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Gymnitidae is a family of Lower to Middle Triassic ammonite cephalopods with evolute, discoidal shells. Hyatt and Smith (1905, p. 114-115) included the Gymnitidae in the suborder Ceratitoidea, which later became the superfamily Ceratitaceae and included in it genera more primitive than ''Gymnites'' as well as the more advanced ''Gymnites''. Those being ''Xenaspis'', ''Flemingites'', and ''Ophiceras''. Derivation as shown in Smith (1932 p. 30) is from ''Xenodiscus''. The more primitive ''Xenaspis'', ''Flemingites'', and ''Ophiceras'', found in Lower Triassic beds in western America have ceratitic sutures. The more developed ''Gymnites'' has deeply digitate ammonitic sutures. Arkell, et al., 1957, in the Treatise on Invertebrate Paleontology, on the other hand included the Gymnitidae in the Pinacocerataceae as the earlier and more primitive of its two families, combining ''Gymnites'' with coeval and more advanced forms. Genera included in the Gymnitidae sensu Arkell include ''Eogymnites'', ''Buddhaites'', ''Japanites'', and of course, ''Gemnites''. The genus ''Xiphogymnites'' was included by Tozer in 1981. Whether to place the Gymnitidae in the Ceratitaceae, as in Hyatt and Smith, combining ''Gymnites'' with its more primitive relatives, or in the Pinacocerataceae, as in Arkell et al., combining ''Gymnites'' with its coeval or more advanced relatives is a matter of perspective which does little to change the overall phylogeny. ==References== * Arkell, et al., 1957. Mesozoic Ammonidea. Treatise on Invertebrate Paleontology, Part L, Ammonoidea. Geol Soc of America and Univ. Kansas Press. *Alpheus Hyatt and James Perrin Smith, 1905. The Triassic Cephalopod Genera of America. U.S. Geological Survey Professional Paper no. 40. (p. 114-115) * James Perrin Smith, 1932. Lower Triassic Ammonoids of North America. U.S. Geological Survey Professional Paper no. 167 (p. 30) 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「Gymnitidae」の詳細全文を読む スポンサード リンク
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