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Belemnitina is a suborder of belemnites, an extinct group of cephalopods. They have been identified as the oldest of all the belemnites and are likely the stockgroup for them. They were extant from the early Jurassic to the early Cretaceous. Their geographic distribution was mostly limited to Europe, although specimens have been described in Japan suggesting they may have been more widespread than previously thought. ==Morphology== They have a broad, rounded pro-ostracum (the section of the guard (the hard internal skeleton commonly fossilised) closest to the head of the living animal). This characteristically lacks longitudinal alveolar canals and the associated splitting surfaces or open fissures. Commonly there are one or more longitudinal furrows in the apical region of the guard, similar furrows may be found in the alveolar region, these would not be accompanied by splitting surfaces or open fissures. ==Distribution== Based on current evidence it appears that the Belemnitina were mostly restricted to Europe prior to the Toarcian (late-early Jurassic), probably in small shelf seas until the Pliensbachian (middle-early Jurassic). After the middle Jurassic they appear to have spread but still remained primarily in Europe. They gradually retreated during the Jurassic, becoming extinct in the Mediterranean area in the Oxfordian (late Jurassic) times. The last remaining members of the Belemnitina disappear from North-western Europe at the Berremian-Aptian boundary (early Cretaceous). 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「Belemnitina」の詳細全文を読む スポンサード リンク
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