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«To cite this version: Aihua Yuan. Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China. Pa- leontology. Universit´ Pierre et Marie ...»

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6 sensu Bless, 1987 在本研究中个体比更早同类分子偏小,但是报道于意大利中二叠世的个体大小介于印度尼 西亚早二叠世和本研究所得个体大小之间;又如 Bairdia altiarcus Chen 1958 在本研究中比产自 华南晚二叠世浅海相地层和意大利中二叠世地层中产出的个体小,但是晚二叠世浅海相地层的 个体要大于中二叠世同类分子。这种相同种在不同时代不同产地出现的个体大小相异的现象, 可能有助于认识该种的系统发育特征。另外,成年个体和幼年个体的共生进一步证明本研究动 物群为原地埋藏。 对介形虫灭绝模式的具体讨论主要在广西东攀和柳桥剖面展开。由于东攀剖面上覆于柳桥 剖面,因此在本部分讨论中将两个剖面连接为一个剖面(东攀&柳桥剖面)(注:中间有地层缺 失)进行讨论。通过对介形虫在东攀&柳桥剖面的属级(37 个属)和种级(82 个种)地层分布 图的分析,提出该动物群存在两次表象灭绝(apparent extinction)。第一次表象灭绝发生在东攀 &柳桥剖面 03DP5 顶到 03DP6 底, 仅有 6 个种存活。第二次表象灭绝发生在 03DP10, 该层之上, 该层为介形虫在东攀&柳桥剖面出现的最后层位,该层之上未发现介形虫。第一次(主要)表象 灭绝面和放射虫的主灭绝幕相对应,同时沉积学、矿物学和地球化学等分析显示该层发生明显 的海平面下降、强烈的火山活动和可能的贫/缺氧等地质事件,TOC 曲线亦在该层出现一个最大 幅度的正偏,所有分析都突出了该层是“事件层”的特征。 最后本文对广西东攀&柳桥、浙江煤山(GSSP)和巢湖剖面的灭绝规律进行了对比讨论。 煤山剖面介形虫动物群的灭绝主幕发生在 25 层底,明显滞后于东攀&柳桥剖面(据 Feng et al.

(2007a)和 Zhang et al.(2007a, b),东攀剖面 03DP6 底对应于煤山剖面 24e 底)。这种滞后 可能反映了介形虫动物群在不同水深条件下灭绝过程的差异。动物群在深水条件下首先灭绝, 可能指示深水环境首先受到灾变事件的影响,比如一些学者提出的深海中二叠世即开始的缺氧 事件到晚二叠世才影响到表层海水。另外值得注意的是,东攀&柳桥和煤山剖面的主要灭绝层都 位于火山成因粘土岩之下,这可能暗示不同水体介形虫动物群灭绝都和强烈的火山活动相关。 安徽巢湖剖面介形虫动物群在从第 5 层底往上分异度和丰度急剧下降,仅见个别介形虫壳体或 碎片。按照剖面对比,巢湖剖面第 5 层底对应于煤山剖面第 25 层底。如果将巢湖第 5 层底视为 介形虫在该剖面的主灭绝层,则同处于下扬子板块的巢湖和煤山剖面的介形虫动物群灭绝具有 同步性。从而说明,古环境(水深)并非决定灭绝过程的唯一要素,古地理位置也不容忽视。 总之,本论文首次对华南晚二叠世深水相介形虫动物群进行了研究,迈出了该时段深水相 介形虫研究的第一步,通过分类学、古环境分析和灭绝模式的讨论,初步系统化地勾勒出了华 南晚二叠世深水相介形虫动物群的整体面貌。当然,这只是晚二叠世深水相介形虫研究的起步, 要全面认识晚二叠世深水相介形虫,进而联系浅海相介形虫动物群,诠释整个介形虫动物群在 晚二叠世的面貌和对事件的响应,还需要未来更多更系统化的工作。 关键词:介形虫,晚二叠世,深水相,华南,古生态,古水深,灭绝 Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China

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As the other marine organisms, ostracods suffered drastic change during the end-Permian mass extinction, the largest event among the “Big Five” in the Phanerozoic history. In South China, Late Permian strata are well and widely exposed, which provides the great availability for related studies.

