ostracods

ostracods Ostracods (technically Ostracoda) are a group of small laterally compressed crustaceans which are enclosed within an exoskeleton or carapace consisting of two valves. The carapace is made of chitin (a hydrocarbon related to cellulose) but is usually reinforced to a greater or lesser extent by calcium carbonate or, less commonly, calcium phosphate. The shape varies but it is usually ovate, kidney-shaped, or bean-shaped. Most ostracods are small, generally 0.3 to 3 mm long, although a few giants are as much as 30 mm in length. The two valves are unequal in size, the larger one partly overlapping the smaller. The valves are hinged along their dorsal edges and are closed by a series of muscles which leave scars on the valve interiors. Ostracods, like other crustaceans, grow by moulting the carapace at regular intervals. During its life each individual thus produces a series of shed carapaces or instars which are graded in both size and morphology. The exoskeletons of some species of ostracods exhibit sexual dimorphism: male and female carapaces differ in size or morphology, or both.

Modern ostracods are classified on the basis of their soft anatomy, particular note being made of the form and number of the appendages and the structure of the reproductive organs, features which are rarely preserved in fossils. Consequently, fossil forms are classified using the characteristics of the carapace, such as overall shape, hinge structure, muscle scar pattern, and ornamentation.

The ostracods first appeared in the early Cambrian but are relatively rare until the Silurian. In later Palaeozoic, Mesozoic, and Cenozoic sequences they can be extremely abundant. They have been widely used biostratigraphically in Jurassic to Pleistocene rock sequences. Most ostracods occupied a restricted, usually benthic, habitat and they therefore tend to have a limited geographical distribution. This makes them of limited use for long-distance correlations. However, where rocks of shallow-marine, brackish, or freshwater origin are interbedded, they are particularly valuable and can be used to correlate marine and non-marine sequences.

Living ostracod species inhabit a wide range of marine, brackish, and freshwater environments. A few species have even adapted to life in humid terrestrial habitats. Their diet includes bacteria, protists of all kinds, living algae and higher plants, and the organic debris that results from the decomposition of plants and animals. Since the majority of Recent genera are present in fossil assemblages as far back as the Miocene and many have close relatives in Mesozoic assemblages, ostracods are widely used in palaeoecological analysis. Even where the biological affinities of a fossil are uncertain, palaeoecology can be inferred from carapace morphology. For example, the carapaces of freshwater species tend in general to be thin and smooth, while those of marine forms are thick and heavily ornamented. There are similar correlations between carapace features and behaviours such as swimming, crawling, and burrowing.

Allan N. Insole

Bibliography

Bignot, G. (1985) Elements of micropalaeontology. Graham and Trotman, London.
Brasier, M. D. (1980) Microfossils. Allen and Unwin, London.