Helcionellid or Helcionelliformes is an

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

of small fossil shells that are universally interpreted as

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

s, though no sources spell out why this taxonomic interpretation is preferred. These animals are first found about in the late

Nemakit-Daldynian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ( ...

age, which is the earliest part of the

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

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

s with rather snail-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

conchifera Conchifera is a subphylum of the phylum Mollusca. It comprises all of the shell-bearing classes of molluscs, such as clams, tusk shells, ammonites, and monoplacophorans. The other subphylum is Aculifera. Non-monoplacophoran conchiferans emerged ...

ns, a group that includes all the well-known modern classes – gastropods,

cephalopods A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, an ...

and

bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

. 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 ere ...

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 Parkhaev, 2013 (syn "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

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 snail-like. However specimens a few centimeters long (1–2 inches) have also been found, mainly

limpet Limpets are a group of aquatic snails that exhibit a conical shell shape (patelliform) and a strong, muscular foot. Limpets are members of the class Gastropoda, but are polyphyletic, meaning the various groups called "limpets" descended indep ...

-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

(lower Early Cambrian), and are a constituent of the small shelly fauna (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