Mammals, with an advanced physiology, maintain a constant body temperature which is independent of that of the environment, and fur and sweat-glands are generally linked with, respectively, conservation and loss of body heat. Physiologically therefore, mammals are advanced over reptiles and are capable of sustained activity in a variety of environmental conditions. In the reptile there are a number of bones in the lower jaw, and the teeth are all simple and pointed. The bones of the skull are loosely connected to each other. In the mammal the lower jaw consists of a single bone, and the teeth are divided into incisors, canines, premolars and molars.
Mammalian teeth are generally much more complicated than reptilian teeth.
In mammals, too, the skull bones are firmly united with each other. These differences between reptiles and mammals are reflected to a certain extent in the bony skeleton in the two groups. Thus, increased locomotory activity in mammals is accompanied by changes in the form and orientation of the limb bones, while the enlarged brain of mammals requires a more sturdy skull than the reptilian one. Furthermore, increased feeding efficiency in mammals in order to maintain physiological processes at a high rate in turn necessitates improved chewing techniques and changes in the structure and arrangement of the teeth. In addition, the chewing process in mammals requires a firm and rigid lower jaw, and while in reptiles each half of the lower jaw consists of several bones, in mammals only a single enlarged bone, the dentary, is present.