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]
2006-2007 taxonomy
According to P. Yu. Parkhaev,[14][15] the order Helcionelliformes is within the subclass
Archaeobranchia Parkhaev, 2001 in the class Gastropoda.
Helcionellids have a single shell, in which the peak forms a distinctive curve.[17] Some have horizontal "inhalent siphons"[12] ("exhaust pipes"[verification needed]) on the concave edges of their shells, and there is debate about whether these pointed forwards or backwards.[16] Most helcionellid fossils that have been collected are only a few millimeters long (1⁄8 inch) 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.[17]
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.[18]
Shell muscles attach near the concave side of the shell.[19]
Fossil record
The earliest helcionellid, in Siberian sections, is Oelandiella.[20][1]
They first appear in the late
Nemakit-Daldynian (lower Early Cambrian),[5] and are a constituent of the
small shelly fauna (SSF). Larger individuals, reaching centimeters in diameter, have also been found.[17]
Helcionellids have been interpreted as juvenile stages of larger limpet-like molluscs.[21]
^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.
^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.
^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.
ISBN978-0-521-40242-2.
^
abWebers, G.F.; Yochelson, E.L. (1989). "Late Cambrian molluscan faunas and the origin of the Cephalopoda". In Crame, J.A. (ed.). Origins and Evolution of the Antarctic Biota. Vol. 47. Geological Society, London: Special Publications. p. 29.
Bibcode:
1989GSLSP..47...29W.
doi:
10.1144/GSL.SP.1989.047.01.04.
S2CID128682549. {{
cite book}}: |journal= ignored (
help)
^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.
ISBN3-925919-72-4.
ISSN0076-2997. 397 pp.
http://www.vliz.be/Vmdcdata/imis2/ref.php?refid=78278