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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Pachydiscus ： ウィキペディア英語版 Pachydiscus ''Pachydiscus'' is an extinct ammonite genus from the Upper Cretaceous with a world-wide distribution, and type for the desmoceratacean family Pachydiscidae. Its type is ''P. neubergicus''. Altogether some 28 species have been described. The shell of ''Pachydiscus'' is compressed and high-whorled, with an oval or flat sided section. Ribs tend to be differentiated into short umbilical and separate ventro-lateral sets, with a smooth area between. ''Pachydiscus'' includes two subgenera, ''P.(Pachysiscus)'' from the Campanian in which the ribs persist, and ''P (Neodesmoceras)'' from the Maastrichtian in which ribs disappear early, leaving an almost smooth shell. == Biostratigraphic significance == The International Commission on Stratigraphy (ICS) has assigned the First Appearance Datum of ''Pachydiscus neubergicus'' as the defining biological marker for the start of the Maastrichtian Stage of the Cretaceous, 72.1 ± 0.2 million years ago. 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「Pachydiscus」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.