Dinosaur TypesArticle | Updated 1 decade ago Dinosaurs diversified into two main groups after they evolved some 230 million years ago in the Triassic Period. One group, called Saurischian dinosaurs includes the four-legged, long-necked, planteating sauropods and their relatives as well as the two-legged theropods. Most theropods were (and some continue to be) carnivores. The other group, called Ornithischian dinosaurs, were herbivores. The key differences between the two groups are in their hip bone and lower jaw structures. Saurischian dinosaurs This group of dinosaurs have a hip structure that resembles that of lizards. The main portion of the pubis bone points forwards or downwards, and is located at an angle to ischium bone. Saurischian hip structure. IMAGE: DORLING KINDERSLEY Ornithischian dinosaurs This group of dinosaurs have a hip structure that superficially resembles that of birds. The main portion of the pubis bone points backwards and is located right along the ischium bone. They also have a bone in their lower jaw, called the predentary bone, that is toothless and forms a beak. Ornithischian hip structure. IMAGE: DORLING KINDERSLEY Triceratops skull indicating predentary bone. IMAGE: DAVID HERRAEZ CALZADA ‹ Cretaceous Greenhouse World Polar Dinosaurs ›