Dinosaur family tree

Dinosaur family tree. Click on image for large version. Source and copyright: Patricia Vickers-Rich.

Dinosaurs were descendants of a primitive group of archosaur reptiles, which lived between 200 and 280 million years ago. These gave rise to the saurischian or 'lizard-hipped' dinosaurs and the ornithischian or 'bird-hipped' dinosaurs, each of which diversified into a great variety of forms over the next 130 million years. The two groups are differentiated by palaeontologists according to the structure of the hip or pelvic girdle.

The saurischians included enormous plant-eating sauropods that walked on all fours such as Mamenchisaurus, and large two-legged carnivorous theropods including Tyrannosaurus rex. Among the ornithischians were the duck-billed hadrosaurs like Tsintaosaurus, and plated stegosaurs such as Stegosaurus. All the ornithischians are thought to have been plant-eaters. The Family Tree here indicates the relationship between many of the better-known dinosaurs.

Saurischia ('lizard-hipped' dinosaurs) and
Ornithischia ('bird-hipped' dinosaurs)

In the Saurischia the hip bones are arranged like those of other reptiles with one of the two bones below the hip joint pointing forwards (the pubis) and the other backwards (the ischium).

The hip bone of a saurischian dinosaur. Source: Museum Victoria, Vertebrate Palaeontology.

In the Ornithischia both of these bones point backwards, though in later forms there is a forward projecting part of the pubis.

The hip bone of an early ornithischian dinosaur. Source: Museum Victoria, Vertebrate Palaeontology.

The hip bone of a later ornithischian dinosaur. Source: Museum Victoria, Vertebrate Palaeontology.

Other differences in the skeletons of the two groups are the presence in the ornithischians of an extra bone at the front of the lower jaw (predentary bone), and the thin bony rods or tendons that support the backbones of the saurischians.