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Volborthellidae
''Volborthella'' is an animal of uncertain classification, whose fossils pre-date . It has been considered for a period a cephalopod. However discoveries of more detailed fossils showed that ''Volborthella''’s small, conical shell was not secreted but built from grains of the mineral silicon dioxide (silica), and that it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living ''Nautilus'' are. This illusion was a result of the laminated texture of the organisms' tests.〔 Therefore, ''Volborthella''’s classification is now uncertain.
It has been speculated that it may in fact represent a sclerite of a larger organism, on the basis of one specimen; however, it may be premature to accept this hypothesis, as the arrangement of sclerites producing this impression may have been occurred by chance.〔 The Ordovician scleritome-bearing ''Curviconophorus'', as well as the Halwaxiids, lobopods and echinoderms, demonstrate the diversity of organisms which may produce a scleritome of this nature.〔 The related ''Campitius'' was originally suggested to be part of a radula rather than a scleritome.
''Volborthella'' is widespread, and a useful biostratigraphic indicator;〔 it has been found in North America, Greenland, Spitsbergen, and northeastern Europe, and is restricted to Lower Cambrian rocks, appearing before the trilobites and co-existing with them for some time.〔
''Volborthella'' has been described as a 'failed attempt in mineralisation'; it constructed its wall by agglutinating grains of sediment, much like some foramanifera, and declined in conjunction with the origin of truly-biomineralising organisms. However, its similarity to ''Salterella'', which contained embedded sediment grains in its mineralised wall, suggests a possible biological relationship - ''Salterella'' may have begun to mineraluse its agglutinated wall. Some ''Volborthella'' specimens do show taphonomic hints that some form of lightly mineralised sheath may have enclosed its agglutinated shell.〔
==References==
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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