Helcionelloida

Helcionelloida; Temporal range: Early Cambrian–Early Ordovician PreꞒ Ꞓ O S D C P T J K Pg N
	Latouchella costata
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Subphylum:	Conchifera
Class:	†Helcionelloida; Peel, 1991
Orders
	Order Helcionelliformes; Order Pelagialliformes; Order Khairkhaniiformes; Order Onychochiliformes; and see text

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.^[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 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 Khairkhaniiformes Parkhaev, 2001

Family Khairkhaniidae Missarzhevsky, 1989

Subclass Dextrobranchia Minichev & Starobogatov, 1975

Order Onychochiliformes Minichev & Starobogatov, 1975

Family Onychochilidae Koken, 1925

References

^ Peel, J. S. (1991). "Functional Morphology of the Class Helcionelloida Nov., and the Early Evolution of the Mollusca". In Simonetta, A. M.; Conway Morris, S (eds.). 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.); 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). Archived 2015-06-10 at the Wayback Machine" 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.