Ostracod
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Ostracoda is a class of the Crustacea, sometimes known as the seed shrimp because of their appearance. Some 50,000 extinct and extant species have been identified, grouped into several orders.
Ostracods are small crustaceans, typically around one mm in size, but varying between 0,2 to 30 mm, laterally compressed and protected by a bivalve-like, chitinous or calcareous valve or "shell". The hinge of the two valves is in the upper, dorsal region of the body.
Ecologically ostracods can be part of the zooplankton, or (most commonly) they are part of the benthos, living on or inside the upper layer of the sea floor. Many ostracods are also found in fresh water and some are known from humid continental forest soils.
Fossils
Ostracods have a long and well-documented fossil record from the Cambrian to the present day. An outline microfaunal zonal scheme based on both foraminifera and ostracoda was compiled by M. B. Hart (1972).
Ostracods have been particularly useful for the biozonation of marine strata on a local or regional scale, and they are invaluable indicators of paleo-environments because of their widespread occurrence, small size, easily-preservable generally-moulted calcified bivalve carapaces, the valves are a commonly found microfossil.
Morphology
The body of an ostracod is encased by two valves, which together form the duplicature. A distinction is made between the valve (hard parts) and the body with its appendages (soft parts).
The body consists of a cephalon (head), separated from the thorax by a slight constriction. The segmentation is unclear. The abdomen is regressed or absent whereas the adult gonads are relatively large. There are 5-8 pairs of appendages. The branchial plates are responsible for oxygenation.
During the ontogeny the epidermis (containing mesodermal tissue) is invaginated ventrolaterally near the cephalon/thorax area. This invagination proceeds upwards and tailwards, until the whole animal is enveloped by lamellae on both sides: the duplicature is formed. The dorsal region never becomes invaginated, and is called the istmus. The mesodermal tissue in the duplicature develops into the vestibulum. The vestibulum makes contact with the body near the istmus. The vestibulum plays a role in oxygenation. The two lamellae surrounding the animal each have an inner and an outer lamella.
