Calcareous nannoplankton evolution and diversity through time

Summary

Planktic microfossils arguably provide the most complete (stratigraphic and taxonomic) record of biodiversity of any group of organisms. The phytoplankton record is of particular significance as it most likely tracks global changes in the climate-ocean system and, in turn, influenced biodiversity and productivity of higher trophic levels of the biosphere. Coccolithophores and associated calcareous nannoplankton first appear in the fossil record in Upper Triassic sediments (~225 Ma) and, despite significant extinctions at the Triassic/Jurassic boundary, the Mesozoic diversity record is one of relatively uniform increase punctuated by short periods of turnover and decline. Rates of speciation that are significantly above background were restricted to the Late Triassic, Early Jurassic and Tithonian-Berriasian intervals. Enhanced rates of extinction occurred at the Triassic/Jurassic, Jurassic/Cretaceous and Cretaceous/Tertiary boundaries.

There is no clear correlation between coccolithophore diversity and Mesozoic climate, as it is currently understood, but the dominant trajectory of diversity increase suggests long-term stability and widespread oligotrophy in photic zone environments. The Neocomian and Campanian-Early Maastrichtian intervals of diversity increase are clearly associated with increased numbers of endemic taxa at both low and high latitudes. These intervals have been interpreted as periods of cooling or cooler climates, and greater differentiation of the photic zone environment may have led to the biogeographic partitioning. Notably, none of the mid-Cretaceous Oceanic Anoxic Events are associated with above-background evolutionary rates or significant taxonomic loss or innovation.

Cenozoic nannoplankton diversity patterns are markedly more variable than those of the Mesozoic, and rates of speciation, extinction and turnover are consistently higher. There is also good correlation between diversity and climate trends, with higher diversities associated with warm climates. This is best illustrated by the Paleogene record, where the Cenozoic diversity maximum, at the Paleo cene/Eocene boundary, coincided with climates of extreme warmth, and significant diversity decline tracked climate cooling through the Late Eocene and into the Oligocene. This relationship between climate and coccolithophore diversity is contrary to that observed in the Mesozoic, and suggests different controls on evolution during the two eras. The Cretaceous record suggests that cooling within a greenhouse-mode climate system may have stimulated diversification at all viable latitudes via biogeographic partitioning. In contrast, the Cenozoic data indicates that cooling tended to drive diversity decrease. This may be explained by the greater magnitude and longevity of Cenozoic cooling, in an icehouse-mode climate system, which prevented coccolithophore diversification at higher latitudes where, instead, diatoms became established as the dominant group of phytoplankton.