Acritarchs are small organic fossils, present from approximately 3,200 million years ago to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.

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.

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.