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''Leonardoceras'' is a nautiloid cephalopod from the lower Middle Ordovician of Nevada (USA), the shell a small slender exogastric cyrtocone with the venter more narrowly rounded than the dorsum, resembling in overall form a small ''Bassleroceras''. Septa are close spaced, the living chamber short, the siphuncle close to the venter. The type species, ''Leonoardoceras parvum'' Flower was found the Antelope Valley limestone, in the north wall of Ikes Canyon in the Toquima Range, Nevada, about 11 feet below the top of the asaphid trilobite beds. No other species were named. Although in basic form ''Leonardoceras'' resembles ''Bassleroceras'', it differs in the siphuncle. bassleroceratids have thick connecting rings inherited from their ellesmeroceratid ancestors, which were passed on to the Tarphycerida. ''Leonardoceras'' has a poorly preserved, narrow siphuncle, apparently with thin connecting rings characteristic of the Oncocerida and later Nautilida. ''Leonoardoceras'' is considered, as of ref. date, to be the most primitive of the Graciloceatidae, the ancestral family of the Oncocerida. ==''References''== *Rousseau H Flower, 1968. Some Additional Whiterock Cephalopods. Memoir 19 Part II. State Bureau of Mines and Mineral Resource. New Mexico Institute of Mining and Technology, Socorro, New Mexico. * Walter C. Sweet, 1964. Nautilodea -Oncocerida. Treatise on Invertebrate Paleontology, Part K. Geological Society of America and University of Kansas Press. 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「Leonardoceras」の詳細全文を読む スポンサード リンク
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