Acritarch

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A supposed Ediacaran embryo contained within an acritarch from the Doushantuo formation

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 classified as 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 1,400 million years ago, and had achieved considerable diversity 100 million years later. Diversity crashed during the Sturtian-Varanger glacial event around 800 million years ago. 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.

Whilst the classification of acritarchs into form genera is entirely artificial, it is not without merit, as the form taxa show similar traits to genuine taxa - for example an 'explosion' in the Cambrian and a mass extinction at the end of the Permian.