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. Ver. The second shift is a significant change in the mean angle values from 35-45o to 15-22o among the Eiffellithus assemblage and the appearance of morphotypes with crossbar angles of <10o recognized as E. eximius s.s. The results of this biometric study indicate that two forms can be reliably recognized and used to increase the biostratigraphic resolution, with the FAD of E. eximius s. Verbeek near the middle to upper Turonian boundary and E. eximius s.s. near the lower to middle Turonian boundary.