Morphological Evolution of Coccolithophores Across the Late Eocene to Early Oligocene

The Eocene–Oligocene Transition (EOT) represents one of the most significant intervals of rapid and long-term global change in the Cenozoic, marked by the onset of Antarctic glaciation, declining atmospheric CO₂, and major reorganizations of ocean circulations. The coccolithophore genus Reticulofenestrais key to understanding late Cenozoic climate change, as it is the most wide-spread lineage（Emiliania and Gephyrocapsa） in the modern ocean. Here, we combine morphometric, evolutionary, and high-resolution analyses of coccoliths from the late middle Eocene to early Oligocene, using marine sediments from the South Atlantic Ocean and mid–low latitude oceans. Morphometric and mixture analysis based on distal shield length, circularity, and central opening ratio enables a systematic comparison between taxonomic morphotypes and statistically defined components. Length and circularity effectively distinguish groups, whereas central opening ratio shows high intra-group variability. A total of nine morphotypes were identified from 6700 data points, providing a consistent basis for tracking morphological change through time. The integrated dataset reveals three evolutionary phases: initial diversification during late Eocene cooling, extinction following peak diversity after the Eocene–Oligocene Transition, and early Oligocene stabilization reflecting adaptation in smaller species. This long-term trajectory is accompanied by increasing size and decreasing abundance of large species during the late Eocene, followed by their disappearance and reduced size variability in small- to medium-sized coccoliths. This transition culminates in the first size reduction (extinction of Reticulofenestra umbilicus) in the Reticulofenestra–Gephyrocapsa–Emiliania lineage (~32 Ma), which reflects a reorganization of coccolithophore communities rather than a simple size decrease. While the long-term reduction in size has been linked to declining atmospheric pCO₂, the combined morphological evidence indicates that the size reduction resulted from the expansion of eutrophic environments following the establishment of overturning circulation and Antarctic ice buildup. In this context, small- to medium-sized coccolithophores with smaller central openings are associated with nutrient-rich waters, whereas larger species with larger openings are linked to oligotrophic conditions. Superimposed on these gradual changes, high-resolution (~6 kyr) data show that size variations ofReticulofenestra, Dictyococcites, and Coccolithus are significantly correlated, indicating homogeneous responses to environmental changes. Spectral analysis reveals obliquity (~40 kyr) and precession (~23 kyr and ~18 kyr) cycles, suggesting that short-term ecological fluctuations periodically altered nutrient conditions in the upper ocean. These orbitally paced variations indicate that coccolith morphology responded sensitively to changes in ocean circulation intensity, ice volume, and upper ocean mixing. By combining quantitative taxonomy, evolutionary trends, and orbital-scale variability, this study shows that coccolith size and morphology record a continuous response to both gradual climate forcing and rapid environmental changes across the Eocene–Oligocene interval.