Helcionellid

Taxobox
fossil_range=fossil_range|Cambrian
regnum=animalia
phylum= Mollusca
subphylum= Conchifera
classis= Helcionelloida
ordo = †Helcionellida

The Helcionellids are Cambrian fossils interpreted as early (or stem group) molluscs.cite journal
author = Steiner, Gerhard
year = 2003
title = Molecular Phylogeny of Scaphopoda (Mollusca) Inferred from 18S Rdna Sequences: Support for a Scaphopoda-Cephalopoda Clade
journal = Zoologica Scripta
volume = 32
pages = 343
doi = 10.1046/j.1463-6409.2003.00121.x] Thay are similar in shape to the end of a jester's shoe, and commonly bear ridges encircling their circumference.

They first appear in the late Nemakit-Daldynian (lower Early Cambrian),cite journal
author = Budd, Graham E
year = 2003
title = The Cambrian Fossil Record and the Origin of the Phyla
journal = Integrative and Comparative Biology
volume = 43
issue = 1
pages = 157–165
doi = 10.1093/icb/43.1.157
url = http://intl-icb.oxfordjournals.org/cgi/content/abstract/43/1/157
format = Free full text] and are a constituent of the small shelly fauna (SSF). Larger individuals, reaching centimeters in diameter, have also been found.citation
author = Mus, M. M.; Palacios, T.; Jensen, S.
year = 2008
title = Size of the earliest mollusks: Did small helcionellids grow to become large adults?
journal = Geology
volume = 36
issue = 2
pages = 175
doi = 10.1130/G24218A.1
url = http://geology.geoscienceworld.org/cgi/content/abstract/36/2/175]

These resemble limpets, with the peak of the shell distinctively curved to form a shape identical to the helcionellids known from the SSF assemblages. Since molluscs grow by adding material to the opening of the shell, it appears that the helcionellids of the SSF are juvenile molluscs, which continued to grow into large limpet like creatures typical of early Cambrian molluscs.

External links

*http://www.palaeos.com/Invertebrates/Molluscs/BasalMollusca/Conchifera/Helcionelloida.html

References