The latest Aalenian–early Bajocian time interval (ca. 171-169 Ma) is marked by a global reorganization of oceanic plates with the Central Atlantic opening and the formation of the Pacific plate. This time interval is also marked by a global geochemical perturbation of δ13C with a negative excursion at the Aalenian/Bajocian boundary and a positive excursion during the early Bajocian. Evolutionary diversifications of marine invertebrate taxa, namely ammonites, radiolarians, and coccolithophorids, are recorded at that time. Concerning coccolithophorids, this interval witnesses the diversification and expansion of the most successful Mesozoic genus: Watznaueria. In this study, we explore the potential environmental, ecological, and biological forcing at the origin of Watznaueria diversification and its effect on the coccolith assemblages through quantification of the absolute and relative abundances of calcareous nannofossils in two Middle Jurassic key sections: Cabo Mondego (Portugal) and Chaudon-Norante (France). In both sections, we find an increase in nannofossil absolute abundance and flux at the beginning of the lower Bajocian, coeval with an increase in absolute and relative abundances of Watznaueria spp., followed by a plateau in the middle and upper part of the lower Bajocian. The increase of Watznaueria spp. is synchronous with a decrease in relative abundance of other major coccolith taxa, whereas the absolute abundance of these species did not decrease. During the climatically driven early Bajocian eutrophication event, Watznaueria spp. integrated into the calcareous nannoplankton community in two successive evolutionary steps involving first W. contracta and W. colaccicchii, and second W. britannica and W. aff. manivitiae. Step 1 was driven by an increase in niche carrying capacities linked to the early Bajocian eutrophication. Step 2 was driven by specific adaptation of the newly evolved Watznaueria species to bloom in nutrient-rich environments not exploited before. These evolutionary events have initiated the 100-Myr reign of Watznaueria over the calcareous nannoplankton community.

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