Helcionellid

{{Automatic taxobox

| taxon = Helcionelliformes

| fossil_range={{fossil range|Fortunian|Early Ordovician|refs={{cite journal | author = Kouchinsky A. V., Bengtson S., Runnegar B. N., Skovsted C. B., Steiner M., Vendrasco M. J. | year = 2012 | title = Chronology of early Cambrian biomineralization | journal = Geological Magazine | volume = 149 | issue = 2| pages = 221–251 | doi=10.1017/s0016756811000720| bibcode = 2012GeoM..149..221K| doi-access = free }}}}

| image = Latouchella_costata.JPG

| image_caption = Latouchella costata restored as a gastropod

| authority = Geyer, 1994{{cite journal|

title=Small shelly fossils from the argillaceous facies of the Lower Cambrian Forteau Formation of western Newfoundland

|author=Christian B. Skovsted |author2= John S. Peel

|journal=Acta Palaeontologica Polonica|year= 2007|pages= 729–748|

issue=4 |volume=52}}

| subdivision_ranks = Subgroups

| subdivision = See text.

| synonyms = Helcionelliformes Golikov & Starobogatov, 1975

}}

Helcionellid or Helcionelliformes is an order of small fossil shells that are universally interpreted as molluscs,{{cite book|author=Nelson R Cabej|date=2019|url=https://books.google.com/books?id=QdO1DwAAQBAJ&dq=Myoscolex&pg=PA152|title=Epigenetic Mechanisms of the Cambrian Explosion|publisher=Elsevier Science|page=152|isbn=9780128143124}} though no sources spell out why this taxonomic interpretation is preferred.{{cite journal|doi=10.1038/s41598-017-16304-6|pmid=29176685 |pmc=5701144 |title=Orthrozanclus elongata n. sp. and the significance of sclerite-covered taxa for early trochozoan evolution | volume=7 |issue=1 |pages=16232 | year=2017|journal=Scientific Reports|author=Zhao Fangchen, Smith Martin R., Yin Zongjun, Zeng Han, Li Guoxiang, Zhu Maoyan|bibcode=2017NatSR...716232Z}} These animals are first found about {{ma|540|530}} in the late Nemakit-Daldynian age, which is the earliest part of the Cambrian period. A single species persisted to the Early Ordovician.{{cite journal

| author = Budd, Graham E

| year = 2003

| title = The Cambrian Fossil Record and the Origin of the Phyla

| journal = Integrative and Comparative Biology

| volume = 43

| issue = 1

| pages = 157–165

| doi = 10.1093/icb/43.1.157

| format = Free full text

| pmid = 21680420

| doi-access = free

}}{{cite journal |author1=Michael Steiner |author2=Guoxiang Li |author3=Yi Qian |author4=Maoyan Zhu |author5=Bernd-Dietrich Erdtmann |year=2007 |title= Neoproterozoic to early Cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the Yangtze Platform (China) |journal= Palaeogeography, Palaeoclimatology, Palaeoecology |volume=254 |issue=1–2 |pages=67–99 |doi= 10.1016/j.palaeo.2007.03.046 |bibcode=2007PPP...254...67S}}{{cite journal|last=Gubanov|first=Alexander P.|author2=Peel, John S.|year=2003|title=Latest Helcionelloid Molluscs from the Lower Ordovician of Kazakhstan |journal=Palaeontology|volume=44|issue=4|pages=681–694|doi=10.1111/1475-4983.00198|doi-access=free}}{{cite journal|last=Peel|first=John S.|author2=Radvan J. Horný|year=2004|title=A new problematic Early Ordovician univalve mollusc from France |journal=Palaeontology|volume=47|issue=6|pages=1629–1639|doi=10.1111/j.0031-0239.2004.00414.x|bibcode=2004Palgy..47.1629P |doi-access=free}} These fossils are component of the small shelly fossils (SSF) assemblages.

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.{{cite journal| doi=10.1038/s41598-017-16304-6 | pmid=29176685 | pmc=5701144 | volume=7 | issue=1 | pages=16232 | title=Orthrozanclus elongata n. sp. and the significance of sclerite-covered taxa for early trochozoan evolution | year=2017 | journal=Scientific Reports | author=Zhao Fangchen, Smith Martin R., Yin Zongjun, Zeng Han, Li Guoxiang, Zhu Maoyan| bibcode=2017NatSR...716232Z }}

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.{{Cite book

| publisher= Cambridge University Press |pages=157–177

}}{{cite journal

| author= Gubanov, A.P. |author2= Peel, J.S.

| title= The early Cambrian helcionelloid mollusc Anabarella Vostokova

| doi=10.1111/1475-4983.00334

|bibcode= 2003Palgy..46.1073G

|s2cid= 84893338

|doi-access=free}} 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.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

