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. Most are viviparous with the developing embryo receiving nutrients from the female across the placenta . A few are oviparous.
Mammals have large brains in comparison to other vertebrate groups and are capable of learning.
Parental care of long duration helps young learn from the parents. The oldest fossil mammals are dated to 220 million years ago. Early mammals coexisted with dinosaurs throughout the Mesozoic era. Most Mesozoic mammals were small, probably insectivorous and nocturnal. Mammals continued to diversify during the Cenozoic.