Prehistoric Life Prehistoric Secrets in Stone The Earliest Fossils The Silurian Reef Trilobites Fish The Coal Forests Early Reptiles and Amphibians Sea Reptiles Flying Reptiles The Teeming Seas Hunting Dinosaurs Biggest Dinosaurs Two-Footed Plant Eaters Plated and Armoured Dinosaurs Horned Dinosaurs The Mammal Age Dawns Grassland Mammals The Ice Age	Reptile Evolution Two main reptile groups evolved in the Permian and Triassic periods. The first was the mammal-like reptiles, such as Cynognathus. The second main group was the thecodontians. They began as crocodile-like animals. These reptiles were to develop into the dinosaurs.   Previous: Back Book: Prehistoric Life Section: Prehistoric Chapter: Early Reptiles and Amphibians