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 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 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:

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 Pelagielliformes 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 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