Dinosaurs are classified into two orders according to the shape of their hip bones. The Saurischia are the 'lizard-hipped' dinosaurs. The other order of dinosaurs have hips shaped more like those of birds, and are called the Ornithischia. The distinction is clear if you look at the skeletons of the two types of dinosaurs, but is more difficult to visualize if you look at the whole animal: many bird-hipped dinosaurs look nothing like birds, while many lizard-hipped dinosaurs walked on their hind legs and looked rather bird-like.

Saurischian dinosaurs are themselves separated into two groups called suborders. The first suborder were the meat-eating theropods. The second suborder were plant-eaters, and are called sauropods. These included Diplodocus, Apatosaurus and Brachiosaurus.

The family tree on this page shows how the saurischian dinosaurs were related to one another. In fact, there is continuing debate about the exact relationship of various theropod groups, and the scheme used here is adapted from several recent sources. As you can see, most of the families of dinosaurs lived in the Jurassic or the Cretaceous periods. The earliest saurischians were found in the Triassic period, and these were the prosauropods and the first coelurosaurs.