There are 4,600+ mammal species in the world and more are still being discovered. Fossil evidence dates back to the Mesozoic Era, Triassic Period which is about 248 million years ago. Mammals are unified under the broad kingdom Animalia and belong to the class Mammalia. What makes mammals distinct from other creatures is their ability to suckle young from mammary glands filled with nourishing milk, the presence of hair, and different skeletal features. Mammals are able to maintain a constant body temperature and give birth to live young. One order of mammals, the Chiroptera (bats), are capable of true winged flight. The order Cetacea includes aquatic mammals like whales, dolphins, and porpoises. As you can see, mammals have a great and wonderful variety of shapes and sizes and they occupy many different niches in the environment.