Helcionelloida

Helcionelloida is an extinct group of ancient molluscs (phylum Mollusca). These are the oldest known conchiferan molluscs, that is, they had a mineralised shell. Some members of this class were mistaken for Monoplacophorans. The class was erected by Peel in 1991.[1]

Helcionelloida
Temporal range: Early Cambrian–Early Ordovician
Latouchella costata
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Class: †Helcionelloida
Peel, 1991
Orders

and see text

Yochelcionella cyrano
Pelagiella atlantoides

Anatomy
Further information: Helcionellid § Morphology

These animals were untorted and they had a coiled, cone-shaped shell. The majority of species were small (shells being about two millimeters in length and or diameter). Modern reconstructions depict them as resembling snails. The shells of some genera, particularly Yochelcionella, also possessed a "snorkel"-like opening which was most likely used for breathing.

2005 taxonomy

The taxonomy of the Gastropoda by Bouchet & Rocroi, 2005[2] categorizes members of this taxon within the Paleozoic molluscs of uncertain systematic position, but that taxonomy to not use the name Helcionelloida.

2006-2007 taxonomy

Taxonomy of helcionelloid according to the opinion of P. Yu. Parkhaev:[3][4]

Gastropoda Cuvier, 1797

Subclass Archaeobranchia Parkhaev, 2001

  • Superfamily Helcionelloidea Wenz, 1938
  • Subfamily Stenothecinae Runnegar & Jell, 1980
  • Subfamily Watsonellinae Parkhaev, 2001
  • Order Pelagiellifomes MacKinnon, 1985

Subclass Divasibranchia Minichev & Starobogatov, 1975

Subclass Dextrobranchia Minichev & Starobogatov, 1975

gollark: This has been explained already.
gollark: I also do this, but:- how often do the search queries contain things you dislike- how hard is it to scroll past it or whatever, given that average queries probably won't bring up much of that
gollark: I do not think search is a significant issue, and the logreading thing can be fixed.
gollark: I mean, you could shunt it to an archive channel via webhook things after however long, but that would have its own issues.
gollark: The precise time is tunable, after some amount of time it would probably cease to be discussed. And why should they *not* exist? The logreading issue is fixable as I said, search... maybe less so, but I'm not sure how many search queries actually turn up that stuff *now* and how big an issue it would be.

References
  1. 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 and the Significance of Problematic Taxa. Cambridge University Press. pp. 157–177. ISBN 978-0-521-40242-2.
  2. 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
  3. P. Yu. Parkhaev (2006) "Adaptive radiation of the Cambrian helcionelloid mollusks (Gastropoda, Archaeobranchia). Archived 2015-06-10 at the Wayback Machine" In: S.V. Rozhnov (ed.) "Evolution of the biosphere and biodiversity. Towards the 70th anniversary of A. Y. Rozanov". 2006. Moscow, pp. 282-296.
  4. P. Yu. Parkhaev (2007). "The Cambrian 'basement' of gastropod evolution". Geological Society, London, Special Publications. 286: 415–421. doi:10.1144/SP286.31.
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