Helcionellid

[4] These animals are first found about 540 to 530 million years ago[3] in the late Nemakit-Daldynian age, which is the earliest part of the Cambrian period.

[5] 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.

Order Helcionelliformes Golikov & Starobogatov, 1975 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.

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.

This speculative restoration of the tiny Helcionellid fossil Yochelcionella shows water flowing in under the shell, over the gills and out through the "exhaust pipe". [ 16 ]