2006-2007 taxonomy

According to P. Yu. Parkhaev,{{cite book |author= P. Yu. Parkhaev |year= 2006 |chapter-url= http://macroevolution.narod.ru/22parkhaev.pdf |chapter= Adaptive radiation of the Cambrian helcionelloid mollusks (Gastropoda, Archaeobranchia) |editor= S.V. Rozhnov |title= Evolution of the biosphere and biodiversity. Towards the 70th anniversary of A. Y. Rozanov |place= Moscow |pages= 282–296 |access-date= 2009-11-03 |archive-date= 2015-06-10 |archive-url= https://web.archive.org/web/20150610205708/http://macroevolution.narod.ru/22parkhaev.pdf |url-status= dead }}{{cite book |author= P. Yu. Parkhaev |year=2007 |title= The Cambrian 'basement' of gastropod evolution |journal= Geological Society, London, Special Publications |volume=286 |issue=1 |pages=415–421 |url= https://books.google.com/books?id=GA7-8JIh9IwC&pg=PA415 |publisher= Geological Society |isbn= 978-1-86239-233-5 |doi= 10.1144/SP286.31 |bibcode= 2007GSLSP.286..415P |s2cid=130979274 }} 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

File:Yochelcionella water flow.png shows water flowing in under the shell, over the gills and out through the "exhaust pipe".{{Cite journal |first1=B. |first2= J. |last2= Pojeta Jr |title = Molluscan Phylogeny: the Paleontological Viewpoint |volume= 186 |last1= Runnegar |journal= Science |issue= 4161 |pages= 311–317 |date=Oct 1974 |jstor = 1739764 |pmid= 17839855 |doi= 10.1126/science.186.4161.311 |bibcode= 1974Sci...186..311R |s2cid= 46429653}}]]

Helcionellids have a single shell, in which the peak forms a distinctive curve.{{Cite journal

|author1= Mus, M.M. |author2= Palacios, T. |author3=Jensen, S. |year= 2008

| title = Size of the earliest mollusks: Did small helcionellids grow to become large adults?

| journal = Geology

| volume = 36

| issue = 2

| pages = 175

| doi = 10.1130/G24218A.1

| url = http://geology.geoscienceworld.org/cgi/content/abstract/36/2/175

|bibcode= 2008Geo....36..175M}} Some have horizontal "inhalent siphons"{{Cite book |doi= 10.1144/GSL.SP.1989.047.01.04 |chapter= Late Cambrian molluscan faunas and the origin of the Cephalopoda |year= 1989 |last1= Webers |first1= G.F. |last2= Yochelson |first2= E.L. |title= Origins and Evolution of the Antarctic Biota |journal= Geological Society, London, Special Publications |editor-last= Crame | editor-first= J.A. |publisher= Geological Society, London: Special Publications |volume= 47 |issue= 1 |page= 29|bibcode= 1989GSLSP..47...29W |s2cid= 128682549 }} ("exhaust pipes"{{Verify source|date=June 2010}}) 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 ({{convert|3|mm|in |frac=8 |abbr=in |disp=out}}) 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.

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.[https://www.researchgate.net/profile/Pavel_Parkhaev/publication/236216830_The_Functional_Morphology_of_the_Cambrian_Univalved_Mollusks-Helcionellids_1/links/02e7e517107b5ae7e4000000/The-Functional-Morphology-of-the-Cambrian-Univalved-Mollusks-Helcionellids-1.pdf The Functional Morphology of the Cambrian Univalved Mollusks—Helcionellids. 1]

Shell muscles attach near the concave side of the shell.{{Cite journal |journal=Paleontological Journal |volume=38 |issue=3 |year=2004 |pages=254–256 |title=New Data on the Morphology of Shell Muscles in Cambrian Helcionelloid Mollusks |author=P. Yu. Parkhaev |url=https://www.researchgate.net/publication/236217138}}

== Fossil record==

The earliest helcionellid, in Siberian sections, is Oelandiella.{{cite journal | author = Gubanov A. P., Peel J. S. | year = 1999 | title = Oelandiella, the earliest Cambrian helcionelloid mollusc from Siberia | journal = Palaeontology | volume = 42 | issue = 2| pages = 211–222 | doi=10.1111/1475-4983.00070| bibcode = 1999Palgy..42..211G | s2cid = 129393470 }}{{cite journal | author = Kouchinsky A. V., Bengtson S., Runnegar B. N., Skovsted C. B., Steiner M., Vendrasco M. J. | year = 2012 | title = Chronology of early Cambrian biomineralization | journal = Geological Magazine | volume = 149 | issue = 2| pages = 221–251 | doi=10.1017/s0016756811000720| bibcode = 2012GeoM..149..221K| doi-access = free }}

They first appear in the late Nemakit-Daldynian (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.{{cite journal

|author1= Mus, M.M. |author2= Palacios, T. |author3=Jensen, S. |year= 2008

| title= Size of the earliest mollusks: Did small helcionellids grow to become large adults?

| url= http://geology.geoscienceworld.org/cgi/content/abstract/36/2/175 | access-date=2008-10-01

|bibcode= 2008Geo....36..175M}}

References

External links

https://web.archive.org/web/20080930105735/http://www.palaeos.com/Invertebrates/Molluscs/BasalMollusca/Conchifera/Helcionelloida.html

Category:Helcionelloida