Overview
Helcionelloida is the name given to an extinct group of ancient molluscs (phylum Mollusca). These are the oldest known conchiferan molluscs, that is, they had a mineralised shell. Some members of this class were mistaken for Monoplacophorans. The class was erected by Peel in 1991.1]
Anatomy
These animals were untorted and they had a coiled, cone-shaped shell. The majority of species were small (shells being about two millimeters in length and or diameter). Modern reconstructions depict them as resembling snails. The shells of some genera, particularly Yochelcionella, also possessed a "snorkel"-like opening which was most likely used for breathing.
2005 taxonomy
The taxonomy of the Gastropoda by Bouchet & Rocroi, 2005[2] categorizes members of this taxon within the Paleozoic molluscs of uncertain systematic position, but that taxonomy to not use the name Helcionelloida.
2006-2007 taxonomy
Taxonomy of helcionelloid according to the opinion of P. Yu. Parkhaev[3][4]:
Gastropoda Cuvier, 1797
Subclass Archaeobranchia Parkhaev, 2001
- Order Helcionelliformes Golikov & Starobogatov, 1975
- 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
- Order Pelagiellifomes MacKinnon, 1985
- Family Pelagiellidae Knight, 1952
- Family Aldanellidae Linsley et Kier, 1984
Subclass Divasibranchia Minichev & Starobogatov, 1975
- Order Khairkhaniifomes Parkhaev, 2001
- Family Khairkhaniidae Missarzhevsky, 1989
Subclass Dextrobranchia Minichev & Starobogatov, 1975
- Order Onychochiliformes Minichev & Starobogatov, 1975
- Family Onychochilidae Koken, 1925
- Order Pelagiellifomes MacKinnon, 1985
- Family Pelagiellidae Knight, 1952
- Family Aldanellidae Linsley et Kier, 1984
Subclass Divasibranchia Minichev & Starobogatov, 1975
- Order Khairkhaniifomes Parkhaev, 2001
- Family Khairkhaniidae Missarzhevsky, 1989
Subclass Dextrobranchia Minichev & Starobogatov, 1975
- Order Onychochiliformes Minichev & Starobogatov, 1975
- Family Onychochilidae Koken, 1925
References
- ^ Peel, JS (1991). "Functional morphology of the Class Helcionelloida nov., and the early evolution of the Mollusca". In Simonetta, AM; Conway Morris, S. The early evolution of Metazoa and the significance of problematic taxa. Cambridge University Press. pp. 157?177. ISBN 978-0-521-40242-2.
- ^ Bouchet P. & Rocroi J.-P. (Ed.); Fr?da J., Hausdorf B., Ponder W., Valdes A. & War?n 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. doi:10.1144/SP286.31. http://books.google.com/books?id=GA7-8JIh9IwC&pg=PA415.
External links
- Atkins C. J. & Peel J. S. (2008). "Yochelcionella (Mollusca, Helcionelloida) from the lower Cambrian of North America". Bulletin of Geosciences 83(1): 23-38 (8 figures). doi:10.3140/bull.geosci.2008.01.023. - slightly different taxonomic placement of Yochelcionellidae.
- http://www.palaeos.com/Invertebrates/Molluscs/BasalMollusca/Conchifera/H elcionelloida.html
Taxonomy
The Class Helcionelloida is further organized into finer groupings including:
- Order (1): Helcionellida
- Species: ZipcodeZoo has pages for 70 species and subspecies in the Class Helcionelloida.
Orders
Helcionellida
The Helcionellids are fossil shells of very small marine mollusks that are preserved as shells. These animals are first found about 540 to 530 million years ago 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. [more]
At least 99 species and subspecies belong to the Order Helcionellida.
More info about the Order Helcionellida may be found here.
References
- ^ Peel, JS (1991). "Functional morphology of the Class Helcionelloida nov., and the early evolution of the Mollusca". In Simonetta, AM; Conway Morris, S. The early evolution of Metazoa and the significance of problematic taxa. Cambridge University Press. pp. 157?177. ISBN 978-0-521-40242-2.
- ^ Bouchet P. & Rocroi J.-P. (Ed.); Fr?da J., Hausdorf B., Ponder W., Valdes A. & War?n 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. doi:10.1144/SP286.31. http://books.google.com/books?id=GA7-8JIh9IwC&pg=PA415.
Sources
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