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]
^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.
^ abNelson 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)
^ abBudd, 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.
^Cite error: The named reference Weber1989 was invoked but never defined (see the help page).
^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
Helcionellid or Helcionelliformes is an order of small fossil shells that are universally interpreted as molluscs, though no sources spell out why this...
concept that the ancestral molluscan radula was mineralized. However, the Helcionellids, which first appear over 540 million years ago in Early Cambrian rocks...
concept that the ancestral molluscan radula was mineralized. However, the Helcionellids, which first appear over 540 million years ago in Early Cambrian rocks...
Oelandia is an extinct genus of basal molluscs of the family helcionellid from the Middle Cambrian of Sweden, China and North America. It has a small,...
Tannuella is a genus of helcionellids known from Lower Cambrian strata. It has a high, septate shell similar to that of Knightoconus . These genera were...
convincingly triploblastic, as all "higher" animals are. Subsequently, the Helcionellids are the first shelly fossils that can be placed in the stem group to...
Tichkaella is a genus of helcionellid from the Middle Cambrian. It has a strongly spiralled, smooth shell with concentric ridges that have low relief....
helcionellid from the Middle Cambrian of New Zealand. It is flattened sideways, with a dividing wall within its shell; it resembles the helcionellid Eotebenna...
Totoralia is a genus of limpet-like helcionellid known from Cambrian deposits including the Burgess Shale. It comprises two species. T. reticulata bears...
proposes the cephalopods and gastropods as sister to the scaphopods with helcionellids as the stem group. A review of deep molluscan phylogeny in 2014 found...
Protowenella is a genus of helcionellid from the Middle Cambrian of Australia. It has a strongly spiralled, smooth shell with concentric ridges that have...
this genus belongs with the gastropods, the monoplacophorans, or the helcionellids, although no firm association with any of these classes has been established...
Lamaureriella is a genus of laterally-flattened curled helcionellid known from Lower Ordovician deposits. The majority of the hundreds of known specimens...
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,...
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