The Helcionellacea is one of two

taxonomic Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...

superfamilies of extinct primitive marine

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 within the order Bellerophontida. This superfamily is thought to be the more primitive of the two superfamilies, the other being the Bellerophontacea. These molluscs are thought to be

sea snail Sea snail is a common name for slow-moving marine gastropod molluscs, usually with visible external shells, such as whelk or abalone. They share the taxonomic class Gastropoda with slugs, which are distinguished from snails primarily by the ...

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military * ...

gastropod 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. T ...

mollusk 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 e ...

s, rather than

monoplacophora Monoplacophora , meaning "bearing one plate", is a polyphyletic superclass of molluscs with a cap-like shell inhabiting deep sea environments . Extant representatives were not recognized as such until 1952; previously they were known only from ...

ns.

Diagnosis

Helcionellaceans are characterised by cap-shaped to bellerophontiform shells; commonly with strong rugae, (transverse plications) clearly defined on both the interior and exterior of the shell; with a septum or septa partitioning off the apex, but lacking the anal emargination that is characteristic of the Bellerophontacea.

Taxonomy

The Helcionellacea includes two families according to the

Treatise A treatise is a formal and systematic written discourse on some subject, generally longer and treating it in greater depth than an essay, and more concerned with investigating or exposing the principles of the subject and its conclusions."Treat ...

, 1960, the

Helcionellidae Helcionellid or Helcionelliformes is an order of small fossil 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 Ne ...

, which is the more primitive, and the

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 ...

, which is the more derived. The Helconellidae (Wenz, 1938) represented by the genus '' Helcionella'' (Grabau & Shimer, 1909) more closely resembles the

n ancestor, and is characterized by elongate cap-shaped shells. The Coreospiridae (Knight, 1947) which includes '' Coreospira'' (Satto, 1936) '' Cycloholcus'' (Knight, 1947) and ''

Latouchella ''Latouchella'' is an extinct genus of marine invertebrate animal, that is considered to be a mollusk and which may be a sea snail, a gastropod. It is a helcionellid from the Tommotian epoch of what is now Siberia. Its tightly coiled, spiral ...

'' (Cobbold, 1921) are more bellerophontiform (shaped like a bellerophont). ''Latauchella'' has the least amount of apical coiling, and more closely resembles species of ''Helcionella''. ''Coreospira'' is tightly coiled while ''Cycloholcus'' is open coiled. Both bear some resemblance to the genus '' Cloudia'' in the

Cyrtolitidae Cyrtolitidae is an extinct family of monoplacophorans in the order Cyrtonellida. Genera * '' Cloudia'' * '' Cyclocyrtonella'' * '' Cyrtolites'' * '' Cyrtonellopsis'' * '' Kolihadiscus'' * '' Neocyrtolites'' * '' Paracyrtolites'' * '' Quasisin ...

, Bellerophontacea. Wagner (2011) reassigned the Helcionellacea (alternatively Helcionelloidea) to the Helcionellida with the following families, Helcionellidae, Garkielladae, and Procarinaridae. Parkhaev 200

added the Igarkiellidae and expanded the Coreospiridae and Helcionellidae with additional genera. The Procarinaridae, represented by '' Procarinaria'' from the

Upper Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

Italy Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical re ...

and the

Czech Republic The Czech Republic, or simply Czechia, is a landlocked country in Central Europe. Historically known as Bohemia, it is bordered by Austria to the south, Germany to the west, Poland to the northeast, and Slovakia to the southeast. The ...

, was tentatively included in the Cyrolitidae by Knight et al., 1960, and this taxon may or may not belong instead to the Helcionellacea. The stratigraphic gap of the intervening

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

suggests otherwise.

References

*J Brooks Knight et al. 1960. Systenatuc Descriptions (Gastropoda), Treatise on Invertebrate Paleontology, Part I, Mollusca 1. Geological Society of America and University of Kansas Press.
Helcoinelloidea
Paleobiology Database. 1-4-13 {{Taxonbar, from=Q5701567 Prehistoric gastropods Gastropod superfamilies Triassic extinctions