What is a dinosaur?

Dinosaurs were a group of land animals that appeared about 235 million years ago and, in one branch, never left. They are defined not by size or ferocity but by quiet details of the skeleton: legs held straight underneath the body rather than sprawled out to the side, and matching features of the hip and ankle. That upright stance is the real signature of the group.

Early on, dinosaurs split into two lineages told apart by the arrangement of their hip bones. The saurischiansA 'lizard-hipped' dinosaur — the theropods and sauropodomorphs., or "lizard-hipped" dinosaurs, include the meat-eating theropodA mostly meat-eating, bipedal dinosaur. Birds are living theropods.s and the long-necked sauropodA giant, long-necked, four-legged plant-eater like Brachiosaurus.s. The ornithischiansA 'bird-hipped' dinosaur. All were plant-eaters (Triceratops, Stegosaurus)., or "bird-hipped" dinosaurs, were all plant-eaters: the horned, plated, armoured, and duck-billed kinds.

Plenty of famous prehistoric animals are not dinosaurs at all. Pterosaurs, the flying reptiles, were close cousins. Plesiosaurs and mosasaurs swam in the seas. Dimetrodon, with its tall sail, was a relative of mammals and died out long before the first dinosaur. Dinosaurs are land animals, full stop. The fliers and swimmers belong to other branches of the reptile family tree.

Almost everything in this guide is a reading of bone. Size, the shape of teeth, where an animal lived, sometimes how it grew: those come straight from fossils. Skin colour, the exact spread of feathers, courtship, the sounds they made: those are reconstructions, the artist's and scientist's best guess. This guide tries to keep the two apart, because the honest version is the more interesting one.