Cell geometry and size strongly determine the physiology of unicellular algae, exerting specific control on isotopic and elemental discrimination during photosynthesis and biomineralisation. In recent years, coccolithophore-derived paleoceanographic proxies have contributed to a better understanding of past variability in sea surface temperature and growth rates of ancient haptophytes, as well as past levels of atmospheric carbon dioxide. Investigations into the mechanisms underlying these biogeochemical proxies underpin the importance of coccolithophorid cell size. Thus, accurate reconstructions of ancient cell size from the fossil record would improve the interpretations of such paleo-proxies. However, this approach is complicated by the fact that the fossil record of coccolithophores is dominated by single coccoliths, rather than by intact coccospheres, formed by interlocking coccoliths that surround living cells.
This paper presents quantitative constraints on coccolith size, coccosphere- and cell diameter, for three main Cenozoic genera of coccolithophore (Reticulofenestra, Cyclicargolithus and Coccolithus) through detailed biometry of rarely fossilized coccospheres. Together, these taxa are most dominant in Cenozoic deep-sea sediments, providing an exquisitely detailed record of their evolution and the bulk of coccolith-carbonate burial through time. Accurate estimates of coccolithophore cell size can be made by relatively simple size measurements of individual coccoliths in fossil assemblages. For the investigated taxa, the number of coccoliths per fossil coccosphere remained relatively constant throughout the Cenozoic, thus offering better control on estimates of cellular calcite production by the ancestors of today’s prolific bloom-forming coccolithophores Emiliania huxleyi, Gephyrocapsa spp. and Coccolithus pelagicus.