The Helcionellids are fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 shells of very small marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 mollusks that are preserved as shells. These animals are first found about in the late Nemakit-Daldynian age, which is the earliest part of the Cambrian

Cambrian

The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 period. A single species persisted to the Early Ordovician. These fossils are component of the small shelly fossils (SSF) assemblages.

These are thought to be early molluscs with rather snail-like shells. They may be the ancestors of the modern 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....

ns, a group that includes all the well-known modern families – gastropods, cephalopods and bivalves. Shelled molluscs therefore predate the earliest fossil trilobite

Trilobite

Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...

s. They have also been considered to represent direct ancestors to the cephalopods.

Parkhaev (2006, 2007) considers these animals to be crown-group gastropods. Previous to the 2006 classification by Parkhaev, helcionellids were classified within the separate class Helcionelloida

Helcionelloida

Helcionelloida is the name given to an extinct group of ancient molluscs . 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...

 or as "Uncertain position (Gastropoda or Monoplacophora)" within "Paleozoic molluscs of uncertain systematic position" according to 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...

2006-2007 taxonomy

According to the opinion of P. Yu. Parkhaev the order Helcionelliformes is within the subclass Archaeobranchia Parkhaev, 2001 in the class Gastropoda.

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

Description

Helcionellids have a single shell, in which the peak forms a distinctive curve. Some have horizontal inhalent siphons ("exhaust pipes") on the concave edges of their shells, and there is debate about whether these pointed forwards or backwards. Most Helcionellid fossils that have been collected are only a few millimeters long and rather snail-like. However specimens a few centimeters long have also been found, mainly limpet-like in shape, although some were laterally compressed and others were tall. The smallest specimens may have been juvenile or larval forms of the larger specimens.

External links

• http://www.palaeos.com/Invertebrates/Molluscs/BasalMollusca/Conchifera/Helcionelloida.html