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The group called dinosaurs includes all the animals that evolved from the very first dinosaur, the common ancestor. Because the fossil record is incomplete, we have not found fossils of this common ancestor; however, based on the characteristics we see in all dinosaurs, we can determine new and unique characteristics it possessed.

A key evolutionary innovation of dinosaurs is that they walk with a fully erect posture, holding their hind legs vertically under their hips. The top of the thighbone (or femur) has a knob or head that sticks out to the side of the rest of the bone and fits into the hip socket (or acetabulum) in the pelvis. The hip socket has a hole in the center, as well as a rim of bone along the upper margin, which helps support the weight of the body on the leg. Since these features are present in all known dinosaurs, we deduce that they first evolved in the common ancestor.

In popular culture, many other kinds of animals, which don’t have this fully developed hip and leg structure, are often incorrectly called dinosaurs. Some examples include Mesozoic marine reptiles, such as plesiosaurs, mosasaurs and ichthyosaurs, as well as a group of Mesozoic flying reptiles called pterosaurs.

This video is part of a series, “Dinosaurs Explained,” produced by the American Museum of Natural History. In the series, Museum paleontologists answer the most frequently asked questions about dinosaurs.