A mammalian order comprising approximately 90 living species of whales, dolphins, and porpoises and their fossil relatives. Like all other mammals but unlike all fish, cetaceans nurse their young with milk produced by the mother, are endothermic (warm-blooded), breathe air, have a lower jaw that consists of a single bony element (the dentary), and have three small bones (hammer, anvil, and stirrup) subserving sound transmission within the ear.