(Figure 2) Distribution of planktonic foraminifera in the upper Paleocene of DSDP Hole 93-605

Good faunal preservation in the upper part of the Planorotatites pseudomenardii Zone at Deep Sea Drilling Project Site 605, northwestern Atlantic, allows a biometric analysis of the upper Paleocene planktonic foraminiferal species Planorotatites pseudomenardii (Belli), a keeled species that probably developed from a middle Paleocene unkeeled Planorotalites form. Multivariate analysis shows a consistent separation of all Planorotatites specimens into two groups, which are differentiated by the presence or absence of a complete keel; other variables are only of minor importance. The keeled group coincides with P. pseudomenardii. We recognize only one unkeeled species, Planorotalites chapmani (Parr), with Planorotalites ehrenbergi (Bolli), Planorotalites imitata (Subbotina), Planorotalites planoconica (Subbotina), Planorotalites troelseni (Loeblich and Tappan), and Planorotalites hausbergensis (Gohrbrandt) as junior synonyms. P. chapmani ranges from the middle Paleocene to at least the top of the upper Paleocene. The morphology of P. pseudomenardii does not change significantly, and although the frequency of Planorotalites is variable, the proportion of P. pseudomenardii to all Planorotalites varies only slightly around 65% in the upper two-thirds of its range at Site 605. However, in the top 1.5 m of its range the proportion of P. pseudomenardii decreases; in the same section, all Planorotalites specimens show a reduction in the size of their tests, suggesting that a temporary change in environmental conditions led to the exit of P. pseudomenardii\ in Magnetozone C24R at Site 605-apparently higher than expected from current standard zonations. Unkeeled Planorotalites, in contrast to R. pseudomenardii, persisted and regained normal size. The entry of P. pseudomenardii at Site 605 cannot be described in the same detail because of low frequencies of Planorotalites specimens and an erratic distribution of P. pseudomenardii in the lower part of its range. Many of the washed residues of the samples from these sediments are dominated by radiolarians, and the poorly preserved foraminiferal faunas may have abundant benthics, indicating carbonate dissolution. The initially low frequencies of P pseudomenardii relative to the unkeeled Planorotalites show a strong negative correlation with the total amount of radiolarians per sample and could be the result of preferential preservation, as well as of the same environmental conditions that caused the abundance of radiolarians.大西洋西北部深海钻探计划（Deep Sea Drilling Project, DSDP）605站位的假门德氏平旋虫（Planorotatites pseudomenardii）带上部地层保存了完好的动物群，使得我们能够对古新世晚期的浮游有孔虫（planktonic foraminifera）物种假门德氏平旋虫（Planorotatites pseudomenardii, Belli）开展生物统计分析（biometric analysis）。该物种为具龙骨类群，其演化可能源自古新世中期的无龙骨平旋虫（Planorotalites）形态类群。 多变量分析（multivariate analysis）显示，所有平旋虫（Planorotatites）标本均可被清晰划分为两组，二者的核心差异在于是否具有完整的龙骨结构，其余变量仅具有次要区分意义。具龙骨类群即对应假门德氏平旋虫。本次研究仅认定一种无龙骨类群：查普曼平旋虫（Planorotalites chapmani, Parr），而埃伦贝格平旋虫（Planorotalites ehrenbergi, Bolli）、拟态平旋虫（Planorotalites imitata, Subbotina）、平锥平旋虫（Planorotalites planoconica, Subbotina）、特勒森平旋虫（Planorotalites troelseni, Loeblich & Tappan）以及豪斯贝格平旋虫（Planorotalites hausbergensis, Gohrbrandt）均为其同物异名（junior synonyms）。查普曼平旋虫的地层分布时限为古新世中期，至少延伸至古新世晚期顶部。 假门德氏平旋虫的形态未发生显著变化；尽管平旋虫属（Planorotalites）的丰度存在波动，但在605站位该物种分布范围的上部三分之二层段中，假门德氏平旋虫占所有平旋虫的比例始终稳定在65%左右，仅存在小幅变化。但在其分布范围的顶部1.5米层段中，假门德氏平旋虫的占比出现下降；同一层段内所有平旋虫标本的壳体（test）尺寸均出现缩减，这表明环境条件的短期变化导致假门德氏平旋虫从605站位的磁极性带C24R（Magnetozone C24R）中消失——该层位的时限明显高于当前标准分带的预期值。与之相对，无龙骨类平旋虫并未消失，其壳体尺寸也恢复至正常水平。 由于605站位假门德氏平旋虫分布下限层段的平旋虫标本丰度较低，且该物种的分布存在不稳定性，因此我们无法对其首次出现的过程开展同等细致的研究。对这些沉积物样品的洗选残渣分析显示，多数残渣以放射虫（radiolarians）为主；有孔虫动物群保存状况较差，且可能富含底栖有孔虫（benthic foraminifera），这指示存在碳酸盐溶蚀（carbonate dissolution）现象。假门德氏平旋虫相较于无龙骨类平旋虫的初始低丰度，与每个样品中放射虫的总数量呈现显著负相关，这一现象可能是选择性保存作用导致的，同时也与造成放射虫大量富集的相同环境条件有关。