What is a mammal?
A mammal is a vertebrate animal with hair and glands to produce milk for feeding its young. Their middle ears include three bones--two of these were jaw bones in their reptilian ancestors. Mammals are also warm-blooded and have a four-chambered heart, but these traits are shared with unrelated animals (birds and theropods). The ancestors of modern mammals lived in the Triassic Period, in the early stages of the Age of Reptiles.
Photo by B D Patterson of Artibeus lituratus in Ecuador, June 2011