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Helcionellid; Temporal range: Fortunian–Early Ordovician PreꞒ Ꞓ O S D C P T J K Pg N
	Latouchella costata restored as a gastropod
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	†Helcionelloida
Subclass:	†Archaeobranchia
Order:	†Helcionelliformes; Geyer, 1994
Subgroups
	See text.
Synonyms
	Helcionelliformes Golikov & Starobogatov, 1975

Helcionellid or Helcionelliformes is an order of small fossil shells that are universally interpreted as molluscs,^[3] though no sources spell out why this taxonomic interpretation is preferred.^[4] These animals are first found about 540 to 530 million years ago^[3] in the late Nemakit-Daldynian age, which is the earliest part of the Cambrian period. A single species persisted to the Early Ordovician.^[5]^[6]^[7]^[8] These fossils are component of the small shelly fossils (SSF) assemblages.^[5]

These are thought to be early molluscs with rather snail-like shells, although they lack any compelling molluscan synapomorphies and thus may not belong to the group.^[9]

They have been alleged to represent ancestors of the modern conchiferans, a group that includes all the well-known modern classes – gastropods, cephalopods and bivalves.^[10]^[11] They have also been considered to represent direct ancestors to the cephalopods.^[12]

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.^[13]

2006-2007 taxonomy

According to P. Yu. Parkhaev,^[14]^[15] 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

This speculative restoration of the tiny Helcionellid fossil *Yochelcionella* shows water flowing in under the shell, over the gills and out through the "exhaust pipe".^[16]

Helcionellids have a single shell, in which the peak forms a distinctive curve.^[17] Some have horizontal "inhalent siphons"^[12] ("exhaust pipes"^{[verification needed]}) on the concave edges of their shells, and there is debate about whether these pointed forwards or backwards.^[16] Most helcionellid fossils that have been collected are only a few millimeters long (1⁄8 inch) and rather snail-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.^[17]

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.^[18]

Shell muscles attach near the concave side of the shell.^[19]

Fossil record

The earliest helcionellid, in Siberian sections, is Oelandiella.^[20]^[1]

They first appear in the late Nemakit-Daldynian (lower Early Cambrian),^[5] and are a constituent of the small shelly fauna (SSF). Larger individuals, reaching centimeters in diameter, have also been found.^[17]

Helcionellids have been interpreted as juvenile stages of larger limpet-like molluscs.^[21]

