Chiasmolithus species in middle Eocene and Oligocene sediments of ODP Holes 105-647A and 120-748B

In this preliminary biometric study of the calcareous nannofossil species Chiasmolithus expansus, Chiasmolithus oamaruensis, and Chiasmolithus altus from the upper middle Eocene to lower Oligocene of Sites 647 and 748, we document a complete gradation of forms among all three species. Chiasmolithus oamaruensis has significantly higher morphologic variance than the other species. The Chiasmolithus population at each site changes from C. expansus to C. oamaruensis and then to C. altus. This may not reflect a true evolutionary sequence because a major reversal in shape change of the central cross-bar structure accompanies this sequence, and because C. altus is morphologically closer to C. expansus than it is to C. oamaruensis. The change in the width of the cross-bar structure is primarily a result of changes in the alignment of the central connecting bar, rather than of changes in the cross-bar angle. At Site 748, two fluctuations in morphology produce sample populations intermediate between all three species. In addition, reported stratigraphic and paleogeographic occurrences of C. oamaruensis and C. altus show different latitudinal distributions. These morphological and distributional patterns may be explained by a continuous morphologic gradient between C. oamaruensis and C. altus, with C. oamaruensis occurring more commonly in cool-water paleoenvironments, and C. altus occurring more commonly in cold-water paleoenvironments. Thus, paleoenvironmental fluctuations at Site 748 may be the cause of the morphologic fluctuations in Chiasmolithus. This hypothesis can be tested against previously proposed evolutionary models by more detailed sampling of sections along a latitudinal transect.本研究针对采自647站位与748站位中始新统上部至渐新统下部地层的钙质超微化石（calcareous nannofossil）物种——扩张叉石藻（Chiasmolithus expansus）、奥马鲁叉石藻（Chiasmolithus oamaruensis）及高叉石藻（Chiasmolithus altus）开展了初步生物计量学研究，记录了三个物种间完整的形态渐变序列。相较于其余两个物种，奥马鲁叉石藻的形态变异度显著更高。各站位的叉石藻种群均呈现出由扩张叉石藻向奥马鲁叉石藻过渡，再向高叉石藻转变的序列。但该序列或并非真实的演化序列：一方面，该序列伴随中央横杆结构的形态变化出现显著反转；另一方面，高叉石藻在形态特征上与扩张叉石藻的相似度更高，而非奥马鲁叉石藻。横杆结构宽度的变化主要源于中央连接杆的排列偏移，而非横杆夹角的改变。在748站位，两次形态波动形成了介于三个物种之间的过渡样本种群。此外，已有文献报道的奥马鲁叉石藻与高叉石藻的地层及古地理产出记录显示，二者存在截然不同的纬度分布格局。上述形态与分布模式可通过奥马鲁叉石藻与高叉石藻之间存在的连续形态梯度加以解释：奥马鲁叉石藻更常见于凉水古环境，而高叉石藻则更多分布于冷水古环境。因此，748站位的古环境波动或为该区域叉石藻种群形态波动的成因。后续可通过沿纬向断面开展更精细的剖面采样，将该假说与此前提出的各类演化模型进行验证对比。