Brachiopods were the most abundant and diverse marine invertebrates of the Palaeozoic. Brachiopods originated in the early Cambrian and diversified in the Ordovician. They dominated the shallow marine environment throughout the Palaeozoic. Although they survived the end-Permian mass extinction they declined through the Mesozoic so that of the 4500 fossil genera known, only 120 exist today.
Brachiopods have a two-valved, hinged shell enclosing their soft tissues. Although brachiopod morphology seems relatively straightforward there are difficulties with their taxonomy because many species that look very similar have a very different internal organization. New techniques have allowed brachiopod classification to be reorganized on a line that more accurately reflects their family tree. The traditional division into two classes, Inarticulata and Articulata, has been superseded by a system separating brachiopods into three subphyla, the simplified characteristics of which are shown in Table 6.1. Linguliformea and Craniiformea may be considered to be inarticulated brachiopods and Rhynchonelliformea to be articulated brachiopods in the older classifications.
Most modern brachiopods live in marginal marine environments attached to the substrate and species tend to be morphologically similar. However, Palaeozoic brachiopods exploited a wider range of marine environments and their morphology was extremely diverse, ranging from erect corallike forms to flattened saucer shapes. The morphology and community structure of brachiopods can be used in the interpretation of past environments.
Table 6.1 Brachiopod classification.
Shell composition Hinge mechanism Pedicle
Organophosphate Lacking teeth and sockets Present Gut with anus
Lacking teeth and sockets Reduced or absent Gut with anus
Teeth and sockets present Present
Gut without anus
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