File:Fossiliferous dolostone with ostracods 3.jpg

English: Dolostone is a crystalline-textured chemical sedimentary rock composed of dolomite (CaMg(CO3)2 - calcium magnesium carbonate). Most dolostones form by chemical alteration of previously-existing limestones. They tend to be fine-grained, vuggy (= having irregularly-sized and irregularly-shaped holes), and have poorly preserved fossils.

The small ovoid fossils in this dolostone sample are ostracods, often jokingly called "lima beans". Ostracods have an Ordovician to Holocene fossil record. Some Cambrian fossils (e.g., bradoriids) were formerly assigned to ostracods. True ostracods are small crustaceans with a calcified, bivalved, hinged carapace ("shell"). Most are microfossil-sized, but some are much larger. Many forms have considerable sexual dimorphism - females can have a different shell form compared with males. Such adult female ostracods are called heteromorphs. The pre-adult ostracods and male adults are called tecnomorphs.

Most ostracods are filter feeders, but some are detritus feeders. They principally consume diatoms (= tiny, 1-celled, photosynthesizing protists) and other small "algal"; forms. Marine, brackish, and freshwater ostracods are known - they can be benthic (on the seafloor) or nektonic (free-swimming). Even terrestrial ostracods are known, but they are rare - they live on water drops on leaves in some jungles. Ostracods occur in large numbers in aquatic settings, but they are not very good swimmers - they are virtually defenseless against fish.

Ostracods are common fossils and have biostratigraphic utility. They are often used by petroleum companies to date sedimentary rocks. They are probably the most economically significant crustacean group. In Mesozoic and Cenozoic rocks, ostracods are second only to foraminifera in abundance and biostratigraphic importance.

Ostracod shells often have prominent ornamentation, consisting of lobes, spines, and processes. The ornamentation is used for taxonomic differentiation. Paleozoic ostracods tend to have more ornamentation than later forms. Some ostracods are just smooth "lima beans".

The hinge line is on the dorsal side of the shell and typically consists of a simple groove or edge. Interlocking teeth structures, as in bivalves or brachiopods, are not present. Generally, there are five pairs of adductor muscles, which are used to close the shell - their presence will leave muscle scars on the shell's interior. Ostracod shells usually have tiny holes through which sensory hairs extended. Transparent to translucent portions of the shell are where eyespots occur. Some ostracods have only one eyespot.

Classification: Animalia, Arthropoda, Crustaca, Ostracoda

Stratigraphy: unknown, but possibly from the Silurian of Ohio

Locality: out-of-place sample from Pickerington, northwestern Fairfield County, central Ohio, USA

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See info. at:

en.wikipedia.org/wiki/Ostracod