Helcionellid

According to P. Yu. Parkhaev, the order Helcionelliformes is within the subclass Archaeobranchia Parkhaev, 2001 in the class Gastropoda. Order Helcionelliformes Golikov & Starobogatov, 1975 • Superfamily Helcionelloidea Wenz, 1938 • Family Helcionellidae Wenz, 1938 • Family Carinopeltidae Parkhaev, 2013 (syn "Igarkiellidae" Parkhaev, 2001) • Family Coreospiridae Knight, 1947 • Superfamily Yochelcionelloidea Runnegar & Jell, 1976 • Family Trenellidae Parkhaev, 2001 • Family Yochelcionellidae Runnegar & Jell, 1976 • Family Stenothecidae Runnegar & Jell, 1980 • Subfamily Stenothecinae Runnegar & Jell, 1980 • Subfamily Watsonellinae Parkhaev, 2001 == Morphology==

'' shows water flowing in under the shell, over the gills and out through the "exhaust pipe". 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. Shell muscles attach near the concave side of the shell. == Fossil record==

The earliest helcionellid, in Siberian sections, is Oelandiella. They first appear in the late Nemakit-Daldynian (lower Early Cambrian), and are a constituent of the small shelly fauna (SSF). Larger individuals, reaching centimeters in diameter, have also been found. Helcionellids have been interpreted as juvenile stages of larger limpet-like molluscs.{{cite journal |author1= Mus, M.M. |author2= Palacios, T. |author3=Jensen, S. |year= 2008 | title= Size of the earliest mollusks: Did small helcionellids grow to become large adults? | journal= Geology |volume= 36 |issue= 2 | page= 175 |doi= 10.1130/G24218A.1 | url= http://geology.geoscienceworld.org/cgi/content/abstract/36/2/175 | access-date=2008-10-01 ==See also==