ARCHOSAURIA
One of the main ways of classifying dinosaurs if by their hip type, either saurischian (meaning "reptile-hipped") or ornithischian (meaning "bird-hipped"). All dinosaurs can be divided into one of these two groups.
Dinosaur hips were made up of three bones that could be found on each side of the pelvis: the ilium, the ischium, and the pubis. In reptile-hipped dinosaurs the ischium pointed backwards and the pubis pointed forwards. With these dinosaurs, the joints and connections between the hip and leg bones were very simple, which made it possible for the dinosaur to carry its legs directly beneath its body. In this fashion, the legs acted like the pillars of a building, capable of holding up a substantial amount of weight.
Among the reptile hipped dinosaurs there were meat-eaters, such as Tyrannosaurus, and plant-eaters, such as Diplodocus.
Bird-hipped dinosaurs, on the other hand, were all plant eaters. Their hips were different from reptile-hipped dinosaurs because their pubis pointed backwards along with the ischium. Bird-hipped dinosaurs included some of the more distinctive-looking dinosaurs, such as the heavily armored Ankylosaurus or the Stegosaurus.