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What makes a dinosaur a dinosaur?

A defining feature of a dinosaur like Velociraptor was a hole in their hip socket (a Velociraptor was a carnivorous dinosaur closely related to birds). Other reptiles, such as lizards and crocodiles, have legs that sprawl out to the side, with thigh bones almost parallel to the ground. They walk and run with a side-to-side motion.
Dilong paradoxus in his native habitat © AMNH
Dilong paradoxus in his native habitat © AMNH
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.
diagram of crocodile hip and dinosaur hip
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 because they share a common ancestor with non-avian (non-flying) dinosaurs, which became extinct about 66 million years ago. They share features such as the hole in the hip socket, the three-toed foot and the s-shaped neck.

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