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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
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113:
481:
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.
751:
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)".
332:
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
337:
1456:
918:
688:
568:
478:-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.
1499:
932:
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.).
1140:
582:
Christian B. Skovsted; John S. Peel (2007). "Small shelly fossils from the argillaceous facies of the Lower
Cambrian Forteau Formation of western Newfoundland".
446:
1181:
941:
336:
or as "Uncertain position (Gastropoda or
Monoplacophora)" within "Paleozoic molluscs of uncertain systematic position" according to the
1457:
https://web.archive.org/web/20080930105735/http://www.palaeos.com/Invertebrates/Molluscs/BasalMollusca/Conchifera/Helcionelloida.html
1080:
615:
1013:
Webers, G.F.; Yochelson, E.L. (1989). "Late
Cambrian molluscan faunas and the origin of the Cephalopoda". In Crame, J.A. (ed.).
136:
1105:
273:, though no sources spell out why this taxonomic interpretation is preferred. These animals are first found about
605:
310:-like shells, although they lack any compelling molluscan synapomorphies and thus may not belong to the group.
1553:
863:"Orthrozanclus elongata n. sp. and the significance of sclerite-covered taxa for early trochozoan evolution"
633:"Orthrozanclus elongata n. sp. and the significance of sclerite-covered taxa for early trochozoan evolution"
1365:
Gubanov A. P., Peel J. S. (1999). "Oelandiella, the earliest
Cambrian helcionelloid mollusc from Siberia".
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1525:
912:
682:
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Bouchet P. & Rocroi J.-P. (Ed.); Frýda J., Hausdorf B., Ponder W., Valdes A. & Warén A. 2005.
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Kouchinsky A. V., Bengtson S., Runnegar B. N., Skovsted C. B., Steiner M., Vendrasco M. J. (2012).
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Runnegar, B.; Pojeta Jr, J. (Oct 1974). "Molluscan
Phylogeny: the Paleontological Viewpoint".
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1075:. Malacologia: International Journal of Malacology, 47(1-2). ConchBooks: Hackenheim, Germany.
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Evolution of the biosphere and biodiversity. Towards the 70th anniversary of A. Y. Rozanov
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474:-like. However specimens a few centimeters long (1–2 inches) have also been found, mainly
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shows water flowing in under the shell, over the gills and out through the "exhaust pipe".
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1106:"Adaptive radiation of the Cambrian helcionelloid mollusks (Gastropoda, Archaeobranchia)"
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Zhao
Fangchen, Smith Martin R., Yin Zongjun, Zeng Han, Li Guoxiang, Zhu Maoyan (2017).
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632:
631:
Zhao
Fangchen, Smith Martin R., Yin Zongjun, Zeng Han, Li Guoxiang, Zhu Maoyan (2017).
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Helcionellids have been interpreted as juvenile stages of larger limpet-like molluscs.
451:
378:
371:
333:
181:
936:(The Significance of Problematic Taxa). Cambridge University Press. pp. 157–177.
1547:
1410:"Size of the earliest mollusks: Did small helcionellids grow to become large adults?"
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1343:
1294:"Size of the earliest mollusks: Did small helcionellids grow to become large adults?"
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846:
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504:(SSF). Larger individuals, reaching centimeters in diameter, have also been found.
329:. They have also been considered to represent direct ancestors to the cephalopods.
112:
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773:
348:
According to P. Yu. Parkhaev, the order Helcionelliformes is within the subclass
17:
1344:"New Data on the Morphology of Shell Muscles in Cambrian Helcionelloid Mollusks"
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401:
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45:
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1017:. Vol. 47. Geological Society, London: Special Publications. p. 29.
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The Functional Morphology of the Cambrian Univalved Mollusks—Helcionellids. 1
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789:"Latest Helcionelloid Molluscs from the Lower Ordovician of Kazakhstan"
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822:"A new problematic Early Ordovician univalve mollusc from France"
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The earliest helcionellid, in Siberian sections, is Oelandiella.
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449:
This speculative restoration of the tiny Helcionellid fossil
317:, a group that includes all the well-known modern classes –
313:
They have been alleged to represent ancestors of the modern
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Shell muscles attach near the concave side of the shell.
709:"The Cambrian Fossil Record and the Origin of the Phyla"
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520:
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taxonomy of the Gastropoda by Bouchet & Rocroi, 2005
1160:. Vol. 286. Geological Society. pp. 415–421.
