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Xenacanthida (or Xenacanthiforms) is an order of prehistoric sharks that appeared during the Lower Carboniferous period. The order includes the families ''Xenacanthidae'', ''Diplodoselachidae'', and ''Orthacanthidae''. The most notable members of the group are the genera ''Xenacanthus'' and ''Orthacanthus''. Some ''Xenacanthus'' may have grown to lengths of . This group of sharks inhabited freshwater environments. Some forms had large serrated spines extending backwards from the neck. ''Xenacanthus'' had characteristic teeth. The base or root of each tooth had a pair of hook-like cusps. Most ''Xenacanthus'' died out at the end of the Permian in the Permian Mass Extinction, with only a few forms surviving into the Triassic period. ==Taxonomy== * Family: Diplodoselachidae Dick, 1981 * * Genus: ''Diplodoselache'' Dick, 1981 * * Genus: ''Dicentrodus'' Traquair, 1888 * * Genus: ''Hagenoselache'' Hampe & Heidkte, 1997 * Family: Orthacanthidae Heyler & Poplin 1989 * * Genus: ''Orthacanthus'' Agassiz, 1843 * Family: Xenacanthidae Fritsch, 1889 * * Genus: ''Plicatodus'' Hampe, 1995 * * Genus: ''Triodus'' Jordan, 1849 * * Genus: ''Xenacanthus'' Beyrich, 1848 * ''incertae sedis'' * * Genus: ''Anodontacanthus'' Davis, 1881 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「Xenacanthida」の詳細全文を読む スポンサード リンク
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