Raw-data to morphometric investigations about the Neogene planktonic foraminifer Globorotalia menardii and related forms from ODP Hole 154-925B (Céara Rise, western tropical Atlantic)

Evolutionary prospection is the study of morphological evolution and speciation in calcareous plankton from selected time-slices and key sites in the world oceans. In this context, the Neogene menardiform globorotalids serve as study objects for morphological speciation in planktic foraminifera. A downcore investigation of test morphology of the lineage of G. menardii-limbata-multicamerata during the past 8 million years was carried out in the western tropical Atlantic ODP Hole 925B. A total of 4669 specimens were measured and analyzed from 38 stratigraphic levels and compared to previous studies from DSDP Sites 502 and 503. Collection of digital images and morphometric measurements from digitized outlines were achieved using a microfossil orientation and imaging robot called AMOR and software, which was especially developed for this purpose. Most attention was given to the evolution of spiral height versus axial length of tests in keel view, but other parameters were investigated as well. The variability of morphological parameters in G. menardii, G. limbata, and G. multicamerata through time are visualized by volume density diagrams. At Hole 925B results show gradual test size increase in G. menardii until about 3.2 Ma. The combination of taxonomic determination in the light microscope with morphometric investigations shows strong morphological overlap and evolutionary continuity from ancestral to extant G. menardii (4–6 chambers in the final whorl) to the descendent but extinct G. limbata (seven chambers in the final whorl) and to G. multicamerata (>=8 chambers in the final whorl). In the morphospace defined by spiral height (dX) and axial length (dY) Globorotalia limbata and G. multicamerata strongly overlap with G. menardii. Distinction of G. limbata from G. menardii is only possible by slight differences in the number of chambers of the final whorl, nuances in spiral convexity, upper keel angles, radii of osculating circles, or by differences in reflectance of their tests. Globorotalia multicamerata can be distinguished from the other two forms by more than eight chambers in the final whorl. It appeared as two stratigraphically separate clusters during the Pliocene. Between 2.88 and 2.3 Ma G. menardii was severely restricted in size and abundance. Thereafter, it showed a rapid and prominent expansion of the upper test size extremes between 2.3 and 1.95 Ma persisting until present. […]

演化探查（Evolutionary prospection）是针对全球海洋选定时间切片与关键站位的钙质浮游生物开展形态演化与物种形成研究的学科方向。本研究以新近纪曼氏型抱球虫类（Neogene menardiform globorotalids）作为浮游有孔虫形态物种形成的研究对象。研究团队在西热带大西洋大洋钻探计划（Ocean Drilling Program, ODP）925B钻孔中，对过去800万年期间曼氏抱球虫-肢状抱球虫-多室抱球虫演化支系的壳体形态开展了岩芯剖面调查。本次研究共从38个地层层面采集并分析了4669枚标本，并与此前深海钻探计划（Deep Sea Drilling Project, DSDP）502、503站位的相关研究成果进行了对比。

研究通过名为AMOR的微化石定向成像机器人（AMOR）及其配套专用软件，完成了数字化图像采集与基于数字化轮廓的形态计量测量。研究重点关注龙骨面视图下壳体螺旋高度与轴向长度的演化规律，同时也对其他形态参数展开了探究。通过体积密度图可视化呈现了曼氏抱球虫、肢状抱球虫与多室抱球虫的形态参数随时间的变化特征。

925B钻孔的结果显示，曼氏抱球虫的壳体尺寸在约3.2 Ma前呈逐步增大趋势。结合光学显微镜下的分类鉴定与形态计量研究结果可知，从祖先类群至现生曼氏抱球虫（最后壳圈具4~6个壳室），再到衍生但已灭绝的肢状抱球虫（最后壳圈具7个壳室），最终到多室抱球虫（最后壳圈壳室数量≥8个），三者存在显著的形态重叠与演化连续性。

在以螺旋高度（dX）与轴向长度（dY）定义的形态空间中，肢状抱球虫与多室抱球虫与曼氏抱球虫存在高度形态重叠。仅通过最后壳圈壳室数量的细微差异、螺旋凸度的细微变化、上龙骨角度、密切圆半径，或是壳体反光率的差异，才能区分肢状抱球虫与曼氏抱球虫。多室抱球虫可通过最后壳圈具8个以上壳室与另外两类区分。在上新世期间，多室抱球虫以两个地层上独立的类群簇形式出现。

在2.88 Ma至2.3 Ma期间，曼氏抱球虫的体型与丰度均大幅受限。此后在2.3 Ma至1.95 Ma期间，其壳体最大尺寸上限呈现快速且显著的扩张，并持续至今。[…]