Fortunately, teeth and jaws can help us to decide where fossils belong. The big change from reptiles to mammals is at the back of the jaw. Reptiles have seven bones in the lower jaw. The dentary, which houses the teeth, the articular, which forms the jaw joint and five other bones. Cynodonts have an additional joint in their jaws.
Mammals have only one bone in the lower jaw, the dentary.
The other bones of the lower jaw have moved into the middle ear to help us to hear. The mammals appeared in the Late Triassic. Megazostrodon is a good example of these early mammals. It was very small, only about 1.5cm long and weighing 20-30g. It is thought that they were nocturnal.