They have an efficient respiratory system which utilizes a diaphragm for ventilation. A four chambered heart segregates the blood stream. Mammary glands secrete milk. Teeth are differentiated into various shapes and sizes which are adapted to chewing many types of food. The jaw apparatus of the ancestral reptiles was also modified during evolution with two of the jaw bones becoming incorporated into the middle ear.
Mammals are dioecious with sexual reproduction and internal fertilization.
Parental care of long duration helps young learn from the parents. Evolution of Mammals. From the synapsid order Therapsida, a relatively unspectacular reptilian group. Mammals diversified after the wave of Cretaceous extinctions There are about 4500 extant species. Extinction of the dinosaurs opened new adaptive zones at the end of the Mesozoic era.