1. Thecodonts. Early reptiles called thecodonts were crocodile-like carnivorous reptiles that lived during the Triassic Period. Dinosaurs, birds, and crocodiles evolved from them.
2. Endothermy and ectothermy. Endothermic animals are able to control their body temperature using their own metabolic processes, while ectothermic animals must use the environment to control their body temperature.
3. Saurischians and ornithischians. Dinosaurs are divided into two main groups based on their hip design. In saurischians, the three hip bones have a three-pronged design and extend out in different directions. In ornithischians, two of the bones extend in the same plane parallel to each other.