Helcionellid

Helcionellid Temporal range: Early Cambrian–Early Ordovician PreЄ Є O S D C P T J K Pg N
File:Latouchella costata.JPG
Latouchella costata restored as a gastropod
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
(unranked):	incertae sedis
Class:	†Helcionelloida
Subclass:	†Archaeobranchia
Order:	†Helcionelliformes Geyer, 1994
Subgroups
See text.
Synonyms
Helcionelliformes Golikov & Starobogatov, 1975

The Helcionellids are fossil shells of very small marine mollusks that are preserved as shells. These animals are first found about 540 to 530 million years ago in the late Nemakit-Daldynian age, which is the earliest part of the Cambrian 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 conchiferans, a group that includes all the well-known modern families – gastropods, cephalopods and bivalves. Shelled molluscs therefore predate the earliest fossil trilobites. 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 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

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 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

Description

This restoration of the tiny Helcionellid fossil Yochelcionella shows water flowing in under the shell, over the gills and out through the "exhaust pipe"

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