Cetacea () is a widely distributed and diverse clade of aquatic mammals that today consists of whales, dolphins, and porpoises. Cetaceans are carnivorous and finned. Most species live in the sea, some in rivers. The name is derived from the Latin cetus "whale", itself from the Greek κῆτος kētos "huge fish".
There are around 89 extant species, which are divided into two groups or parvorders, the Odontoceti or toothed whales, a group of more than 70 species that includes the dolphins, porpoises, belugas, narwhals, sperm and beaked whales, and the Mysticeti or baleen whales, of which there are now 15 species. The extinct ancestors of modern whales are the Archaeoceti.
While cetaceans were historically thought to have descended from mesonychids, molecular evidence supports them as a relative of Artiodactyls (even-toed ungulates). Cetaceans belong to the order Cetartiodactyla (formed by combining Cetacea + Artiodactyla) and their closest living relatives are hippopotamuses and other hoofed mammals (camels, pigs, and ruminants), having diverged about 50 million years ago.
Cetaceans range in size from the 1 m (3 ft 3 in) and 50 kg (110 lb) Maui's dolphin to the 29.9 m (98 ft) and 173,000 kg (381,000 lb) blue whale, which is also the largest animal ever known to have existed. Several species exhibit sexual dimorphism. They have streamlined bodies and two (external) limbs that are modified into flippers. Though not as flexible or agile as seals, cetaceans can swim very quickly, with the killer whale able to travel at 56 kilometres per hour (35 mph) in short bursts and the fin whale able to cruise at 48 kilometres per hour (30 mph). Dolphins are able to make very tight turns while swimming at high speeds. The hindlimbs of cetaceans are internal, and are thought to be vestigial. Baleen whales have short hairs on their mouth, unlike the toothed whales. Cetaceans have well-developed senses—their eyesight and hearing are adapted for both air and water, and baleen whales have a tactile system in their vibrissae. They have a layer of fat, or blubber, under the skin to maintain body heat in cold water. Some species are well adapted for diving to great depths. Read more...