Sea creatures - Monsters

A general term for marine mammals, including Cetacea, Sirenia, Pinnipedia, Desmostylidae, and some of the Mustelidae in the Carnivora order. There are marine mammals that remain on land their entire lives, and those that come onto land at least during breeding season. The former includes about 76 species of Cetacea, including whales and dolphins, and about 5 species of Sirenia, including dugongs and manatees, which are believed to have branched off from the ancestors of ungulates (extinct animals called condylarths). On the other hand, the latter includes approximately 34 species of pinnipeds, such as sea lions, fur seals, sea lions, walruses, and seals, which branched off from carnivora; the Desmostylidae order (extinct), which resembles Sirenia but has large hind legs; sea otters, which are carnivores of the Mustelidae family; some river otters; and the sea mink of North America (Maine), which became extinct around 1860.

Cetaceans, which appeared in the lower Eocene (early part) of the Tertiary Period, are the most highly adapted to marine life, with no hind limbs, tails that have been transformed into flippers, and no hair except around the mouth. Sirenia, which appeared in the middle Eocene (middle part), are less adapted to marine life, with sparsely haired bodies, mobile forelimbs at the carpals, and sometimes claws. Desmostylidae, which are very similar to Sirenia, and Pinnipedia, which appeared much later in the Miocene, have densely haired bodies and flippers for their hind limbs. Marine mammals of the Mustelidae family are thought to have moved into the sea even later.

[Yoshinori Imaizumi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

海にすむ哺乳(ほにゅう)類の総称で、クジラ目、海牛目、鰭脚(ききゃく)目、デスモスチルス目、食肉目イタチ科の一部が含まれる。海獣には終生陸にあがらないものと、少なくとも繁殖期には陸にあがるものとがある。前者には、クジラ、イルカなどのクジラ目約76種と、有蹄(ゆうてい)類の先祖（顆節(かせつ)類という絶滅動物）から分かれ出たと推定されるジュゴン、マナティーなどの海牛目約5種とがある。一方、後者には、食肉類から分かれ出たアシカ、オットセイ、トド、セイウチ、アザラシなどの鰭脚目約34種、海牛目に似るがりっぱな後肢のあるデスモスチルス目（絶滅）、食肉目イタチ科のラッコ、一部のカワウソ、1860年ごろ絶滅した北アメリカ（メーン州）のウミミンクなどが含まれる。

　第三紀始新世下部（前期）に出現したクジラ目はもっとも高度に海生に適応していて、後肢がなく、尾がひれに変わり、口の周りを除いて毛がない。始新世中部（中期）に出現した海牛目は海生への適応度がやや低く、体に毛がまばらに生え、前肢は手根部が可動性で、ときにつめを備えている。海牛目によく似たデスモスチルス目、および鰭脚目ははるかあとの中新世に現れたもので、後者は体に毛を密に生じ、後肢がひれになっている。イタチ科の海獣はさらに遅く海に進出したものと思われる。

［今泉吉典］

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