Although Archaeopteryx is by far the best-known of the transitional fossils, it is not the only one, or even the best. The fossil transition from reptile to mammal is one of the most extensive and well-studied of all the transitions, and detailed series of fossils demonstrate how this transition was accomplished. It is not, therefore, surprising that the creationists do not talk much about the reptile-mammal series, and when they do, most of what they say is demonstrably untrue. The mammals are believed to have evolved from a class of Permian and Triassic reptiles known as therapsids. In particular, mammals are distinguished from reptiles by a number of skeletal traits.
Reptiles have a much larger number of individual bones in their skulls than do mammals.
In reptiles, the teeth are all of the same shape, and although they vary slightly in size, they all have the same simple cone-shaped form. In reptiles, the lower jaw is made up of a number of different bones, and the jaw joint is formed between the quadrate bone in the skull and the angular bone in the jaw. In mammals, by contrast, the lower jaw is made up of a single bone, the dentary, which articulates with the squamosal bone in the skull to form the jaw joint. Reptiles also have a single bone in the middle ear, the stapes. Finally, the reptilian skull is attached to the spine by a single point of contact, the occipital condyle.