Mammals are warm-blooded vertebrates with the unique features of fur and milk glands. The first mammals evolved from reptiles about 210 million years ago during the Triassic Period. Early mammals were small, shrew-like creatures. Mammals did not flourish until the mass extinction event that killed off the dinosaurs approximately 65 million years ago.
Within 10 million years of the extinction event, mammals evolved rapidly to fill every vacant ecological niche left by the dinosaurs. Mammals not only became the dominant vertebrates on land, but also ventured into the water (as whales and dolphins) and the air (as bats). Since the extinction of the dinosaurs, mammals have become very diverse and successful.