Previous studies on Late Permian shallow water ostracod faunas in South China have been evolved in the taxonomy, biostratigraphy and palaeoenvironment. These studies have greatly increased our knowledge on Late Permian shallow water ostracods. However, the absence of studies on contemporary deep water ostracods makes it difficult to comprehensively understanding the Late Permian ostracods during that significant geological time.

This dissertation will focus on the latest Permian deep water ostracod faunas in South China and aims to enrich our understanding on Late Permian ostracods. This is the first study on the Late Permian deep water ostracods in China and on the latest Permian deep water ostracods worldwide. For the time being, the only available data on Permian deep water ostracods were from the Early Permian of Indonesia and Middle Permian-Wuchiapingian of Italy. Thus this work will fill the gap in the history of lastest Permian deep water ostracod study.

In this dissertation, four sections from the deep water strata in South China (Hunan-Guizhou-Guangxi basin and Lower Yangtze basin) are studied in detail on ostracod taxonomy, paleoecology (paleobathymetry and oxygen level) and “extinction” process.

The ostracod taxonomy was carried out as the preliminary work. The diverse faunas are represented by 43 genera and 128 species. Two new species Bairdia dongpanensis and Spinomicrocheilinella anterocompressa were described. One new genus Denticupachydomella n.gen.





and two new species Pseudobythocypris guiqianensis n.sp. and Denticupachydomella spinosa n.sp. are proposed. In general, the ostracod faunas are dominated by small and thin-shelled individuals, although there are also some heavily shelled and strongly ornamented representatives. Many specimens are in very poor preservation and did not provide information for identifying. The recognized ostracods belong to Palaeocopida, Podocopida and Myodocopida. The typical Paleozoic species dominated the faunas accompanying with several Mesozoic forms (e.g. Abrobairdia, Lobobairdia). Compared with previous studies, 19 common species were reported from the Late Devonian-Late Permian strata of Europe, North America and Southeast Asia. Thus the studied faunas have a rather high endemic rate (85.2%).

The paleobathymetry is generally interpreted according to the families/superfamilies with the paleoecological significance. The triangular model, proposed by Lethiers & Raymond (1991), is adopted for precisely tracing the paleobathymetric variation along each studied section and between studied sections. In this model, the paleobathymetry is suggested by virtue of the relative proportion of paleopsychrospheric and neritic species. The paleopsychrospheric species mean the ostracods with the following characters: archaic, smooth/delicately ornamented, thin-shelled and/or having one to four spines. According to the definition, in the studied faunas, 38 species are regarded to be paleopsychrospheric. They belong to the spinose Bairdiidae, Bythocytheridae, Tricorninidae, Berounellinidae, Rectonariidae, Pachydomellidae, Healdiidae, Quasillitidae, Polycopidae, Discoidella and the two undetermined podocopid species. The beds/sub-beds yielding relatively abundant and diverse ostracods are analysed. The analyses based on 14 sub-beds in the Dongpan Section indicated the frequent variations of the paleobathymetry. The few/barren beds in the Shaiwa, Liuqiao and Chaohu Sections were insufficient to trace the paleobathymetric variations along the section. But the comparison between sections displays, among the studied faunas, the Dongpan fauna was yielded in the deepest habitats from the outer shelf to bathyal environments, the Shaiwa fauna was in the next place and indicated a inner shelf to upper slope environment, then the Chaohu fauna dominated by thin-shelled and elongated bairdiids may represent the open-marine environments, and the last, the Liuqiao fauna indicated the shallowest normal marine environments by the presence of heavily shelled and strongly ornamented bairdiids and other typical neritic species. The paleobathymetric interpretation based on ostracods was well supported by other evidences (radiolarians, sedimentology, mineralogy and geochemistry). The comparison also implied the necessity of integration with other evidence when the triangular model is applied. Some local geoevents (e.g. turbid current) may influence the original fauna and thus distort the meaning of the assemblage.