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1221:
1093:
http://www.vliz.be/Vmdcdata/imis2/ref.php?refid=78278
500:(lower Early Cambrian), and are a constituent of the
1073:
Classification and nomenclator of gastropod families
1474:
381:
Parkhaev, 2013 (syn "Igarkiellidae" Parkhaev, 2001)
754:Palaeogeography, Palaeoclimatology, Palaeoecology
529:"Chronology of early Cambrian biomineralization"
607:Epigenetic Mechanisms of the Cambrian Explosion
1157:The Cambrian 'basement' of gastropod evolution
787:Gubanov, Alexander P.; Peel, John S. (2003).
8:
1408:Mus, M.M.; Palacios, T.; Jensen, S. (2008).
1292:Mus, M.M.; Palacios, T.; Jensen, S. (2008).
1015:Origins and Evolution of the Antarctic Biota
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917:: CS1 maint: multiple names: authors list (
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957:Gubanov, A.P.; Peel, J.S. (November 2003).
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1115:. Moscow. pp. 282–296. Archived from
959:"The early Cambrian helcionelloid mollusc
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291:period. A single species persisted to the
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352:Parkhaev, 2001 in the class Gastropoda.
820:Peel, John S.; Radvan J. Horný (2004).
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295:. These fossils are component of the
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287:, which is the earliest part of the
269:that are universally interpreted as
716:Integrative and Comparative Biology
25:
610:. Elsevier Science. p. 152.
359:Golikov & Starobogatov, 1975
847:10.1111/j.0031-0239.2004.00414.x
243:Golikov & Starobogatov, 1975
135:
934:The Early Evolution of Metazoa
496:They first appear in the late
302:These are thought to be early
1:
1031:10.1144/GSL.SP.1989.047.01.04
584:Acta Palaeontologica Polonica
460:only a few millimeters long (
1250:10.1126/science.186.4161.311
774:10.1016/j.palaeo.2007.03.046
1570:
887:10.1038/s41598-017-16304-6
657:10.1038/s41598-017-16304-6
418:Runnegar & Jell, 1980
397:Runnegar & Jell, 1976
41:Fortunian–Early Ordovician
1111:. In S.V. Rozhnov (ed.).
554:10.1017/s0016756811000720
425:Runnegar & Jell, 1980
411:Runnegar & Jell, 1976
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132:Scientific classification
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27:Extinct order of molluscs
1342:P. Yu. Parkhaev (2004).
1154:P. Yu. Parkhaev (2007).
1104:P. Yu. Parkhaev (2006).
126:restored as a gastropod
1387:10.1111/1475-4983.00070
1348:Paleontological Journal
988:10.1111/1475-4983.00334
806:10.1111/1475-4983.00198
707:Budd, Graham E (2003).
604:Nelson R Cabej (2019).
456:
1526:Paleobiology Database
448:
729:10.1093/icb/43.1.157
297:small shelly fossils
276:540 to 530
1426:2008Geo....36..175M
1379:1999Palgy..42..211G
1310:2008Geo....36..175M
1242:1974Sci...186..311R
1166:2007GSLSP.286..415P
1023:1989GSLSP..47...29W
979:2003Palgy..46.1073G
879:2017NatSR...716232Z
838:2004Palgy..47.1629P
766:2007PPP...254...67S
649:2017NatSR...716232Z
545:2012GeoM..149..221K
533:Geological Magazine
299:(SSF) assemblages.
867:Scientific Reports
637:Scientific Reports
502:small shelly fauna
457:
395:Yochelcionelloidea
344:2006-2007 taxonomy
241:Helcionelliformes
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1540:
1513:Open Tree of Life
1476:Helcionelliformes
1468:Taxon identifiers
1434:10.1130/G24218A.1
1318:10.1130/G24218A.1
1236:(4161): 311–317.
1183:978-1-86239-233-5
943:978-0-521-40242-2
498:Nemakit-Daldynian
470:inch) and rather
409:Yochelcionellidae
357:Helcionelliformes
282:Nemakit-Daldynian
278:million years ago
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35:Helcionellid
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323:cephalopods
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1439:2008-10-01
1420:(2): 175.
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1126:2009-11-03
1091:. 397 pp.
963:Vostokova"
961:Anabarella
511:References
441:Morphology
428:Subfamily
421:Subfamily
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319:gastropods
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1266:17839855
997:84893338
905:29176685
738:21680420
675:29176685
327:bivalves
304:molluscs
289:Cambrian
271:molluscs
234:Synonyms
225:See text
169:Mollusca
165:Phylum:
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145:Domain:
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1306:Bibcode
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476:limpet
355:Order
267:shells
264:fossil
258:is an
1531:70795
1505:12654
1500:IRMNG
1391:S2CID
1270:S2CID
1254:JSTOR
1188:S2CID
1120:(PDF)
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