| 翻訳と辞書 |
Squamata
The Squamata, or the scaled reptiles, are the largest recent order of reptiles, comprising all lizards and snakes. With over 9,000 species, it is also the second-largest order of vertebrates, after the perciform fish. Members of the order are distinguished by their skins, which bear horny scales or shields. They also possess movable quadrate bones, making it possible to move the upper jaw relative to the neurocranium. This is particularly visible in snakes, which are able to open their mouths very wide to accommodate comparatively large prey. They are the most variably sized order of reptiles, ranging from the dwarf gecko (''Sphaerodactylus ariasae'') to the green anaconda (''Eunectes murinus'') and the now-extinct mosasaurs, which reached lengths of .
Among the other reptiles, squamates are most closely related to tuataras, which superficially resemble lizards.
== Evolution ==
Squamates are a monophyletic sister group to the tuatara. The squamates and tuatara together are a sister group to crocodiles and birds, the extant archosaurs. Fossils of the squamate sister group, the Rhynchocephalia, appear in the Early Triassic, meaning that the lineage leading to squamates must have existed as well. Modern squamates probably originated in the mid Jurassic,〔 when fossil relatives of geckos and skinks and snakes〔Michael Caldwell ''et al.'' "The oldest known snakes from the Middle Jurassic-Lower Cretaceous provide insights on snake evolution", ''Nature Commynications'', 27 January 2015, summarized in (''Christian Science Monitor'', Joseph Dussault "How did snakes evolve? Fossil discovery holds clues." ): accessed 28 January 2015〕 appear; other groups, including iguanians and varanoids, first appear in the Cretaceous period. Also appearing in the Cretaceous are the polyglyphanodonts, a lizard group of uncertain affinities, and the mosasaurs, a group of predatory, marine lizards that grew to enormous sizes. At the end of the Cretaceous, squamates suffered a major extinction at the K-T boundary which wiped out polyglyphanodonts, mosasaurs, and a number of other groups.
The relationships of squamates have been debated. Although many of the groups originally recognized on the basis of morphology are still accepted, our understanding of their relationships to each other has changed radically as a result of studies of DNA. From morphological data, the iguanians were long thought to be the most ancient branch of the tree 〔 however studies of the DNA suggest that the geckos represent the most ancient branch. Iguanians are now united with snakes and anguimorphs in a group called the ''Toxicofera''. DNA also suggests that the various limbless groups- snakes, amphisbaenians, and dibamids- are unrelated, and instead arose independently from lizards.
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「Squamata」の詳細全文を読む
スポンサード リンク
| 翻訳と辞書 : 翻訳のためのインターネットリソース |
Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.
|
|