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Pinnipeds (), (from Latin ''pinna'' fin and ''pes, pedis'' foot) commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic marine mammals. They comprise the extant families Odobenidae (whose only living member is the walrus), Otariidae (the eared seals: sea lions and fur seals), and Phocidae (the earless seals, or true seals). There are 33 extant species of pinnipeds, and more than 50 extinct species have been described from fossils. While seals were historically thought to have descended from two ancestral lines, molecular evidence supports them as a monophyletic lineage (descended from one ancestral line). Pinnipeds belong to the order Carnivora and their closest living relatives are bears and musteloids (weasels, raccoons, skunks and red pandas), having diverged about 50 million years ago. Seals range in size from the and Baikal seal to the and southern elephant seal, which is also the largest carnivoran. Several species exhibit sexual dimorphism. They have streamlined bodies and four limbs that are modified into flippers. Though not as fast in the water as dolphins, seals are more flexible and agile. Otariids use their front limbs primarily to propel themselves through the water, while phocids and walruses use their hind limbs. Otariids and walruses have hind limbs that can be pulled under the body and used as legs on land. By comparison, terrestrial locomotion by phocids is more cumbersome. Otariids have visible external ears, while phocids and walruses lack these. Pinnipeds have well-developed senses—their eyesight and hearing are adapted for both air and water, and they have an advanced tactile system in their whiskers or vibrissae. Some species are well adapted for diving to great depths. They have a layer of fat, or blubber, under the skin to keep warm in the cold water, and, other than the walrus, all species are covered in fur. Although pinnipeds are widespread, most species prefer the colder waters of the Northern and Southern Hemispheres. They spend most of their lives in the water, but come ashore to mate, give birth, molt or escape from predators, like sharks and killer whales. They feed largely on fish and marine invertebrates; but a few, like the leopard seal, feed on large vertebrates, such as penguins and other seals. Walruses are specialized for feeding on bottom-dwelling mollusks. Male pinnipeds typically mate with more than one female (polygyny), although the degree of polygyny varies with the species. The males of land-breeding species tend to mate with a greater number of females than those of ice- or water-breeding species. Male pinniped strategies for reproductive success vary between defending females, defending territories that attract females and performing ritual displays or lek mating. Pups are typically born in the spring and summer months and females bear almost all the responsibility for raising them. Mothers of some species fast and nurse their young for a relatively short period of time while others take foraging trips at sea between nursing bouts. Walruses are known to nurse their young while at sea. Seals produce a number of vocalizations, notably the barks of California sea lions, the gong-like calls of walruses and the complex songs of Weddell seals. The meat, blubber and fur coats of pinnipeds have traditionally been used by indigenous peoples of the Arctic. Seals have been depicted in various cultures worldwide. They are commonly kept in captivity and are even sometimes trained to perform tricks and tasks. Once relentlessly hunted by commercial industries for their products, seals and walruses are now protected by international law. The Japanese sea lion and the Caribbean monk seal have become extinct in the past century, while the Mediterranean monk seal and Hawaiian monk seal are ranked Critically Endangered by the International Union for Conservation of Nature. Besides hunting, pinnipeds also face threats from accidental trapping, marine pollution, and conflicts with local people. ==Taxonomy== |2= }} |2=grey seal }} |2= }} |2=hooded seal }} |2=bearded seal }} |2= |2=Ross seal }} }} |2= }} |2= }} |2= |2= }} |2= |3= }} |2= |2=Steller sea lion }} }} |2=northern fur seal }} |label2=Odobenidae |2= walrus }} }} }} |caption=Cladogram showing relationships among the pinnipeds, combining several phylogenetic analyses.〔 〕 }} The German naturalist Johann Karl Wilhelm Illiger was the first to recognize the pinnipeds as a distinct taxonomic unit; in 1811 he gave the name Pinnipedia to both a family and an order.