acritarch

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Acritarchs () are organic microfossils, known from the Archean eon of the Precambrian to the present. The classification is a catch all term used to refer to any organic microfossils that cannot be assigned to other groups. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion. Many acritarchs likely represent resting cysts of single-celled marine phytoplanktonic algae, similar to those produced by living dinoflagellates.