[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.