This class comprises over 5000 diverse species, including our own, that can be found across the world. The smallest mammals are found among shrews or bats while the largest is identified as the blue whale. Mammals can be found in many different environments and biomes whether land, aquatic, tropical, or polar. Although vastly different, all mammals share three characteristics: 3 middle ear bones, hair, and production of milk by mammary glands. There are three major groups of mammals that are united by embryonic development features: Monotremes (egg laying mammals), Marsupials, and placental mammals.