What makes a dinosaur a dinosaur?

A defining feature of dinosaurs was a hole in their hip socket. Other reptiles, such as lizards and crocodiles, have legs that sprawl out to the side, with thigh bones almost parallel to the ground so they walk and run with a side-to-side motion.

Dinosaurs, on the other hand, stood with their legs positioned directly under their bodies. A hole in their hip socket permitted this upright stance, which allowed dinosaurs to run faster and with greater endurance than other reptiles of the same size.

Another trait that sets dinosaurs apart is that they walked on their tiptoes all the time. This condition is common in many mammals—some of which can be quite fast, such as cats and dogs—but is unique among reptiles.

A third important trait of dinosaurs is that, whereas primitive reptiles have five toes and fingers, the first dinosaurs used only three toes to stand on the ground (we call this condition a functionally tridactyl foot). The first dinosaurs did have five toes on each foot, but two of them were tiny and did not reach the ground. Something similar happened with their hands—the two outer digits (the ring and pinky fingers) were very small. In some dinosaurs, the two tiny fingers even disappeared completely.

During the Age of Dinosaurs, there were other reptiles living on land and in the seas, including pterosaurs (flying reptiles), and plesiosaurs (ocean-dwelling reptiles). But they did not have a hole in their hip socket and so were not dinosaurs.

Modern birds are classified as a type of dinosaur and share features such as the hole in the hip socket, the three-toed foot, and an s-shaped neck.