Brachiopods look superficially very similar to bivalves (Chapter 9), with both organisms having two shells, usually made from calcite and frequently ornamented with radial ribs. This similarity is the consequence of sharing a similar lifestyle; most species of each group are sessile filter feeders living in the shallow marine environment. As such they represent an example of evolutionary convergence.
There is a simple way to distinguish between almost all brachiopods and bivalves, related to their symmetry (Fig. 6.4). Both have bilateral symmetry, as do most groups of animals. Bivalves grow a left and right shell with the line of symmetry along the margins of the valves. Brachiopods grow a front and back (dorsal and ventral) shell, with the line of symmetry cutting each valve in half. Although there are some extreme adaptations to both groups that involve the two valves developing irregular shapes, these usually still show some vestige of their inherited symmetry so that this rule of thumb tends to be a good tool in discriminating between the two groups.
Line of symmetry
Line of symmetry
Fig. 6.4 Symmetry of brachiopod and bivalve shells. (a) Brachiopod showing dorsal and ventral valves with the line of symmetry bisecting the valves. (b) Bivalve with the line of symmetry along the plane of contact of the valves. (c) Left view of a brachiopod showing asymmetric valves. (d) Right view of a bivalve shell showing the asymmetry of a single valve.
A small (about 2 cm from the beak to the anterior edge), smooth, phosphatic brachiopod known as a "living fossil" as its morphology has not changed significantly since the Ordovician. Fully infaunal, it lives in burrows with its anterior edge close to the sediment-water interface. The pedicle anchors the brachiopod to the mud whilst the valves rotate and grind through the sediment. Modern Lingula mainly exploit marginal habitats, but fossil Lingula are known from shelf and basin environments.
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