Helcionelloida
Helcionelloida is an extinct group of ancient molluscs. 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.
Anatomy
These animals were untorted and they had a coiled, cone-shaped shell. The majority of species were small. 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 categorizes members of this taxon within thePaleozoic 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:Gastropoda Cuvier, 1797
Subclass Archaeobranchia Parkhaev, 2001
- Order Helcionelliformes Golikov & Starobogatov, 1975
- Order Pelagiellifomes MacKinnon, 1985
- Order Khairkhaniiformes Parkhaev, 2001
- Order Onychochiliformes Minichev & Starobogatov, 1975