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Armenoceratidae is a family of early Paleozoic nautiloid cephalopods belonging to the Order Actinocerida.〔Flower 1957.Studies of the Actinoceratida.; Memoir 2; New Mexico Bureau of Mines and Mineral Resources, Socorro, NM〕 〔Teichert 1964. Actinoceratoidea, Treatise on Invertebrate Paleontology, part K.(Nautiloidea)〕 The Armenoceratidae, established by Troedsson (1926) 〔 is characterized by large, straight or slightly curved shells and large siphuncles with strongly expanded segments between the septa. Septal necks are short and abruptly recurved along brims. Radial canals in the endosiphuncular canal system are typically arched, curving forward and backward from near the septal foramina (openings) to connect with the parispatium on either side of the middle of each segments.〔 The parispatium is the narrow opening between the inner side of the connecting rings in actinocerids and the internal siphuncular deposits that grow forward and back from the region of the septal openings. The Armenoceratidae have their beginning in ''Armenoceras'' which first appeared near the beginning of the Chazyan, 2nd stage of the Middle Ordovician in older established chronologies, in northeastern China, Manchuria.,〔〔Flower 1976 Ordovician Cephalopod Faunas and Their Role in Correlation. in Basset M. C. (ed) The Ordovician System. Palaeonological Association.〕 derived from ''Wutinoceras''. Additional genera include ''Nybyoceras'' and ''Selkirkoceras'', respectively from the upper Middle and Upper Ordovician, and ''Monocyrtoceras'', ''Elrodoceras'', and ''Megadiscoceras'' from the Silurian,〔 The Armenoceratidae, ''Armenoceras'', ''Nybyoceras'', and ''Selkirkoceras'' first appear in North America in the diverse cephalopod Redriveran faunas of the early Upper Ordovician 〔〔 but are no longer found during that stage in Asia. ''Selkirkoceras'' is a large armenocerid from the Upper Ordovician with a blunt, flattened, somewhat breviconic shell. Among the Silurian genera, ''Elrodoceras'' has a large shell with the apical part slightly curved, otherwise is straight, and a siphuncle that is narrower than in ''Armenoceras''. ''Monocyrtoceras'' has a siphuncle like that of ''Elrodoceras'', but the entire shell is gently and evenly curved. ''Megadiscosorus'' is similar to ''Armenoceras'' but more breviconic and slightly exogastrically cyrtoconic and with the siphuncle in contact with the ventral wall. The Armenoceratidae most likely gave rise through ''Armenoceras'' to ''Gonioceras'' in the Chazyan, and later in the Middle Ordovician possibly through either an early ''Armenoceras'' or ''Nybyoceras'' to ''Actinoceras'' ==''References''== 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「Armenoceratidae」の詳細全文を読む スポンサード リンク
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