; infraclass Eutheria, cohort Mutica)
An order that comprises the one extinct (Archaeoceti) and two extant (Odontoceti and Mysticeti) suborders of whales. The earliest whales (Archaeoceti) are known from Eocene rocks in Africa and South Asia (see Basilosaurus; Ambulocetus), and are descended from early Artiodactyla, probably more closely related to hippopotamus (Hippopotamidae) than to other groups. Of the two existing groups of whales, the Odontoceti (toothed whales) can be traced back to ancestral forms in the upper Eocene, while the first definitive Mysticeti (baleen whales) occur in Oligocene strata. Whales are streamlined, almost hairless, entirely aquatic. The fore limbs are modified to form paddles without visible digits, the hind limbs are absent, the pelvis is vestigial, except in some Archaeoceti. The tail fin is horizontal and used for propulsion. The skull is modified, with the nasal openings far back on the dorsal surface except in Physeteridae. The diet comprises fish and molluscs (Odontoceti) or mainly plankton (Mysticeti).
Subjects: Zoology and Animal Sciences.