The dentary becomes progressively larger during the evolution of mammal-like reptiles to mammals. The two main groups of dinosaurs, the Saurischia and Ornithischia, are recognized on the basis of their pelvic structure. Rocks which contain fossils were formed from the mud, clay or sand which covered the plants or animals. Ornithischian dinosaurs included only plant-eating types. Thus primitive features in the skeleton can link an animal to an ancestor which lived perhaps many millions of years earlier.
Fossils occur only in sedimentary rocks.
Mammal-like reptiles had a single temporal opening, whereas dinosaurs and crocodiles had two. Primitive reptiles such as the parieasaur Bradysaurus retained the original, closed temporal region of ancestral reptiles. Therocephalians probably gave rise to the more advanced Cynodontia before becoming extinct in Triassic times. ZONE - a layer of sedimentary rock containing the same types of fossils throughout. On the basis of reptile fossils the Karoo has been divided into a number of zones, lying one on top of the other.