PLANKTON EVOLUTION DURING THE BAJOCIAN (MIDDLE JURASSIC, 170—168 MA): A MAJOR TRANSITION IN THE MODERN EUKARYOTIC PHYTOPLANKTON

The Bajocian of the Middle Jurassic was a critical interval in the evolution of the modern plankton during which the dinoflagellates and coccolithophores diversified whilst the planktonic foraminifera appeared for the first time. Our detailed study of the dinoflagellate cyst record has revealed that the Early Bajocian was characterised by low dinoflagellate cyst diversity and the dominance of the genus Dissiliodinium. The unusually large species D. giganteum dominated assemblages around the outer region of the European epicontinental seaway, whereas in Australia the large species D. caddaense was dominant. It was during this interval that the coccolithophore genus Watznaueria diversified and coccolith abundance dramatically increased. Sucheras-Marx et al. (2015) proposed that this was caused by an increase in nutrient levels, as this period is marked by the collapse of carbonate production and the onset of radiolarite sedimentation in the western Tethys. The synchronous appearance of Dissiliodinium supports this as the size and high abundance of this genus strongly suggests that it formed blooms in response to eutrophic conditions. Moreover, a positive carbon isotope excursion at this time indicates enhanced productivity. Geochemical and mineralogical data suggest this phase of eutrophication was caused by a period of global warming, leading to enhanced weathering (and nutrient flux) in the mid-latitudes. Dissiliodinium declined in abundance as cyst-forming dinoflagellates radiated rapidly through the middle—Late Bajocian. This transition from low diversity/high abundance to high diversity/low abundance was synchronous with the appearance of planktonic foraminifera and a major turnover within Tethyan ammonites. These changes occurred during a time of climatic cooling which may have led to a reduction in weathering rates and thus nutrient flux. However, radiolarite sedimentation continued in the western Tethys throughout this interval suggesting that high nutrient levels persisted, at least within Europe. The synchronous diversification of dinoflagellates, appearance of planktonic foraminifera and ammonite turnover indicates a coupling between phytoplankton, zooplankton and metazoan evolution at this time, the nature of which remains unclear.