Deciphering processes controlling mid-Jurassic coccolith turnover

• Significant Jurassic nannofossil assemblage shift indicated by Lotharingius and Watznaueria
• New Early–Middle Jurassic δ13C and δ18O data indicate environmental changes.
• Coccolithophore assemblage turnover occurred in 1.5 Myr around Aalenian/Bajocian boundary.
• Watznaueria with a cross proliferation potentially related to ocean opening?
• Watznaueria diversification linked to lower temperatures and enhanced oceanic fertility.

The Middle Jurassic is characterized by major changes within the fossil coccolithophorid community, with a transition from Lotharingius-dominated to Watznaueria-dominated assemblages, concomitant with a significant increase in the pelagic carbonate production. The mechanisms that triggered this overturn remain poorly understood. Here, we present a compilation (new and previously published data) of Lotharingius and Watznaueria abundances through the Early–Middle Jurassic transition. Alongside this, trends in newly-acquired and literature-derived carbon and oxygen isotope data were used to represent paleoceanographic indicators, such as nutrient and temperature changes. The nannofossil data show a rapid (around 1.5 Myr) turnover around the Aalenian–Bajocian transition. Across the Aalenian/Bajocian boundary, assemblages dominated by Lotharingius spp. give way to assemblages dominated by Watznaueria spp., coinciding with a peak in a particular morphological group of Watznaueria (species with a cross in the central area). The proliferation of this morphogroup occurred during a time of oceanic opening and rearrangement of ocean circulation. This led on to the evolution of pioneering coccolithophorid taxa, but also to extinctions in several marine groups. In the Early Bajocian, the proliferation of two other morphogroups (Watznaueria without a central-area structure and Watznaueria with a bar) corresponds to the major diversification of Watznaueria, and the beginning of its Mesozoic dominance. The Watznaueria diversification and dominance are associated with radiation in other marine groups, and these biotic changes occurred during a time of putative enhanced oceanic fertility and relatively low temperatures. This study suggests that restructuring of fossil coccolithophorid communities may be favored during short turnover intervals related to major paleoceanographic change.