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The Helcionellids are fossil shells of very small marine mollusks that are preserved as shells. These animals are first found about in the late Nemakit-Daldynian age, which is the earliest part of the Cambrian period. A single species persisted to the Early Ordovician. These fossils are component of the small shelly fossils (SSF) assemblages.〔 These are thought to be early molluscs with rather snail-like shells. They may be the ancestors of the modern conchiferans, a group that includes all the well-known modern families – gastropods, cephalopods and bivalves. Shelled molluscs therefore predate the earliest fossil trilobites. They have also been considered to represent direct ancestors to the cephalopods. Parkhaev (2006, 2007) considers these animals to be crown-group gastropods. Previous to the 2006 classification by Parkhaev, helcionellids were classified within the separate class Helcionelloida or as "Uncertain position (Gastropoda or Monoplacophora)" within "Paleozoic molluscs of uncertain systematic position" according to the taxonomy of the Gastropoda by Bouchet & Rocroi, 2005〔Bouchet P. & Rocroi J.-P. (Ed.); Frýda J., Hausdorf B., Ponder W., Valdes A. & Warén A. 2005. ''Classification and nomenclator of gastropod families''. Malacologia: International Journal of Malacology, 47(1-2). ConchBooks: Hackenheim, Germany. ISBN 3-925919-72-4. . 397 pp. http://www.vliz.be/Vmdcdata/imis2/ref.php?refid=78278〕 == 2006-2007 taxonomy == According to the opinion of P. Yu. Parkhaev the order Helcionelliformes is within the subclass Archaeobranchia Parkhaev, 2001 in the class Gastropoda. Order Helcionelliformes Golikov & Starobogatov, 1975 * Superfamily Helcionelloidea Wenz, 1938 * *Family Helcionellidae Wenz, 1938 * *Family Igarkiellidae Parkhaev, 2001 * *Family Coreospiridae Knight, 1947 * Superfamily Yochelcionelloidea Runnegar & Jell, 1976 * *Family Trenellidae Parkhaev, 2001 * *Family Yochelcionellidae Runnegar & Jell, 1976 * *Family Stenothecidae Runnegar & Jell, 1980 * * *Subfamily Stenothecinae Runnegar & Jell, 1980 * * *Subfamily Watsonellinae Parkhaev, 2001 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「helcionellid」の詳細全文を読む スポンサード リンク
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