Bivalves have inhabited the Earth for over 500 million years.
They first appeared in the Middle Cambrian, about 300 million
years before the dinosaurs. They flourished in the Mesozoic and
Cainozoic eras, and abound in modern seas and oceans; their shells
litter beaches across the globe.
In life, bivalve shells are made from layers of crystals of
the minerals calcite or aragonite, which are the two different mineral forms of calcium carbonate (CaCO3). Fossil bivalves were formed
when the sediment in which they were buried hardened into rock.
Many closely resemble living forms which helps us to understand
how they must have lived.
Bivalves, which belong to the phylum Mollusca and class Bivalvia,
have two hard, usually bowl-shaped, shells (called valves)
enclosing the soft body. The valves are the parts usually found
as fossils, but decay of the elastic hinge tissue that joins
them means that they are rarely preserved together.
The inside surface of a bivalve shell is marked by the attachment
areas of the muscles and ligament responsible for opening and
closing the valves. These features, with the teeth and sockets
of the hinge, are important for classification.
The strongly recurved form of the bivalve Gryphaea is
popularly known as the Devil's toe-nail, and 17th and 18th century
Scottish accounts show that its possession was believed to cure
arthritis.
In the Late Cretaceous, rudist bivalves with strangely-shaped
conical right valves up to 50 centimetres high, formed coral-
like reefs across southern Europe and north Africa, but like
many other fossil groups, they suddenly became extinct at the
end of the Cretaceous Period.
