Morphometrics of the calcareous nannofossil genus Eiffellithus during the Turonian

The evolution of the calcareous nannofossil E. turriseiffelii - E. eximius lineage in the Turonian from the western tropical Atlantic and Indian Ocean indicates identical eiffellithid forms with morphological change in the rotation of the crossbar, relative to the longitudinal axis, from an "x" (E. turriseiffelii) to a "+" (E. eximius). Within these indistinguishable eiffellithid assemblages, neither the size nor shape show significant change during the morphological transition of the crossbar orientation.

The rotation of the crossbar toward an axial alignment results in two shifts in the mean angle values. Prior to the first shift, the mean angles of the eiffellithid assemblage have values between 35o and 45o with no forms with crossbar angles of <20o. The first step occurs at the appearance of specimens with crossbar angles between 20o and 10o marking the first appearance datum (FAD) of E. eximius s.