【作者】 袁爱华；

【导师】 冯庆来； Sylvie Crasquin-Soleau；

【作者基本信息】 中国地质大学 ， 古生物学与地层学， 2008， 博士

【摘要】 晚二叠世末期,发生了显生宙以来最大规模的生物灭绝事件,介形虫也同其他海洋生物一样遭受了重创。在晚二叠世地层广泛连续出露的华南,前人已对浅海相介形虫动物群在该时期的分类学、生物地层学和古环境分析等方面进行了详细的研究,积累了丰富的资料,也极大地增加了对华南晚二叠世介形虫的认识。但是,同时期深水相介形虫动物群的研究目前还未见有专门的研究报道,对于介形虫这一对其生存环境响应敏感的微体生物而言,其个体特征和生态组合都因环境不同而各异,因此深水相介形虫资料的缺乏,必将影响对介形虫动物群在晚二叠世末期这一重大地史转折期的特点的全面认识。本论文即着眼于华南晚二叠世末期深水相介形虫动物群,选取位于湘黔桂盆地(广西东攀剖面、广西柳桥剖面和贵州晒瓦剖面)和下扬子盆地(安徽巢湖剖面)的四个深水相剖面进行系统分类学、古生态学和灭绝模式的探讨,旨在通过本研究,丰富对晚二叠世介形虫的整体认知。这是对我国晚二叠世深水相介形虫的首次研究:同时,目前在全球范围内,二叠纪深水相介形虫的研究也仅见于印度尼西亚(帝汶岛)早二叠世和意大利(西西里岛)中二叠世—吴家坪阶地层,因此本研究也是世界上首次对长兴阶深水相介形虫动物群开展的研究,将填补该时段内深水相介形虫研究的空白。经系统分类学研究,共鉴定43属128种,其中包括2个新种(已发表)Bairdia dongpanensis和Spinomicrocheilinella anterocompressa,1个提名(未发表)新属Denticupachydomella n.gen.以及2个提名新种Pseudobythocypris guiqianensis n.sp.和Denticupachydomella spinosa n.sp.。总体上,所研究动物群以薄壳、小个体的介形虫为主,兼有壳体较厚、壳饰发育的分子(如部分土菱介科分子);壳体多为单瓣壳,且总体保存情况不好,相当一部分壳体因保存太差而无法鉴定。从分类单元来看,动物群由古足目介、速足目介和丽足目介分子共同组成,其中速足目介所占比例最高,呈现出以古生代分子为主同时含有少量中生代分子(如:Abrobairdia,Lobobairdia)的混合动物群面貌。动物群表现出明显的地方化,地方种所占比例达85.2%,仅有19个种曾见报道于欧洲、北美和东南亚的晚泥盆世—晚二叠世地层。通过介形虫科及超科所指示的生态意义,对介形虫动物群进行了初步的环境分析;进而,采用Lethiers和Raymond(1991)提出的三角比例模型对各剖面内部及剖面之间进行了古水深变化的推测、追踪和对比。该模型基于动物群中古冷水圈型介形虫和浅海介形虫分异度的相对比例来反映古水深。古冷水圈型介形虫,即之前所认为的图林根型分子(本论文对采用“古冷水圈”而不使用“图林根”一词的理由进行了阐述,并建议介形虫工作者为避免和地质年代阶名的混淆而不再使用“图林根”一词),以薄壳、光滑或带有精美壳饰和具1—4个刺的古代分子为典型代表,一般认为可指示稳定、低能的较深水环境,古冷水圈型分子所占比例越大,一般即认为水体深度越大。根据定义,本研究中38个种可以看作古冷水圈型介形虫,它们分别属于刺状Bairdiidae、Bythocytheridae、Tricorninidae、Berounellinidae、Rectonariidae、Pachydomellidae、Healdiidae、Quasillitidae、Polycopidae、Discoidella和两个速足目未定种。对广西东攀剖面14个介形虫产出相对丰富的亚层在三角比例模型图中的投点显示,东攀剖面在研究区段内发生了频繁的水深变化。