Width analysis of 50 specimen from Eiffellithus coccolith from ODP Core 207-1260A@en

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.西热带大西洋与印度洋土仑阶（Turonian）钙质超微化石（calcareous nannofossil）E. turriseiffelii-E. eximius支系的演化序列显示，该支系的艾费尔颗石类（eiffellithid）个体存在横棒相对纵轴旋转的形态变化：从“x”型（E. turriseiffelii）转变为“+”型（E. eximius）。在这些难以区分的艾费尔颗石类组合中，横棒取向发生形态转变的全过程中，个体的大小与形态均未出现显著变化。横棒向轴向对齐方向旋转，导致其平均角度值出现两次偏移。首次偏移发生前，该艾费尔颗石类组合的平均角度介于35°至45°之间，未出现横棒角度小于20°的个体。第一次偏移对应横棒角度介于10°至20°的标本首次出现，标志着韦贝克定义的E. eximius s. Ver.的首现面（first appearance datum, FAD）。第二次偏移则表现为艾费尔颗石属（Eiffellithus）组合的平均角度值从35°~45°显著转变为15°~22°，同时出现横棒角度小于10°的形态型，即狭义E. eximius（E. eximius s.s.）。本次生物计量学研究结果表明，可通过这两种形态型实现可靠区分，进而提升生物地层分辨率：韦贝克定义的E. eximius首现面位于土仑阶中-上分界附近，而狭义E. eximius首现面则位于土仑阶下-中分界附近。