Early Eocene evolutionary trajectories within the <i>Toweius</i> genus: insights from a newly identified species in the equatorial Atlantic

Abstract. The genus Toweius was a globally prominent 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 1452 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 2 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 in a sub(circular) Toweius ancestor.