The Helcionellids are fossil animals first found about 540 to 530 million years ago in the late Nemakit-Daldynian age, which is the earliest of the Cambrian period and singles species persisted to the Early Ordovician.[1][2][3][4] These fossils are component of the small shelly fossils (SSF) assemblages.[1] They are thought to be early molluscs with rather snail-like shells, and may be the ancestors of the modern conchiferans, a group that includes all the well-known modern families – gastropods, cephalopods and bivalves.[5][6] Shelled molluscs therefore predate the earliest fossil trilobites.[7]
Parkhaev (2006, 2007) consider them as gastropods. Previous to 2006 classification by Parkhaev were helcionellids 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[8]
[edit] 2006-2007 taxonomy
According to the opinion of P. Yu. Parkhaev[9][10] is order Helcionelliformes within the subclass Archaeobranchia Parkhaev, 2001 in the class Gastropoda.
Order Helcionelliformes Golikov & Starobogatov, 1975
[edit] Description
This restoration of the tiny Helcionellid fossil
Yochelcionella shows water flowing in under the shell, over the gills and out through the "exhaust pipe"
[7] Helcionellids have a single shell, in which the peak forms a distinctive curve. [11] Some have horizontal "exhaust pipes" on the concave edges of their shells, and there is debate about whether these pointed forwards or backwards.[7] Most Helcionellid fossils that have been collected are only a few millimeters long and rather snail-like. However specimens a few centimeters long have also been found, mainly limpet-like in shape, although some were laterally compressed and others were tall. The smallest specimens may have been juvenile or larval forms of the larger specimens.[11]
[edit] References
- ^ a b Budd, Graham E (2003). "The Cambrian Fossil Record and the Origin of the Phyla" (Free full text). Integrative and Comparative Biology 43 (1): 157–165. doi:10.1093/icb/43.1.157. http://intl-icb.oxfordjournals.org/cgi/content/abstract/43/1/157.
- ^ Michael Steiner, Guoxiang Li, Yi Qian, Maoyan Zhu and Bernd-Dietrich Erdtmann (2007). "Neoproterozoic to early Cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the Yangtze Platform (China)". Palaeogeography, Palaeoclimatology, Palaeoecology 254: 67–99. doi:10.1016/j.palaeo.2007.03.046.
- ^ Gubanov, Alexander P.; Peel, John S. (2003). "Latest Helcionelloid Molluscs from the Lower Ordovician of Kazakhstan". Palaeontology 44 (4): 681-694. doi:10.1111/1475-4983.00198. http://www3.interscience.wiley.com/journal/119019210/abstract.
- ^ Peel, John S.; Radvan J. Horný (2004). "A new problematic Early Ordovician univalve mollusc from France". Palaeontology 47 (6): 1629-1639. doi:10.1111/j.0031-0239.2004.00414.x. http://www3.interscience.wiley.com/journal/118760753/abstract.
- ^ Peel, J.S. (1991). "Functional morphology of the Class Helcionelloida nov., and the early evolution of the Mollusca". in Simonetta, A.M. and Conway Morris, S. The Early Evolution of Metazoa and the Significance of Problematic Taxa. Cambridge University Press. pp. 157–177. ISBN 0521402425.
- ^ Gubanov, A.P., and Peel, J.S. (November 2003). "The early Cambrian helcionelloid mollusc Anabarella Vostokova". Palaeontology 46 (5): 1073–1087. doi:10.1111/1475-4983.00334.
- ^ a b c Runnegar, B. and Pojeta, J. (1974). "Molluscan phylogeny: the paleontological viewpoint". Science 186 (4161): 311–7. doi:10.1126/science.186.4161.311. PMID 17839855. http://www.sciencemag.org/cgi/content/abstract/186/4161/311. Retrieved 2008-07-30.
- ^ 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. ISSN 0076-2997. 397 pp. http://www.vliz.be/Vmdcdata/imis2/ref.php?refid=78278
- ^ P. Yu. Parkhaev (2006) "Adaptive radiation of the Cambrian helcionelloid mollusks (Gastropoda, Archaeobranchia)." In: S.V. Rozhnov (ed.) "Evolution of the biosphere and biodiversity. Towards the 70th anniversary of A. Y. Rozanov". 2006. Moscow, pp. 282-296.
- ^ P. Yu. Parkhaev (2007). "The Cambrian ‘basement’ of gastropod evolution". Geological Society, London, Special Publications 286: 415–421. http://books.google.com/books?id=GA7-8JIh9IwC&pg=PA415.
- ^ a b Mus, M. M.; Palacios, T.; Jensen, S. (2008), "Size of the earliest mollusks: Did small helcionellids grow to become large adults?", Geology 36 (2): 175, doi:10.1130/G24218A.1, http://geology.geoscienceworld.org/cgi/content/abstract/36/2/175
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