Edentates - hinshirui (English spelling)

A general term for animals belonging to the order Edentata in the class Mammalia. Members of this order Edentata are primitive eutherians that evolved in America. They either have no teeth or their teeth are simple cone-shaped and lack enamel, and no deciduous teeth occur on the premaxilla. They have one more pair of articular processes on the thoracic and lumbar vertebrae than other mammals, the acromion and coracoid process of the scapula are abnormally long and often fused to each other, and the ischium is fused to the frontmost caudal vertebra. They have 6 to 9 cervical vertebrae. The braincase (the part of the skull that holds the brain) is low and cylindrical, the premaxilla is reduced, and they often have a frontonasal bone. The zygomatic arches are often incomplete. They have no cecum or if they do have one it is small, and they have a single uterus. The testes remain in the inguinal or abdominal cavity. The Palaeanodonta suborder flourished in North America from the Paleocene to the Oligocene of the Tertiary Period, and the Xenarthra suborder evolved from it. The latter is divided into the Pilosa infraorder, which lacks a shell, and includes the Anteaters and Sloths, and the Cingulata infraorder, which includes the Armadillos and other sloths with developed shells. The Pilosa terrestrial sloths include the cow-sized Megalonyx (Megalonychidae), the rhinoceros-sized Mylodon (Mylodontidae), and the elephant-sized Megatherium (Giant slothidae), while the Cingulata infraorder includes the gigantic Glyptodon (Glyptodontidae), which weighed about 2 tons.

[Yoshinori Imaizumi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

哺乳(ほにゅう)綱貧歯目に属する動物の総称。この目Edentataの仲間は、アメリカで進化した原始的な真獣類で、歯がないか、あっても単純な円錐(えんすい)形でエナメル質を欠き、乳歯は前顎(ぜんがく)骨には生じない。胸椎(きょうつい)と腰椎の関節突起がほかの哺乳類より一対多く、肩甲骨の肩峰と烏口(うこう)突起が異常に長く、しばしば互いに癒着し、坐骨(ざこつ)は最前位の尾椎と癒着する。頸椎(けいつい)は6～9個。頭骨の脳筺(のうきょう)（脳を入れる部分）は低く円筒形、前顎骨は縮小し、しばしば前鼻骨がある。頬弓(きょうきゅう)は多くは不完全。盲腸はないか、あっても小さく、子宮は単一。精巣は鼠径(そけい)部か腹腔(ふくこう)内にとどまる。北アメリカの第三紀暁新世から漸新世にかけて栄えたパレアノドン亜目Palaeanodontaと、それから分かれて進化した異節亜目Xenarthraがあり、後者は、アリクイ科、ナマケモノ科などからなる甲のない有毛下目Pilosaと、アルマジロ科などの甲が発達したものを含む被甲下目Cingulataに分けられる。有毛下目の地上性のナマケモノ類には、ウシ大のメガロニクス（メガロニクス科）、サイ大のミロドン（ミロドン科）、体重約5トンでゾウと同大のメガテリウム（オオナマケモノ科）などが、被甲下目には体重約2トンのグリプトドン（グリプトドン科）のように巨大なものがあった。

［今泉吉典］

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