What Is a Brachiopod? Two Shells That Are Not a Clam
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Brachiopods have two shells and look like clams, and they are a separate phylum that dominated the sea floor for hundreds of millions of years before being displaced. Telling them apart from bivalves is a matter of symmetry and it explains a great deal about how they lived.
How to tell them apart
A bivalve mollusc, meaning a clam, mussel or oyster, has two shells that are mirror images of each other, with the plane of symmetry running between them, so the left shell matches the right. A brachiopod has two shells of different size and shape, with the plane of symmetry running through the middle of each shell rather than between them, so each valve is symmetrical in itself and the two differ from one another. That single distinction identifies them reliably and reflects a completely different body plan, since the two groups are not closely related and their similarity is convergent. Brachiopods are generally attached to the seabed by a fleshy stalk emerging through a hole in the larger shell, and inside they carry a lophophore, a coiled or looped structure bearing ciliated tentacles that generates a water current and filters food from it, which is a feeding organ found in several unrelated groups and not in molluscs at all.
Why they matter to geology
They are among the most useful fossils for Palaeozoic rocks and their properties explain why:
- •Extremely abundant in shallow marine deposits, with some limestone beds consisting largely of their shells
- •Durable calcite shells that survive transport and preserve well, including fine surface ornament
- •Rapid evolution with distinctive forms, which supports biostratigraphic zonation
- •Wide geographic distribution, allowing correlation between continents
- •Shell chemistry that records ocean temperature and composition, since the calcite incorporates isotopes reflecting the water it formed in, which makes them a standard archive for reconstructing ancient climate
- •Growth lines recording seasonal and sometimes daily increments, which have been used to estimate the number of days in a year in the distant past and independently confirm the slowing of Earth's rotation
How they lived
The body plan dictates a fairly narrow way of life and the variation within it is instructive. Most species attach to the seabed by a stalk and sit slightly raised, filtering water drawn past the lophophore, which suits firm substrates and moderate currents and rules out soft mud where a stalk has nothing to grip. Some groups abandoned the stalk and lay free on the sediment, using a wide flat shell to spread their weight, and others cemented themselves directly to rock or to other shells. A few burrowed, with the lingulids using a long muscular stalk to anchor in a vertical burrow in soft sediment, which is a strategy that has persisted essentially unchanged for a very long time. The shells are opened by muscles rather than by a springy ligament, which is the reverse of a bivalve arrangement, and the hinge structures differ between groups sufficiently to be a primary basis for classification. Feeding is entirely passive, with no ability to pursue anything, which is part of why the group fares badly where competitors can burrow or move.
The great displacement
Brachiopods dominated the Palaeozoic sea floor, outnumbering bivalves substantially, and after the end-Permian extinction the positions reversed permanently, with bivalves dominating ever since and brachiopods reduced to a small number of species in restricted habitats. Explaining that reversal has generated a long argument. The competition hypothesis holds that bivalves are simply better, being more mobile, able to burrow, more metabolically efficient and able to exploit a wider range of food, and that they outcompeted brachiopods once given the opportunity. The mass extinction hypothesis holds that the extinction hit brachiopods disproportionately for reasons unrelated to competition, and that bivalves inherited the space rather than winning it, which is supported by analyses showing that the two groups' diversity trajectories do not look like a competitive replacement but like differential survival of a catastrophe. The current consensus leans towards the second, which is a case of a chance event rather than a superior design determining which group inherited the world.
What survives and where
Several hundred species persist, occupying habitats where bivalves are scarce, including cold deep water, caves, crevices and areas of hard substrate where a stalked attachment works better than burrowing. They are slow-growing, long-lived and metabolically frugal, which suits low-food environments and is consistent with the account of their retreat. One genus, Lingula, has a fossil record extending back to the Cambrian and a body form that has changed remarkably little, which made it a standard example of a living fossil until molecular work showed that the modern species are not the ancient ones and the lineage has continued evolving, which is the usual correction to that label. Living brachiopods are studied for their shell chemistry, since specimens of known age and water conditions calibrate the isotopic methods applied to fossils, so the survivors are scientifically useful out of all proportion to their ecological importance.
The takeaway
The two shells differ from each other and each is symmetrical in itself, which is the reverse of a clam and identifies the group immediately. They dominated Palaeozoic seabeds and their shells date rock and record ancient ocean temperature. The reversal with bivalves after the end-Permian extinction now looks like differential survival of a catastrophe rather than one group outcompeting the other.