page_title: Whale
section_title: Evolution
hierarchical_section_title: Whale / Taxonomy and evolution / Evolution
caption_attribution_description: Skeleton of a basilosaurus cetoides in the Sant Hall of Oceans in the Smithsonian Museum of Natural History in Washington, D.C. It is the only genuine specimen currently exhibited anywhere in the world. In proportion to its body size, Basilosaurus cetoides had extremely small - 2 foot (0.6 m) long hind limbs which had limited mobility, and could only assume 2 positions: against the body, or belly. Scientists believe the limbs were used during sex, to help grasp the female.
context_page_description: Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. They are an informal grouping within the infraorder Cetacea, usually excluding dolphins and porpoises. Whales, dolphins and porpoises belong to the order Cetartiodactyla, which consists of even-toed ungulates. Their closest living relatives are the hippopotamuses, having diverged about 40 million years ago. The two parvorders of whales, baleen whales and toothed whales, are thought to have split apart around 34 million years ago. Whales consist of eight extant families: Balaenopteridae, Balaenidae, Cetotheriidae, Eschrichtiidae, Monodontidae, Physeteridae, Kogiidae, and Ziphiidae.
  Whales are fully aquatic, open ocean creatures, and feed, mate, give birth, suckle and raise their young at sea. Whales range in size from the 2.6 metres and 135 kilograms dwarf sperm whale to the 29.9 metres and 190 metric tons blue whale, which is the largest known creature that has ever lived. The sperm whale is the largest toothed predator on earth. Several species exhibit sexual dimorphism, in that the females are larger than males.
context_section_description: Whales are descendants of land-dwelling mammals of the artiodactyl order (even-toed ungulates). They are related to the Indohyus, an extinct chevrotain-like ungulate, from which they split approximately 48 million years ago. Primitive cetaceans, or archaeocetes, first took to the sea approximately 49 million years ago and became fully aquatic 5–10 million years later. What defines an archaeocete is the presence of anatomical features exclusive to cetaceans, alongside other primitive features not found in modern cetaceans, such as visible legs or asymmetrical teeth. Their features became adapted for living in the marine environment. Major anatomical changes included their hearing set-up that channeled vibrations from the jaw to the earbone (Ambulocetus 49 mya), a streamlined body and the growth of flukes on the tail (Protocetus 43 mya), the migration of the nostrils toward the top of the cranium (blowholes), and the modification of the forelimbs into flippers (Basilosaurus 35 mya), and the shrinking and eventual disappearance of the hind limbs (the first odontocetes and mysticetes 34 mya).
  Whale morphology shows a number of examples of convergent evolution, the most obvious being the streamlined fish-like body shape. Other examples include the use of echolocation for hunting in low light conditions — which is the same hearing adaptation used by bats — and, in the rorqual whales, jaw adaptations, similar to those found in pelicans, that enable engulfment feeding.
  Today, the closest living relatives of cetaceans are the hippopotamuses; these share a semi-aquatic ancestor that branched off from other artiodactyls some 60 mya. Around 40 mya, a common ancestor between the two branched off into cetacea and anthracotheres; nearly all anthracotheres became extinct at the end of the Pleistocene 2.5 mya, eventually leaving only one surviving lineage – the hippopotamus.
  Whales split into two separate parvorders around 34 mya – the baleen whales (Mysticetes) and the toothed whales (Odontocetes).
caption: Basilosaurus skeleton
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