Cetacea

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Cetacea

Cetacea is a widely distributed and diverse infraorder of fully aquatic marine mammals. There are 88 extant species of cetaceans. The two suborders of cetaceans, Mysticeti and Odontoceti, are thought to have split up 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. https://en.wikipedia.org/wiki/Cetacea The Cetacea are one of the most distinctive and highly specialized orders of mammals. They include the largest animal that has ever lived, the blue whale; the highly intelligent and communicative dolphins; the tusked narwhals and blind river dolphins and singing humpback whales — nearly eighty living species in all. Although hunting and other human activities have endangered most cetacean species, the outlook for many is improving. Whales are highly specialized for life in the water. Most species are marine, but dolphin species are found in the Yangtze, Amazon, Paraná, Indus and Ganges Rivers. Whales have streamlike bodies with highly compressed neck vertebrae, dorsal fins, and a tail with two finlike flukes arranged horizontally. Modern whales have greatly elongated anterior skull bones, and the nostrils are located on the top of the head, forming the blowhole. The forelimbs are specialized to form flippers, and the hind limbs and pelvis are extremely small and do not normally extend out of the body wall of the animal. Modern whales may be divided into two subgroups: the Odontoceti (toothed whales) and the Mysticeti (baleen whales). Odontocetes bear teeth — typically numerous and peglike, although sometimes modified, as in the single tusk of the narwhal or the odd, fanglike lower teeth of the beaked whales. Many odontocetes can navigate by echolocation, producing sound waves using a complex system of nasal sacs and passages, and using the echoes to navigate. Dolphins Whale and porpoises are examples of odontocetes, as are belugas, narwhals, killer whales, sperm whales, and beaked whales. Baleen whales lack teeth completely as adults (although teeth are present in fetal baleen whales). They feed by straining small marine organisms out of the water using plates of baleen, a hornlike substance that forms filaments that hang down from the roof of the mouth. Blue, right, humpback, minke, gray, and fin whales are well-known examples of baleen whales. Some baleen whales, most famously the humpack whales, are known for the strange and complex songs they produce; their function is not clear, but unlike toothed whales, baleen whales do not use their songs for echolocation. The fossil record of whales is rich, and recent discoveries have shed much light on the origin of the Cetacea. http://www.ucmp.berkeley.edu/mammal/cetacea/cetacean.html RD

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