贵州晒瓦、广西柳桥和安徽巢湖剖面因丰富层位太少而不足以对单个剖面进行古水深变化的追踪。但是剖面之间的对比清晰地显示出各剖面所反映的古环境之间的关系。在所研究剖面中,广西东攀剖面指示最深的水体环境,变化于外陆棚到半深海环境;晒瓦剖面次之,可能沉积于内陆棚到斜坡上部环境;巢湖剖面以光滑、薄壳、壳体狭长的土菱介分子占绝对优势,因此指示的可能是正常的开放海环境;从介形虫动物群组成来看,柳桥剖面应该是最浅水体环境下的沉积,该剖面产出部分厚壳和壳饰复杂的土菱介分子(部分为中生代先驱分子)和其他典型滨浅海型介形虫。通过介形虫动物群分析得出的上述结论得到了来自其他学科研究(放射虫研究、沉积学、矿物学和地球化学等)的证实。但是在与其他研究成果对比中,也对三角比例模型的应用提出了限定条件,该模型自身不能去除某些地区性的事件影响,比如在广西东攀剖面,浊流的注入有极大的可能带入浅海区的分子,导致浅海介形虫比例突然增加,深水动物群结构发生重组,从而影响对动物群原始面貌的真实反映。这也同时说明,在进行古环境恢复研究时,开展多学科研究的重要性。本研究首次在深水介形虫动物群中利用介形虫的给养方式进行了海水古含氧量水平的分析。该模式之前均应用于浅海底栖介形虫动物群,基本原理是含氧量水平和滤食类分子在动物群中所占比例相关。本研究选取介形虫产出较为丰富的层位进行分析,结果显示,总体上,所研究动物群产于常氧—富氧环境。对广西东攀剖面的19个(亚)层进行分析表明,其中仅有03DP4这一层位,含有62.5%的滤食类分子,指示贫/缺氧环境。该结论与由微量元素和有孔虫分析得出的结论相一致,从而初步证明利用该模式进行深水介形虫动物群重建含氧量水平的可行性。但是,应该指出的是,在广西东攀剖面,稀土元素Ce异常和遗迹化石所指示的贫/缺氧层位和上述三者并不吻合,因此,深水环境含氧量水平的重建目前仍相对困难,需要更多工作的验证。在介形虫灭绝部分,对混生、分异度和丰度变化、小型化等几个和灭绝相关的问题进行了讨论。本研究动物群以古生代常见分子占优势,同时含有一些中生代先驱分子,呈现出混生的特点,在这一方面与同时代浅海相介形虫动物群具有可比性。但是,深水相动物群的组成仍在很大程度上有别于浅海相,表现在其古生代分子中含有部分具有很长延限带的古冷水圈型分子,这些分子多报道于晚泥盆世—早石炭世地层。这种组成上的区别必然会导致不同水体条件下介形虫动物群灭绝模式的差异。动物群的分异度和丰度在各剖面没有明显的沿剖面从下向上逐步降低的趋势,但是在二叠系—三叠系界线层未发现介形虫个体,在二叠系—三叠系界线之下分异度和丰度发生突然降低(为零)。在所研究动物群中,未观察到个体大小随层位变化而变小即“小型化”现象,对19个常见种(曾报道于欧洲、北美和东南亚的晚泥盆世—晚二叠世)进行个体大小的统计,结果亦未显示小型化特征,甚至本研究中部分介形虫壳体要大于其更早的同类分子。有些壳体比其更早的部分同类分子个体偏小,本研究倾向于将之视为幼年个体,原因是部分种在不同时代和产地的更早同类分子个体大小各异,比如Bairdia ? sp.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,该层之上,仅有6个种存活。第二次表象灭绝发生在03DP10,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。更多还原

【Abstract】 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 for latest Permian deep water ostracods in taxonomy, paleoenvironment (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).更多还原