Helcionelloida is 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.

Anatomy

These animals were

untorted Torsion is a gastropod synapomorphy which occurs in all gastropods during larval development. Torsion is the rotation of the visceral mass, mantle, and shell 180˚ with respect to the head and foot of the gastropod. This rotation brings the mant ...

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, 2005Bouchet 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. . ISSN 0076-2997. 397 pp. http://www.vliz.be/Vmdcdata/imis2/ref.php?refid=78278 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: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. Gastropoda Cuvier, 1797 Subclass Archaeobranchia Parkhaev, 2001 *Order Helcionelliformes Golikov & Starobogatov, 1975 :*Superfamily Helcionelloidea Wenz, 1938 ::*Family Helcionellidae Wenz, 1938 ::*Family Igarkiellidae Parkhaev, 2001 ::*Family

Coreospiridae Coreospiridae is an extinct family of Paleozoic molluscs of uncertain position taxonomically. They might be snails ( Gastropoda), Helcionelloida, or they might be Monoplacophora. 2005 taxonomy The taxonomy of the Gastropoda by Bouchet & R ...

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 *Order Pelagiellifomes MacKinnon, 1985 :*Family Pelagiellidae Knight, 1952 :*Family Aldanellidae Linsley et Kier, 1984 Subclass Divasibranchia Minichev & Starobogatov, 1975 *Order Khairkhaniiformes Parkhaev, 2001 :*Family Khairkhaniidae Missarzhevsky, 1989 Subclass Dextrobranchia Minichev & Starobogatov, 1975 *Order Onychochiliformes Minichev & Starobogatov, 1975 :*Family Onychochilidae Koken, 1925

References

External links

* Atkins C. J. & Peel J. S. (2008). "''Yochelcionella'' (Mollusca, Helcionelloida) from the lower Cambrian of North America". ''

Bulletin of Geosciences The ''Bulletin of Geosciences'' is a peer-reviewed scientific journal publishing original research papers, review articles, and short contributions. It covers all aspects of palaeoenvironmental geology, including palaeontology, stratigraphy, s ...

'' 83(1): 23-38 (8 figures). . - slightly different taxonomic placement of Yochelcionellidae. *https://web.archive.org/web/20080930105735/http://www.palaeos.com/Invertebrates/Molluscs/BasalMollusca/Conchifera/Helcionelloida.html {{Taxonbar, from=Q675080 Mollusc classes Prehistoric protostome classes