Helcionelloida is the name given to an extinct group of ancient molluscs (phylum Mollusca

Mollusca

The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...

). These are the oldest known conchifera

Conchifera

Conchifera is a taxonomic term. It is a subphylum of the phylum Mollusca. It comprises all of the shell-bearing classes of molluscs, including the snails, clams, tusk shells, chitons, ammonites, monoplacophorans, and so on....

n molluscs, that is, they had a mineralised shell. Some members of this class were mistaken for Monoplacophora

Monoplacophora

Monoplacophora, meaning "bearing one plate", is a polyphyletic class of mollusks with a cap-like shell, living on the bottom of deep sea. Extant representatives were unknown until 1952; previously they were known only from the fossil record.- Definition :...

ns. The class was erected by Peel in 1991.

Anatomy

These animals were untorted

Torsion (gastropod)

Torsion is an anatomical event which takes place during the very early part of the life of snails and slugs of all kinds. In other words, 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˚...

 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

Yochelcionella

Yochelcionella is an extinct genus of basal molluscs which lived during the Tommotian epoch, the first epoch of the Cambrian period. This genus is often reconstructed to resemble snails ....

, also possessed a "snorkel"-like opening which was most likely used for breathing.

2005 taxonomy

The taxonomy of the Gastropoda by Bouchet & Rocroi, 2005

Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)

The taxonomy of the Gastropoda as it was revised by Philippe Bouchet and Jean-Pierre Rocroi is currently the most up-to-date overall system for classifying gastropod mollusks...

 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

Gastropoda

The Gastropoda or gastropods, more commonly known as snails and slugs, are a large taxonomic class within the phylum Mollusca. The class Gastropoda includes snails and slugs of all kinds and all sizes from microscopic to quite large...

Cuvier, 1797

Subclass Archaeobranchia Parkhaev, 2001

• Order Helcionelliformes Golikov & Starobogatov, 1975
• Superfamily Helcionelloidea Wenz, 1938

• Family Helcionellidae Wenz, 1938
• Family Igarkiellidae
  Igarkiellidae
  Igarkiella is the sole genus of Igarkiellidae, an extinct family of Paleozoic fossil molluscs of uncertain position.- 2005 taxonomy :The taxonomy of the Gastropoda by Bouchet & Rocroi, 2005 categorizes Igarkiellidae in the superfamilia Scenelloidea within thePaleozoic molluscs of uncertain...
  
   Parkhaev, 2001
• Family Coreospiridae
  Coreospiridae
  Coreospiridae is an extinct family of Paleozoic molluscs of uncertain position taxonomically. They might be snails or they might be monoplacophorans, Gastropoda or Monoplacophora.- 2005 taxonomy :...
  
   Knight, 1947

• Superfamily Yochelcionelloidea
  Yochelcionelloidea
  Yochelcionelloidea is an extinct superfamily of paleozoic molluscs of uncertain position . The earliest yochelcionellids are known from the Middle Tommotian, but they are most diverse from the Botomian through the early Middle Cambrian.- 2005 taxonomy :The taxonomy of the Gastropoda by Bouchet &...
  
   Runnegar & Jell, 1976

• Family Trenellidae
  Trenellidae
  Trenella is the sole genus in Trenellidae, an extinct family of paleozoic Yochelcionellids.- 2005 taxonomy :The taxonomy of the Gastropoda by Bouchet & Rocroi, 2005 categorizes Trenellidae in the superfamilia Yochelcionelloidea within the...
  
   Parkhaev, 2001
• Family Yochelcionellidae Runnegar & Jell, 1976
• Family Stenothecidae
  Stenothecidae
  Stenothecidae is an extinct family of univalved Cambrian molluscs. .- Morphology :...
  
   Runnegar & Jell, 1980

• Subfamily Stenothecinae Runnegar & Jell, 1980
• Subfamily Watsonellinae Parkhaev, 2001

• Order Pelagiellifomes MacKinnon, 1985
• Family Pelagiellidae
  Pelagiellidae
  Pelagiellidae is an extinct family of paleozoic molluscs.- 2005 taxonomy :The taxonomy of the Gastropoda by Bouchet & Rocroi, 2005 categorizes Pelagiellidae in the superfamilia Pelagielloidea within the...
  
   Knight, 1952
• Family Aldanellidae
  Aldanellidae
  Aldanellidae is an extinct family of paleozoic molluscs that have been assigned to the Gastropod stem group but may also belong to a paraphyletic "Monoplacophora".- 2005 taxonomy :...
  
   Linsley et Kier, 1984

Subclass Divasibranchia Minichev & Starobogatov, 1975

• Order Khairkhaniifomes Parkhaev, 2001
• Family Khairkhaniidae
  Khairkhaniidae
  Khairkhaniidae is an extinct family of Paleozoic fossil molluscs of uncertain position. They are thought to belong to either the Gastropoda or Monoplacophora.They possess planispiral coiled shells with a columnar microstructure.- 2005 taxonomy :...
  
   Missarzhevsky, 1989

Subclass Dextrobranchia Minichev & Starobogatov, 1975

• Order Onychochiliformes Minichev & Starobogatov, 1975
• Family Onychochilidae
  Onychochilidae
  Onychochilidae is an extinct family of small, upper Cambrian to lower Devonian, hyperstrophically coiled molluscs, generally with smooth whorls and elongate apertures, commonly regarded as gastropods....
  
   Koken, 1925

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