THE EVOLUTION OF PHYTOPLANKTON DURING THE EARLY MESOZOIC

Phytoplankton are drifting microscopic plants and are the most abundant marine organisms. Modern phytoplankton floras emerged during the Triassic Period and underwent major radiations in the Middle Jurassic (Bajocian). The inception of the modern phytoplankton came after an interval of particularly low diversity during the Carboniferous and Permian, when sea levels were particularly low. The dinoflagellate cyst fossil diversity record rose and fell in congruence with global sea levels and temperature fluctuations. It is hypothesised that high sea levels result in significantly greater ecospace for coccolithophores and dinoflagellates, which thrive in epicontinental sea settings. The coccolithophores and the dinoflagellates were badly affected by the end-Triassic mass extinction and recovered slowly during the Early Jurassic. Both groups also declined during the Toarcian Oceanic Anoxic Event, some 11 myr before the Bajocian radiations. The latter phenomena have been linked to continental breakup, sea level rise and a positive shift in isotopically light carbon isotopes.