〔Scheffer, (p. 52. )〕 American zoologist Joel Asaph Allen reviewed the world's pinnipeds in an 1880 monograph, ''History of North American pinnipeds, a monograph of the walruses, sea-lions, sea-bears and seals of North America''. In this publication, he traced the history of names, gave keys to families and genera, described North American species and provided synopses of species in other parts of the world. In 1989, Annalisa Berta and colleagues proposed the unranked clade Pinnipedimorpha to contain the fossil genus ''Enaliarctos'' and modern seals as a sister group. Pinnipeds belong to the order Carnivora and the suborder Caniformia (known as dog-like carnivorans). Of the three extant families, the Otariidae and Odobenidae are grouped in the superfamily Otarioidea,〔(【引用サイトリンク】publisher=Paleobiology Database )〕 while the Phocidae belong to the superfamily Phocoidea.〔(【引用サイトリンク】publisher=Paleobiology Database )〕 Otariids are also known as eared seals due to the presence of pinnae. These animals rely on their well-developed fore-flippers to propel themselves through the water. They can also turn their hind-flippers forward and "walk" on land.〔Riedman, p. 64.〕 The anterior end of an otariid's frontal bones extends between the nasal bones, and the supraorbital foramen is large and flat horizontally. The supraspinatous fossas are divided by a "secondary spine" and the bronchi are divided anteriorly.〔 Otariids consist of two types: sea lions and fur seals. Sea lions are distinguished by their rounder snouts and shorter, rougher pelage, while fur seals have more pointed snouts, longer fore-flippers and thicker fur coats that include an undercoat and guard hairs. The former also tend to be larger than the latter.〔Riedman, pp. 68–70.〕 Five genera and seven species (one now extinct) of sea lion are known to exist, while two genera and nine species of fur seal exist. While sea lions and fur seals have historically been considered separate subfamilies (Otariinae and Arctocephalinae respectively), a 2001 genetic study found that the northern fur seal is more closely related to several sea lion species. This is supported by a 2006 molecular study that also found that the Australian sea lion and New Zealand sea lion are more closely related to ''Arctocephalus'' than to other sea lions.〔 Odobenidae consists of only one living member: the modern walrus. This animal is easily distinguished from other extant pinnipeds by its larger size (exceeded only by the elephant seals), nearly hairless skin and long upper canines, known as tusks. Like otariids, walruses are capable of turning their hind-flippers forward and can walk on land. When moving in water, the walrus relies on its hind-flippers for locomotion, while its fore-flippers are used for steering. In addition, the walrus lacks external ear flaps.〔Riedman, pp. 82–83.〕 Walruses have pterygoid bones that are broad and thick, frontal bones that are V-shaped at the anterior end and calcaneuses with pronounced tuberosity in the middle.〔 Phocids are known as true or "earless" seals. These animals lack external ear flaps and are incapable of turning their hind-flippers forward, which makes them more cumbersome on land. In water, true seals swim by moving their hind-flippers and lower body from side to side.〔 Phocids have thickened mastoids, enlarged entotympanic bones, everted pelvic bones and massive ankle bones. They also lack supraorbital processes on the frontal and have underdeveloped calcaneal tubers.〔 A 2006 molecular study supports the division of phocids into two monophyletic subfamilies: Monachinae, which consists of ''Mirounga'', ''Monachus'' and Lobodontini; and Phocinae, which includes ''Pusa'', ''Phoca'', ''Halichoerus'', ''Histriophoca'', ''Pagophilus'', ''Erignathus'' and ''Cystophora''. In a 2012 review of pinniped taxonomy, Berta and Morgan Churchill suggested that, based on morphological and genetic criteria, there are 33 extant species and 29 subspecies of pinnipeds, although five of the latter lack sufficient support to be conclusively considered subspecies. They recommend that the genus ''Arctocephalus'' be limited to ''Arctocephalus pusillus'', and they resurrected the name ''Arctophoca'' for several species and subspecies formerly placed in ''Arctocephalus''.〔 More than 50 fossil species have been described. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Pinniped」の詳細全文を読む スポンサード リンク
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