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Knightoconus
''Knightoconus antarcticus'' is a Cambrian monoplacophoran thought to represent an ancestor to the cephalopods. It had a chambered, conical shell, but lacked a siphuncle. The absence of this siphuncle has been taken as evidence against cephalopod ancestry, as the factors that would influence the siphuncle to penetrate preexisting septal chambers remains unknown. The prevailing argument suggests that a strand of tissue remained attached to the previous septum as the mollusc moved forwards and deposited its next septum, producing an obstacle to the complete closure of the septum and becoming mineralised itself. Ten or more septa are found in mature individuals, occupying around a third of the shell - septa form very early and have been found in specimens as small as 2 mm in length. Unlike monoplacophoran fossils, there is no evidence of muscle scarring in ''Knightoconus'' fossils.〔 Scars from the closely related ''Hypseloconus'' have been used to determine its orientation. ''Knightoconus'' started life with an exogastric shell, that becomes endogastric as the organism grew.
Alternate taxonomy is: Tergomya, Kiringellida, Hypseloconidae.〔(The Paleobiology Database ). Accessed 6 August 2009.〕
== See also ==
* Evolutionary history of cephalopods
* The Late Cambrian fossil ''Plectronoceras'' is now thought to be among the earliest cephalopod fossils.
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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