Bivalves (Phylum Mollusca)

© Craven & Pendle Geological Society

Dunbarella

Dunbarella

 Posidonia

Posidonia

Carbonicola

Carbonicola sp

Bivalves have been around a very long time! They have been recorded in Mid-Cambrian times and some have survived to the present day. The numerous closely related living forms have enabled palaeontologists to gain a good understanding of ancient types.

These molluscs either swam or burrowed into the soft marine sediments.  Some even bore into a much harder substrate. They usually have two mirror-imaged shells (Gryphae is one example that doesn't) so unlike brachiopods their line of symmetry runs between both valves.  The bivalve shell is usually made out of calcium carbonate. In Carboniferous times however, molluscs such as Carbonicola adapted to live in fresh water rivers and lakes.

Bivalves were a source of food for other creatures that inhabited the sea such as gastropds, crustaceans, starfish and fish. In order to protect themslves from predators, bivalves were able to burrow into the sea bed (substrate) while others grew thick shells with spines on. The shape of the bivalve would usually indicate the mode of life e.g. the spectacular Dunbarella would more than likely have adapted to survive in muddy sea floors.

In the Pennsylvanian deposits, the bivalve Carbonicola would have been a useful tool for geologists' in their search for coal-bearing rocks, due to its fresh water existence.