References

^ ^a ^b Kouchinsky A. V., Bengtson S., Runnegar B. N., Skovsted C. B., Steiner M., Vendrasco M. J. (2012). "Chronology of early Cambrian biomineralization". Geological Magazine. 149 (2): 221–251. Bibcode:2012GeoM..149..221K. doi:10.1017/s0016756811000720.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ Christian B. Skovsted; John S. Peel (2007). "Small shelly fossils from the argillaceous facies of the Lower Cambrian Forteau Formation of western Newfoundland". Acta Palaeontologica Polonica. 52 (4): 729–748.
^ ^a ^b Nelson R Cabej (2019). Epigenetic Mechanisms of the Cambrian Explosion. Elsevier Science. p. 152. ISBN 9780128143124.
^ Zhao Fangchen, Smith Martin R., Yin Zongjun, Zeng Han, Li Guoxiang, Zhu Maoyan (2017). "Orthrozanclus elongata n. sp. and the significance of sclerite-covered taxa for early trochozoan evolution". Scientific Reports. 7 (1): 16232. Bibcode:2017NatSR...716232Z. doi:10.1038/s41598-017-16304-6. PMC 5701144. PMID 29176685.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ ^a ^b ^c Budd, Graham E (2003). "The Cambrian Fossil Record and the Origin of the Phyla" (Free full text). Integrative and Comparative Biology. 43 (1): 157–165. doi:10.1093/icb/43.1.157. PMID 21680420.
^ Michael Steiner; Guoxiang Li; Yi Qian; Maoyan Zhu; Bernd-Dietrich Erdtmann (2007). "Neoproterozoic to early Cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the Yangtze Platform (China)". Palaeogeography, Palaeoclimatology, Palaeoecology. 254 (1–2): 67–99. Bibcode:2007PPP...254...67S. doi:10.1016/j.palaeo.2007.03.046.
^ Gubanov, Alexander P.; Peel, John S. (2003). "Latest Helcionelloid Molluscs from the Lower Ordovician of Kazakhstan". Palaeontology. 44 (4): 681–694. doi:10.1111/1475-4983.00198.
^ Peel, John S.; Radvan J. Horný (2004). "A new problematic Early Ordovician univalve mollusc from France". Palaeontology. 47 (6): 1629–1639. Bibcode:2004Palgy..47.1629P. doi:10.1111/j.0031-0239.2004.00414.x.
^ Zhao Fangchen, Smith Martin R., Yin Zongjun, Zeng Han, Li Guoxiang, Zhu Maoyan (2017). "Orthrozanclus elongata n. sp. and the significance of sclerite-covered taxa for early trochozoan evolution". Scientific Reports. 7 (1): 16232. Bibcode:2017NatSR...716232Z. doi:10.1038/s41598-017-16304-6. PMC 5701144. PMID 29176685.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ Peel, J.S. (1991). "Functional morphology of the Class Helcionelloida nov. and the early evolution of the Mollusca". In Simonetta, A.M.; Conway Morris, S. (eds.). The Early Evolution of Metazoa (The Significance of Problematic Taxa). Cambridge University Press. pp. 157–177. ISBN 978-0-521-40242-2.
^ Gubanov, A.P.; Peel, J.S. (November 2003). "The early Cambrian helcionelloid mollusc Anabarella Vostokova". Palaeontology. 46 (5): 1073–1087. Bibcode:2003Palgy..46.1073G. doi:10.1111/1475-4983.00334. S2CID 84893338.
^ ^a ^b Webers, G.F.; Yochelson, E.L. (1989). "Late Cambrian molluscan faunas and the origin of the Cephalopoda". In Crame, J.A. (ed.). Origins and Evolution of the Antarctic Biota. Vol. 47. Geological Society, London: Special Publications. p. 29. Bibcode:1989GSLSP..47...29W. doi:10.1144/GSL.SP.1989.047.01.04. S2CID 128682549. {{cite book}}: |journal= ignored (help)
^ Bouchet 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. ISBN 3-925919-72-4. ISSN 0076-2997. 397 pp. http://www.vliz.be/Vmdcdata/imis2/ref.php?refid=78278
^ P. Yu. Parkhaev (2006). "Adaptive radiation of the Cambrian helcionelloid mollusks (Gastropoda, Archaeobranchia)" (PDF). In S.V. Rozhnov (ed.). Evolution of the biosphere and biodiversity. Towards the 70th anniversary of A. Y. Rozanov. Moscow. pp. 282–296. Archived from the original (PDF) on 2015-06-10. Retrieved 2009-11-03.{{cite book}}: CS1 maint: location missing publisher (link)
^ P. Yu. Parkhaev (2007). The Cambrian 'basement' of gastropod evolution. Vol. 286. Geological Society. pp. 415–421. Bibcode:2007GSLSP.286..415P. doi:10.1144/SP286.31. ISBN 978-1-86239-233-5. S2CID 130979274. {{cite book}}: |journal= ignored (help)
^ ^a ^b Runnegar, B.; Pojeta Jr, J. (Oct 1974). "Molluscan Phylogeny: the Paleontological Viewpoint". Science. 186 (4161): 311–317. Bibcode:1974Sci...186..311R. doi:10.1126/science.186.4161.311. JSTOR 1739764. PMID 17839855. S2CID 46429653.
^ ^a ^b ^c Mus, M.M.; Palacios, T.; Jensen, S. (2008). "Size of the earliest mollusks: Did small helcionellids grow to become large adults?". Geology. 36 (2): 175. Bibcode:2008Geo....36..175M. doi:10.1130/G24218A.1.
^ The Functional Morphology of the Cambrian Univalved Mollusks—Helcionellids. 1
^ P. Yu. Parkhaev (2004). "New Data on the Morphology of Shell Muscles in Cambrian Helcionelloid Mollusks". Paleontological Journal. 38 (3): 254–256.
^ Gubanov A. P., Peel J. S. (1999). "Oelandiella, the earliest Cambrian helcionelloid mollusc from Siberia". Palaeontology. 42 (2): 211–222. Bibcode:1999Palgy..42..211G. doi:10.1111/1475-4983.00070. S2CID 129393470.
^ Mus, M.M.; Palacios, T.; Jensen, S. (2008). "Size of the earliest mollusks: Did small helcionellids grow to become large adults?". Geology. 36 (2): 175. Bibcode:2008Geo....36..175M. doi:10.1130/G24218A.1. Retrieved 2008-10-01.