A review of Paleozoic phytoplankton biodiversity: Driver for major evolutionary events?

Phytoplankton form the base of most marine trophic chains and studying their past diversity at regional and global scales can provide valuable insights into the evolution of marine ecosystems and climate history. Using a new database of more than 4000 species of acritarchs and prasinophytes, a comprehensive investigation of the taxonomic diversity trajectories of this marine (micro)phytoplankton throughout the Paleozoic is performed for the first time. This dataset compiles data from published literature, including taxonomic, geographic and stratigraphic information at the stage resolution. Our results highlight five major temporal trends in phytoplankton diversity variation: (i) an initial plateau of moderate richness during the early and middle Cambrian, followed by (ii) a sharp increase from the late Cambrian to the Middle Ordovician, which records the highest Paleozoic diversity of organic-walled phytoplankton (OWP); then, (iii) a protracted decrease during the Late Ordovician to Middle Devonian; (iv) a slight peak in diversity during the Late Devonian, before (v) falling to the lowest richness recorded during the Carboniferous and Permian. The role of phytoplankton during major biotic events is discussed: While phytoplankton evolution may have been a factor in enabling the “Cambrian Explosion”, we do not find a strong relationship between the diversity changes of the phytoplankton and this event and we thus refute the notion that it might have been a major driver of radiations during this interval. However, a strong increase in phytoplankton diversity coincides with the Great Ordovician Biodiversification Event (GOBE), indicating that the profound changes of marine phytoplankton, and thus of the base of marine food webs, enabled diversifications throughout marine ecosystems. A decrease in phytoplankton diversity during the Lower and Middle Devonian points against the hypothesis of phytoplankton triggering the proposed “Devonian Nekton Revolution”. By comparing the results with paleoenvironmental parameters, several factors are found to be possibly related to the long-term diversity trends: Our results indicate that paleogeography and sea-level changes were probably the main drivers of phytoplankton diversity patterns throughout the Paleozoic, while increases in sediment influx provided facilitating conditions for phytoplankton diversification. Atmospheric CO2 concentration as well as temperature and related sea ice cover are found to be further important controlling factors for phytoplankton diversity.