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''Bactroceras'' is a genus of the family Baltoceratidae, orthoceroid cephalopods that lived during the early Middle Ordovician, from about 472—464 mya, existing for approximately 8 million years.〔 Nautiloidea – Ellesmerocerida by W.M Furnish and Brian F. Glensister; Treatise on Invertebrate Paleontology Part K R.C. Moore (ed) Univ.Kans. press〕 ==Taxonomy== ''Bactroceras'' was named by Holm (1898). Its type is ''Bactroceras avus''. It was assigned to the ''Baltoceratidae'' by Furnish and Glenister(1964)〔 and retained there by Evans (2005).〔D. H. Evans. 2005. The Lower and Middle Ordovician cephalopod faunas of England and Wales. Monograph of the Palaeontographical Society 623:1-81〕 Some paleontologists have regarded Bactroceras as an early bactritid because of its spherical apex and ventral siphuncle.〔 〕 However, a more recent study has argued that the shell of ''Bactroceras'' has important differences from those of true bactritids. For instance, the first shell chamber of ''Bactroceras'' resembles that of other Ordovician orthocerids, such as ''Archigeisonoceras'' and ''Hedstroemoceras'': it is about 10 mm in diameter and is short, forming a spherical cap. True bactritids more strongly resemble late Silurian and Devonian orthocerids, whose first chamber is only about 5 mm across. Moreover, there is a large stratigraphic gap of nearly 50 million years between ''Bactroceras'' and the next orthocones with a spherical apex and ventral siphuncle.〔 〕 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「Bactroceras」の詳細全文を読む スポンサード リンク
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