For the oxygen level reconstruction, the FF% (percentage of the filter-feeders) model is attempted here for the first time in the deep-water fauna. This model is based on the alimentation mode of the benthic ostracods. According to this model, the oxygen level is associated with the percentage of the filter-feeders. In this study, the analyses are carried out for beds/sub-beds with relatively abundant and diverse individuals. The general oxic conditions can be inferred from the analyses. In the Dongpan Section, 19 bed/sub-beds were analysed the proportion of filter-feeders. The bed 03DP4, yielding 62.5% filter-feeders, was the unique dysoxic horizon according to the relationship between FF% and oxygen level proposed by Lethiers & Whatley (1994). This interpretation appeared accordant with the results by trace elements and foraminifera. Thus the application of FF% model seems reliable. But it should be mentioned that the oxygen level reconstruction in deep water environments seems still very difficult, because not all evidence from different methods support the same horizons. More work is needed in oxygen level reconstructing.

Before discussing the concrete “extinction” process, some hotspots related to the extinction event were evolved in this dissertation. As the contemporary shallow water ostracod faunas, the “mixed” phenomenon was also reflected by the latest Permian deep water ostracod faunas, which were dominated by typical Paleozoic species accompanying with Mesozoic forms. But the deep water faunas differed from their shallow contemporary by including the long-ranging paleopsychrospheric species, reported from the Late Devonian-Early Carboniferous strata. The difference in composition may result in various “extinction” process. The change of diversity and abundance in studied sections did not show continuous decline along the studied interval from the bottom up. But below the Permian-Triassic boundary, the diversity and abundance showed sudden decline because no ostracod was found from the topmost of Upper Permian and lowermost Triassic. The discussion on miniaturization was also carried out. In the studied faunas, the change in individual size was not observed along the section. The comparison of the 19 common species between the studied faunas and previous occurrences also did not display the miniaturization. Some individuals found in this study were even larger than their ancestors. Thus no general miniaturization occurred in the latest Permian deep water ostracods. The presence of different sizes of intraspecies individuals reflects the ontogenic lineage among different occurrences. The smaller individuals than the ancient ones are considered as the instars. The coexistence of instars and adults supports the preservation in situ of the studied faunas.

The concrete discussion on ostracod “extinction” was involved in the Dongpan and Liuqiao joint section due to the clear stratigraphical relationship between the two sections. According to the specific and generic distribution, two apparent extinction horizons were proposed for the Dongpan and Liuqiao faunas. The First/Major apparent extinction horizon was located at the top of 03DP5 to the bottom of 03DP6. Above this horizon only 6 ostracod species survived. The Second apparent extinction horizon was in 03DP10, above which all ostracod taxa disappeared in the Dongpan Section. The First apparent extinction horizon is corresponding to the first crisis of radiolarians, the regression, the strong volcanic activities and possible anoxia/dysoxia. And the TOC curve showed the largest positive excursion in this horizon. All studies highlighted this “event horizon”.

The “extinction” process was compared between the Dongpan and Meishan Sections. The delay of the “extinction” in the Meishan Section was revealed. In the Dongpan Section, the First/Major apparent extinction took place at the bottom of 03DP6, which is corresponding to the boundary of 24d and 24e in the Meishan Section. In the Meishan Section, the major ostracod “extinction” occurred at the bottom of the bed 25. The delay of “extinction” in the Meishan Section may indicate that the deep water area was earlier affected by the catastrophic events, such as the anoxia. Both the major “extinction” horizon in the Dongpan and Meishan Section was underlied the volcanic ash bed. This may indicate that the ostracod “extinction” in both sections were associated with the volcanic activities. The comparison between Dongpan, Meishan and Chaohu faunas displays the role of paleogeography in the “extinction”.

In the Chaohu Section, the bed CH5 is corresponding to the bed 25 of the Meishan Section (stratigraphical correlation based on the “sandwich” PTBST) and ostracods rapidly declined in diversity and abundance at the bottom of bed CH5. This synchronism between the Chaohu and Meishan indicated that the paleoenvironment was not the only definitive factor for the “extinction” process. The paleogeography also played an important role. The reason resulting in the similarities and differences by the paleogeography is still pendent. However, without doubt, the variety of “extinction” between the different paleoenvironments and paleogeographical localities, indicates the necessity of the research on deep water ostracods, comparing the shallow water contemporaries with relatively abundant data (although systematic collation and revision are necessary).

Key words: ostracoda, Late Permian, deep-water, South China, paleoecology, paleobathymetry, extinction

Contents

Preface·······································································································································································1



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