What are brachiopods?
Brachiopods are a type of marine ‘shellfish’ having their soft body enclosed in a pair of shells. In this respect they superficially resemble the familiar bivalve molluscs that include clams and mussels, but they are not related to these at all. The two shells of brachiopods are different in size (unlike most bivalves), and the similarity in shape of some forms to Roman oil lamps has led to their being sometimes called ‘lamp-shells’. Brachiopods may be attached to the sea floor by a variety of means, including a fleshy stalk (called a pedicle), spines, or a kind of cement; or they may lie free on the sediment surface. The shell can be opened to allow the animal to pump seawater into it, using a large feathery structure called the lophophore which also filters small food particles from the seawater and extracts oxygen. In some types of brachiopods the two shells have an interlocking hinge, but in other types the shells lack a hinge and are simply held together by muscles.
Brachiopod, Spirifer sp., Devonian Period (410-354 mya)
Artist: Kate Nolan / Source: Museum Victoria
Where are fossil brachiopods found?
Brachiopods were one of the dominant forms of life in the oceans for much of the Palaeozoic Era, from 545 to 251 million years ago (mya), during which time they evolved many different forms. At the end of the Palaeozoic they suffered massive extinctions, and although they again became abundant during the following Mesozoic Era (251-65 mya), they never recovered their former variety. Fossil brachiopods are so diverse and widespread that they are often used by palaeontologists to determine the age of rocks in which they are found. Brachiopods are very common in Middle Palaeozoic rocks of central and eastern Victoria, and are also found in Tertiary limestones.
Fossil brachiopods, Archaeorthis waratahensis; Early Ordovician (c. 485 my), Waratah Bay, Victoria
Source: Museum Victoria
Living brachiopods and their relatives
Living brachiopods can still be found in many marine environments (for example, several species inhabit the sediments of Bass Strait and Westernport Bay), but they are most common in deep and cold waters and so are unlikely to be seen. Fewer than 400 living species of brachiopods have been discovered — a far cry from the number of fossil species, of which at least 30 000 are known. Some brachiopods alive today are almost unchanged in appearance from some of the earliest forms that appeared during the Cambrian Period at the beginning of the Palaeozoic.
Brachiopods are related to bryozoans, another group of marine invertebrates that are more commonly seen though possibly not recognized (see Museum Victoria Information Sheet No. 10138). Although bryozoans and brachiopods appear very different, their relationship is indicated by the presence of a similar water filtering mechanism (the lophophore) and by other features.