Acritarch
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Acritarchs are small organic structures found as fossils. In general, any small, non-acid soluble (i.e. non carbonate, non-siliceous) organic structure that can not otherwise be accounted for is an acritarch. Most acritarchs are likely the remains of single-celled organisms, especially the planktonic algae. They are found in sedimentary rocks from the present back into the Precambrian. They are typically isolated from siliciclastic sedimentary rocks using hydrofluoric acid but are occasionally extracted from carbonate-rich rocks. They are excellent candidates for index fossils used for dating rock formations in the Paleozoic Era and when other fossils are not available. Because most acritarchs are thought to be marine, they are also useful for palaeoenvironmental interpretation. Acritarchs include the remains of a wide range of quite different kinds of organisms - ranging from the egg cases of small metazoans to resting cysts of many different kinds of chlorophyta (green algae). It is likely that some acritarch species represent the resting stages (cysts) of algae that were ancestral to the dinoflagellates. The nature of the organisms associated with older acritarchs is generally not clear, though many are probably related to unicellular marine algae. In theory, when the biological source (taxon) of an acritarch does become known, that particular microfossil is removed from the acritarchs and classified with its proper group.
Acritarchs are known from 1400 Ma and had achieved considerable diversity by 1300 Ma. Diversity crashed during the Sturtian-Varanger glacial event around 800 Ma. Diversity increased again during the Ediacaran period. Diversity declined suddenly at the end of the Precambrian. The acritarchs show their greatest diversity during the Cambrian, Ordovician, Silurian and Devonian. The nature of some Acritarchs can be identified by their structure. A few can be tentatively identified by the presence of specific chemicals associated with the fossils.
