Length analysis of 50 specimen from Eiffellithus coccolith from sediment core TDP-31

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.

热带西大西洋与印度洋土仑阶（Turonian）地层中，钙质超微化石（calcareous nannofossil）的艾菲尔石藻属（Eiffellithus）E. turriseiffelii-E. eximius支系演化历程显示，该支系的艾菲尔石藻类（eiffellithid）整体形态一致，但横棒相对纵轴的旋转方式存在形态转变：从“x”型（对应E. turriseiffelii）转变为“+”型（对应E. eximius）。在这些难以区分的艾菲尔石藻类组合中，横棒取向发生形态转变的过程中，组合内个体的大小与形状均无显著变化。横棒向轴向对齐的旋转会导致平均角度值出现两次偏移。第一次偏移发生前，艾菲尔石藻类组合的平均角度介于35°~45°之间，未出现横棒角度小于20°的标本。第一次偏移事件对应横棒角度介于10°~20°的标本首次出现，这标志着Verbeek定义的E. eximius（E. eximius s. Ver.）的首现层位（first appearance datum, FAD）。第二次偏移则表现为艾菲尔石藻属组合的平均角度值从35°~45°显著变为15°~22°，同时出现横棒角度小于10°的形态类型，即严格意义上的E. eximius（E. eximius s.s.，sensu stricto）。本次生物计量学研究结果表明，可通过两类可靠的形态类型提升生物地层分辨率：Verbeek定义的E. eximius的首现层位大致位于土仑阶中-上交界附近，而严格意义上的E. eximius的首现层位则靠近土仑阶下-中交界。