Taxonomy and evolution Porpoises, along with whales and dolphins, are descendants of land-living ungulates (hoofed animals) that first entered the oceans around 50 million years ago (Mya). During the Miocene (23 to 5 Mya), mammals were fairly modern, meaning they seldom changed physiologically from the time. The cetaceans diversified, and fossil evidence suggests porpoises and dolphins diverged from their last common ancestor around 15 Mya. The oldest fossils are known from the shallow seas around the North Pacific, with animals spreading to the European coasts and Southern Hemisphere only much later, during the Pliocene. ORDER Artiodactyla Infraorder Cetacea Parvorder Odontoceti toothed whales Superfamily Delphinoidea Family Phocoenidae – porpoises Genus †Haborophocoena H. toyoshimai Genus Neophocaena N. phocaeniodes – Indo-Pacific finless porpoise N. sunameri – East Asian finless porpoise N. asiaeorientalis – Yangtze finless porpoise Genus †Numataphocoena N. yamashitai Genus Phocoena P. phocoena – harbour porpoise P. sinus – vaquita P. dioptrica – spectacled porpoise P. spinipinnis – Burmeister's porpoise Genus Phocoenoides P. dalli – Dall's porpoise Genus †Semirostrum S.ceruttii Genus †Septemtriocetus S. bosselaersii Genus †Piscolithax P. aenigmaticus P. longirostris P. boreios P. tedfordi Recently discovered hybrids between male harbour porpoises and female Dall's porpoises indicate the two species may actually be members of the same genus. Biology Anatomy Porpoises have a bulbous head, no external ear flaps, a non-flexible neck, a torpedo shaped body, limbs modified into flippers, and a tail fin. Their skull has small eye orbits, small, blunt snouts, and eyes placed on the sides of the head. Porpoises range in size from the and Vaquita to the and Dall's porpoise. Overall, they tend to be dwarfed by other cetaceans. Almost all species have female-biased sexual dimorphism, with the females being larger than the males, although those physical differences are generally small; one exception is Dall's porpoise. Odontocetes possess teeth with cementum cells overlying dentine cells. Unlike human teeth, which are composed mostly of enamel on the portion of the tooth outside of the gum, whale teeth have cementum outside the gum. Porpoises have a three-chambered stomach, including a fore-stomach and fundic and pyloric chambers. Porpoises, like other odontocetes, possess only one blowhole. Breathing involves expelling stale air from the blowhole, forming an upward, steamy spout, followed by inhaling fresh air into the lungs. All porpoises have a thick layer of blubber. This blubber can help with insulation from the harsh underwater climate, protection to some extent as predators