The toothed whales (systematic name Odontoceti) form a parvorder of the artiodactyl infraorder Cetacea, including sperm whales, beaked whales, dolphins, and porpoises. As the name suggests, the parvorder is characterized by the presence of teeth rather than the baleen of other whales. Seventy-three species of toothed whales are described. They are thought to have split from baleen whales, parvorder Mysticeti, around 34 million years ago. Whales and dolphins, the paraphyletic groups of Cetacea, as well as porpoises, belong to the clade Cetartiodactyla with even-toed ungulates; their closest living relatives are the hippopotamuses which diverged about 40 million years ago.