To move through the air, they flapped large winglike webs of skin that stretched from their elongated front legs to their hind legs. The great difference between the reptiles and the birds and mammals that followed them is warm blood. Reptiles are cold-blooded, which means that they must seek warmth for their bodies from an external source. Birds with teeth lived from about 150 million to 65 million years ago. These creatures might have evolved from flying reptiles, replacing their skin flaps with feathers.
Mammals represent an advance from birds because of their method of reproduction.
Birds lay eggs that must be protected until they hatch. Many of the early mammals were ancestors of familiar animals of today. Ancient horses had three toes and were not much larger than sheep. There were also little camels that had no humps and small rhinoceroses without horns. Mammoths and mastodons resembled elephants but were larger.