Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Mollusca

Nearly all small shelled molluscs from the Early Cambrian are fragile, but they are well represented in small shelly faunas appearing first in Stage 1. Similar to Gastropoda, these SSFs have coiled shells, though the authors suggest they are not likely to be crown forms, and small shells that have been assigned to Bivalvia that may also represent stem groups. There are also groups that appear likely to be molluscs or mollusc-related, that are entirely extinct: Rostroconchia, a form resembling bivalves, though with a single shell; various clades that are univalved among what was previously called Monoplacophora; and possibly Probivalvia, that has 2 articulaed shells, though doesn’t much resemble Bivalvia in gross shape. Hyolihida, which is generally regarded as a separate extinct phylum that is close to Mollusca, which have coiled shells the apertures of which are closed by an operculum, a second flattish shell. The hyolithids, tending to be measured in millimetres, are especially common among SSFs, and are also found in assemblages of other types, that lasted about 280 My, some later forms reaching 40 cm in length.

Sources & Further reading

1. Erwin, Douglas H., & Valentine, James W., 2013, The Cambrian Explosion: The Construction of Animal Biodiversity, Roberts & Co., Greenwood Village, Colorado