Quantifying intrageneric morphological variability and evolution in Coccolithus across the Early Eocene Climatic Optimum in the equatorial Atlantic (ODP Site 1258)

The genus Coccolithus includes some of the most widespread and abundant coccolithophore species during the early Eocene. While the early Eocene saw major evolutionary turnover within the calcareous nannoplankton, in particular during the Early Eocene Climatic Optimum (EECO; ~53–49 Ma), Coccolithus remained consistently common throughout the extremely warm EECO and the subsequent cooling interval. Notably, early Eocene Coccolithus exhibited substantially broader morphological variability than its modern representatives, spanning wide ranges in coccolith size and shape. This high intrageneric diversity may in part explain why this taxon remained ecologically prominent. Different Coccolithus species/morphotypes and their specific traits could reveal what selective pressures favoured this group across climatic extremes. Here, we quantify intrageneric morphological variability by combining species-level assemblage counts with coccolith biometry in 53 deep-sea sediment samples from ODP Site 1258 (Demerara Rise, equatorial Atlantic). This dataset provides an opportunity to better understand the adaptive flexibility and resilience of the Coccolithus lineage during the early Eocene. For example, preliminary data reveal that small species were more common (C. pauxillus and C. pelagicus <5 μm) during the EECO, while a shift towards larger species and morphotypes (e.g., C. formosus and C. pelagicus >5 μm) is observed afterwards. Supported by biometric analysis, these patterns indicate long-term community shifts in mean cell size and associated physiological strategies under prolonged greenhouse conditions.