Mammals have existed for the past 200 million years. According to fossil evidence, they evolved from a mammallike reptile group. These reptile ancestors were small, active carnivores. They were equipped with several specialized types of teeth. Their limbs were positioned more directly under the trunk as opposed to spreading out to the sides. Both of these characteristics are prominent features of mammals.
For the past 70 million years mammals have been the dominant animals in terrestrial ecosystems. Because they are warm-blooded, they have an advantage over their reptilian ancestors. Warm-bloodedness makes them capable of longer terms of sustained activity than cold-blooded organisms can undertake. This allows them to exploit environments that would be inhospitable to cold-blooded animals. (See also animal; animal behavior;animal migration; animals, domesticated.)
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