Acidification and reduced carbonate saturation of the oceans are measureable responses to anthropogenic emissions of carbon dioxide into the atmosphere [1]. As major pelagic producers of CaCO3 in the modern ocean, the sensitivity of coccolithophores (single-celled phytoplankton) to changes in surface water chemistry is of particular relevance for ocean biogeochemical cycles and climate feedback systems (for example, ref. 2). Coccolithophores build exoskeletons from individual CaCO3 (calcite)

plates-coccoliths-that cover the cell surface and form a protective barrier (the coccosphere). Our current understanding of coccolithophore responses to ocean acidification (OA) is predominantly based on calcification rate experiments (coccolith calcite production per cell, per unit time), which indicate complex, often species-or strain-specific, impacts (for example, refs 3, 4, 5). There is also experimental evidence that coccolithophore species have the ability to evolve and adapt to the acidifiying carbonate chemistry conditions that are projected for the future, over the relatively short timescales of multiple generations (100-1,000 s of generations) [6]. The geological record of fossil coccolithophores is remarkably complete (stratigraphically and taxonomically) throughout the past 220 million years [7], and therefore provides a valuable means by which to test hypotheses of coccolithophore response both to long-term environmental change and abrupt climate perturbations. However, interrogation of the geological record for the specific impact of OA on coccolithophore calcification is challenging, because calcification rate cannot readily be determined in fossil populations, and therefore direct comparison with responses measured in modern culture and field experiments is inhibited.