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Sactoceras
''Sactoceras'' is an extinct nautiloid cephalopod that lived during the Ordovician and Silurian in what would become North America, Europe, and Asia.〔(PaleoBiology Database: ''Sactoceras'', basic info )〕
==Taxonomy==
The genus ''Sactoceras'' was named by Hyatt (1884). The type species is ''Orthoceras richteri'' Barrande, 1874, which comes from the Upper Silurian Ludlow Series of the Prague Basin, Czech Republic.〔 Evans, David H. 2000. A Cephalopod Fauna from the Middle Ordovician of Saudi Arabia. Palaeontology, Vol. 43, Part 3, 2000, pp. 573-589. 〕 Hyatt named ''Sactoceras'' for actinocerid species in which siphuncle segments were much reduced in diameter relative to the diameter of the shell, noting that the "siphon becomes approximately reduced . . . with age", which he interpreted as a "degradational senile shrinking". Barrande (according to Foord (1888)) considered this to indicate a reverson to the simple siphuncle of the Orthoceratidae, which might be taken to indicate a relationship to the Proteoceratidae.
Evans (2000)〔 considered ''Sactoceras'' to be an orthocerid, placing it in the Pseudorthoceratoidea (originally Pseudorthoceratacea) of Flower and Caster, 1935, and in the family Sactoceratidae of Troedsson, 1926. The Pseudorthocerataceae is now generally regarded as a separate order, the Pseuorthocerida, within the superorder Orthoceratoidea of Wade, 1988.〔 Wade, Mary 1988. Nautiloids and their descendants: cephalopod classification in 1988. New Mexico Bureau of Mines and Mineral Resources Memoir 44, October 1988. 〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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