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Brachiopods
Brachiopods (Brachiopoda) are sea-living, double-shelled animals that first lived in Palaeozoic times. Some species of brachiopods still survive, although they are now
quite rare.
Brachiopods were most common in the Palaeozoic Era, unlike bivalves (see page 29) which have become common mainly since the Mesozoic Era. Brachiopods have been used as index fossils for Jurassic and Cretaceous rocks.
The two shells, or valves, of a brachiopod are distinguished by being symmetrical (in a mollusc they are asymmetrical). The two valves are strongly convex, with one valve (the ventral) valve being much larger than the other. Some lived in shallow waters, attached to rocks
by a stem-like structure called a pedicle. Others lived just below the surface of sandy beaches. Some had spines
to help them remain stable enough to live on shallow, wave-washed beaches.
The earliest brachiopods are from the Cambrian Period, but they became much more common and varied in the Silurian Period (the Age of the Brachiopods).
(Below) The main characteristics of a brachiopod viewed from above (top) and the side (bottom).
Symmetrical along valves from above or below
Not symmetrical between valves from side
Ventral valve
Growth line
Dorsal valve
Ventral valve
(Right) The hard parts (shown in red) are normally preserved in fossil brachiopods.
Dorsal valve
Muscles
Pedicle for attachment to rock
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