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. In mammals, the occipital condyle is double-faced. A small third eye was still generally present in the top of the skull, but its opening was a tiny one. In many therapsids, the occipital condyle became double, as in the mammals. In many respect, the tritylodont skull was very mammalian in its features.