Mammals developed from ancient reptiles 200 million years ago. Their rise and subsequent proliferation coincided with the decline of the reptiles on the planet. Body hair, mammary glands, live birth, a highly efficient 4-chambered heart, and a constant, raised body temperature are all general characteristics of mammals that have allowed this group to replace reptiles as the dominant animal group on the planet. Mammals can be found on every continent and in every ocean.
To learn more about these creatures, follow the links below.