Early Eocene Paleoclimate and Marine Phytoplankton Evolution

Paper I The early Eocene Climatic Optimum (EECO; ~53–49 Ma) was the warmest sustained global warming episode of the Cenozoic, accompanied by major alterations in terrestrial and marine biota. Here we detail changes in low-latitude calcareous nannofossil assemblages at two tropical sites in the equatorial Atlantic (ODP Site 1258) and Pacific (ODP Site 865), across the EECO and the subsequent early to middle Eocene cooling transition (EMET, ~49-47 Ma). Our results show that the evolutionary turnover between Toweius and Reticulofenestra is characterized by a two-step decline in Toweius, marking peak warmth during the EECO (52-50 Ma). Tropical nannofossil assemblages reveal a broad acme of Discoaster during the same time, as previously reported at several sites, but also uniquely show distinct abundance increases in Campylosphaera, Umbilicosphaera, and Calcidiscus. The decline of these taxa and the rapid expansion of Reticulofenestra characterized the conclusion of the EECO and the start of global temperature decline ~49 Ma. Multivariate statistical analysis of these and nine other nannofossil datasets from higher latitudes reveals that the occurrence and prevalence of specialist taxa constitutes a distinct tropical signature in addition to the previously identified latitudinal expansion of (sub)tropical taxa during the EECO. This pronounced compositional contrast was likely enabled by enhanced thermal stratification, sluggish ocean circulation, and intensified hydrological dynamics of the tropics during the EECO. Global cooling and more vigorous meridional overturning circulation after the EECO likely explain increased connectivity, biogeographical restructuring, and the establishment of the first Reticulofenestra-dominated coccolithophore communities. Paper II The genus Toweius was a globally distributed group of coccolithophores that thrived during the late Paleocene and early Eocene. Characterized by circular to elliptical coccoliths with three concentric tubes, Toweius is notable for its evolutionary linkage and abundance turnover with Reticulofenestra during the Early Eocene Climatic Optimum (EECO; 53­–49 Ma). This study focuses on a newly identified Toweius morphotype (herein labeled Toweius type II) from the tropics, which lacks the typical inner and middle tubes found in known Toweius species (Toweius type I). The unique morphology of Toweius type II raises important questions about its origin and evolutionary significance. Biometric analysis of 1,462 individual specimens, combined with light (LM) and scanning electron microscopy (SEM), reveals that coccolith size, central area-to-rim ratio, and shape serve as reliable diagnostic criteria for distinguishing between morphotypes. Our results show that Toweius type II predominantly comprises small (~3 µm) and thin (sub)circular coccoliths with a larger central area, lower mass estimate, and thus reduced calcification, compared to Toweius type I. These distinctive traits, particularly the light calcification and a wider central area, may reflect a genetically regulated physiological adaptation that enabled Toweius type II to persist under extreme EECO conditions for an additional two million years following the decline of Toweius type I. Furthermore, the co-occurrence of this morphotype alongside rare transitional forms before the EECO suggests that it may not merely be an ecophenotypic response to environmental stress. Instead, we propose that Toweius type II evolved gradually through phyletic speciation, involving the complete loss of the inner R-unit tubes within a sub(circular) Toweius ancestor.