Helcionellid or Helcionelliformes is an order of small

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

shells that are universally interpreted as molluscs, though no sources spell out why this taxonomic interpretation is preferred. These animals are first found about in the late Nemakit-Daldynian age, which is the earliest part of the Cambrian period. A single species persisted to the

Early Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. ...

. These fossils are component of the small shelly fossils (SSF) assemblages. These are thought to be early molluscs with rather

snail A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class G ...

-like shells, although they lack any compelling molluscan synapomorphies and thus may not belong to the group. They have been alleged to represent ancestors of the modern conchiferans, a group that includes all the well-known modern classes –

gastropods The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. The ...

, cephalopods and bivalves. 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 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 ...

or as "Uncertain position (Gastropoda or Monoplacophora)" within "Paleozoic molluscs of uncertain systematic position" according to the taxonomy of the Gastropoda by Bouchet & Rocroi, 2005Bouchet 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. . . 397 pp. http://www.vliz.be/Vmdcdata/imis2/ref.php?refid=78278

2006-2007 taxonomy

According to 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

Carinopeltidae ''Carinopelta'' is a genus within Carinopeltidae, an extinct family of Cambrian molluscs of uncertain position. It is in the superfamily Scenelloidea. Taxonomy The taxonomy of the Gastropoda by Bouchet & Rocroi, 2005Bouchet P. & Rocroi J.-P ...

Parkhaev, 2013 (syn "Igarkiellidae" Parkhaev, 2001) **Family

Coreospiridae Coreospiridae is an extinct family of Paleozoic molluscs of uncertain position taxonomically. They might be snails ( Gastropoda), Helcionelloida, or they might be Monoplacophora. 2005 taxonomy The taxonomy of the Gastropoda by Bouchet & R ...

Knight, 1947 * Superfamily

Yochelcionelloidea Yochelcionelloidea is an extinct superfamily of paleozoic molluscs of uncertain position ( Gastropoda or Monoplacophora). The earliest yochelcionellids are known from the Middle Tommotian, but they are most diverse from the Botomian through the ...

Runnegar & Jell, 1976 **Family

Trenellidae ''Trenella'' is the sole genus in the Trenellidae, an extinct family of paleozoic Yochelcionellids. 2005 taxonomy The taxonomy of the Gastropoda by Bouchet & Rocroi, 2005Bouchet P. & Rocroi J.-P. (Ed.); Frýda J., Hausdorf B., Ponder W., Val ...

Parkhaev, 2001 **Family

Yochelcionellidae ''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. ''Yochelcionella'' is the type genus of the family ...

Runnegar & Jell, 1976 **Family

Stenothecidae Stenothecidae is an extinct family of fossil univalved Cambrian molluscs which may be either gastropods or monoplacophorans. The name of this taxon should not be confused with that of the class Stenothecoida, a group of problematic Cambrian ...

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

Morphology

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

-like. However specimens a few centimeters long (1–2 inches) 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. Inside the shell are a series of longitudinal ridges stretching to the apex. Some people reckon that they are to do with controlling water currents in a mantle cavity; others think that they are to do with support or muscle attachment. Shell muscles attach near the concave side of the shell.

Fossil record

The earliest helcionellid, in Siberian sections, is Oelandiella. They first appear in the late Nemakit-Daldynian (lower Early Cambrian), and are a constituent of the

small shelly fauna The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambri ...

(SSF). Larger individuals, reaching centimeters in diameter, have also been found. Helcionellids have been interpreted as juvenile stages of larger limpet-like molluscs.

References

External links

*https://web.archive.org/web/20080930105735/http://www.palaeos.com/Invertebrates/Molluscs/BasalMollusca/Conchifera/Helcionelloida.html {{Taxonbar, from=Q5701568 Helcionelloida