made of keratin, like claws or fingernails, and do not fossilize often, but the bones they are attached to do fossilize, preserving evidence of the beak. In many plant-eating dinosaurs, the front of the jaw has a different texture than the rest of the skull, and has many small openings for blood vessels and nerves. This structure is typical of the jaws of of living beaked animals. It provides a rich blood supply to the beak, allowing it to grow with the animal. The areas highlighted in the photos are the beak attachment sites of a hadrosaur dinosaur, left, and a chicken, right.