Helcionelloida

Cladus: Eukaryota
Supergroup: Opisthokonta
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladus: Nephrozoa
Cladus: Protostomia
Cladus: Spiralia
Cladus: Lophotrochozoa
Phylum: Mollusca
Classes: Helcionelloida
Ordine: Helcionellida - Pelagiallifomes - Khairkhaniifomes - Onichochiliformes

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 created 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. They 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 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

* 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

1. ^ Peel, JS (1991). "Functional morphology of the Class Helcionelloida nov., and the early evolution of the Mollusca". The early evolution of Metazoa and the significance of problematic taxa. (Cambridge: Cambridge University Press): 157–177.
2. ^ 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
3. ^ 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.
4. ^ 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.

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/Helcionelloida.html
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