The diversity of the Permian phytoplankton

Abstract

The Paleozoic fossil record of organic-walled microphytoplankton is represented by the acritarchs, organic-walled microfossils (OWMs) considered mostly as the resting cysts of phytoplanktonic organisms, although the biological affinities of the acritarchs are, by definition, unknown. Acritarchs appear in the Precambrian and reach their highest diversity in the Lower and Middle Paleozoic (Cambrian to Devonian). After a drastic diversity drop in the Devonian, they are considered to be of very low diversity in the Upper Paleozoic (Carboniferous to Permian), where some authors claim the presence of a ‘phytoplankton blackout’ due to nutrient depletion in the oceans. Here we present a synthesis of the Permian fossil record of acritarchs. The revision shows that Permian acritarch descriptions have largely been neglected, compared to other palynomorph groups, such as spores and pollen grains. While larger organic-walled cysts, as known from the Lower and Middle Paleozoic, are usually absent, many smaller acritarchs are commonly found in Permian palynological assemblages. In addition, several OWMs interpreted as fungal spores have been described, but also possible green algae, including chlorophycean, prasinophycean or zygnematophyceaen algae. During most of the Permian stages, OWMs show genus richness of about 20 to 30 genera. Some genera, such as Micrhystridium and Veryhachium, have been reported in over 40 publications. Nevertheless, many Permian acritarchs still need to be documented in detail, and additional systematical studies, in particular of the very small taxa, are needed to fully understand the diversity and significance of Permian phytoplankton.