The coccolithophore Reticulofenestra is key to understanding late Cenozoic climate change, but taxonomic identification remains challenging. This study uses morphometric and mixture analysis to evaluate taxonomic criteria for late Eocene to early Oligocene Reticulofenestra. The method enables a systematic comparison between the taxonomic morphotypes and the components recognized through Gaussian mixture modeling of parameters of distal shield length, circularity, and central opening ratio. At the species level, a total of nine morphotypes were studied from 6700 data points collected across three study sites in the mid–low latitude Atlantic–Indian Oceans. The results demonstrate that length and circularity effectively distinguish groups, validating their uses as primary taxonomic criteria. Nevertheless, central opening ratio exhibits high intra–group variability, limiting its standalone application. In addition, our integrated data reveal three evolutionary phases: (1) initial diversification during late Eocene cooling; (2) extinction following peak diversity after the Eocene–Oligocene Transition; and (3) early Oligocene stabilization reflecting adaptation in smaller species. This work provides statistical references for the validation of semi-quantitative morphologic descriptions, which may help establish a robust framework for identifying this ecologically vital lineage and enhance our understanding of phytoplankton responses to past environmental changes. Future genomic studies will be crucial for elucidating genotype-phenotype-environment relationships in Reticulofenestra evolution.
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