【作者】 李丽霞；

【导师】 冯洪真；

【作者基本信息】 南京大学 ， 古生物学与地层学， 2013， 博士

【副题名】大坪期均分笔石动物群的系统分类、生物地层及多样性演变

【摘要】 奥陶纪生物大辐射是显生宙以来一次具有重大演化意义的海洋生物宏演化事件,也是古生代演化动物群宏演化进程中的首次大规模辐射事件。作为奥陶纪海洋中的优势门类之一,笔石在奥陶纪得到重大发展,不仅多样性急剧增加、演化迅速,其生态空间也得到极大地扩张。从全球角度来看,弗洛期—大坪期是均分笔石动物群最重要的辐射演化时期,大量均分笔石类群在这一时期起源并迅速分异演化,成为该时期生物类群的主体。在这一全球性辐射事件中,均分笔石动物群不仅是辐射的主力军,更直接关系到奥陶纪三大笔石动物群的演替,起着承前启后的作用。随着奥陶纪生物大辐射研究在全球范围内的广泛开展,全球奥陶纪笔石辐射演化的研究已取得一系列成果,揭示了奥陶纪生物辐射事件的基本特征,确立了其辐射演化型式、规模及起始时限,就辐射事件的环境背景和控制因素作了大量针对性研究。然而,目前的研究中仍存在一些问题亟待解决。首先,细致的实例研究较少,在进行多样性数据统计过程中,多以前人工作所积累数据为依据,对于基础资料可靠性的关注度有待提高；其次,虽然我国华南奥陶系已建立了相对完整的笔石带序列,但是早奥陶世末期至中奥陶世早期的笔石带普遍存在地层跨度大、带化石不具有全球可对比性等问题,大大增加了在笔石多样性对比过程中由于地层对比差异而产生的误差。再次,在进行多样性数据统计时所使用的时间单位精度偏低,笔石辐射演化过程中细节变化往往被掩盖。此外,研究程度的区域性差异导致江南区中部及西南部多样性数据相对较少,严重影响了江南区乃至整个华南笔石多样性数据采集的完整性。在华南,奥陶系发育良好,分布范围较广,地层序列完整、连续且出露较好；同时具备连续过渡的台地、斜坡、盆地三种沉积环境,其中的斜坡区是笔石辐射演化的关键区域,也是开展奥陶纪笔石多样性演变研究的理想地区。隶属江南区的湖南益阳南坝剖面弗洛阶和大坪阶非常发育,笔石产出丰富、分异度高,且大多因黄铁矿化而呈立体保存。更为重要的是,该剖面从下奥陶统特马道克阶至中奥陶统大坪阶的含笔石地层出露连续、完整,是进行早—中奥陶世笔石系统古生物、生物地层学研究的理想剖面,同时也是奥陶纪笔石辐射演化研究的关键地区。针对上述奥陶纪笔石辐射演化研究中存在的问题,本文系统研究了南坝剖面的弗洛期—大坪期均分笔石动物群,并对其多样性演变进行了实例分析。论文鉴定描述了弗洛期—大坪期笔石26属81种,其中反称笔石3属3种,均分笔石23属78种,并对均分笔石动物群中具有生物地层意义的各属种进行了系统描述和详细讨论。以此为基础,同时参考其他地区相关属种的参数,以始端发育特征为主要分类依据,对广义的对笔石属(Didymograptus M’Coy,1851)进行了厘定。认为Acrograptus、Baltograptus、Corymbograptus、Didymograptellus及Expansograptus均应独立成属,不再隶属于对笔石属(Didymograptus M’Coy)的范畴。除去上述各属之外的狭义对笔石(Didymograptus s. str.)也可分为4种不同的类型,其中始端发育型式为artus型,第一个胞管(th11)从亚胎管中部生出、下斜或下曲伸展的类型在本研究中被定名为Norvegiograptus gen. nov.。同时,在巅峰笔石属(Corymbograptus)中建立新亚种Corymbograptus v-deflexus cruciatus subsp. nov.。基于细致的笔石系统古生物研究,在南坝剖面弗洛期—大坪期地层中自下而上确立了7个笔石带,即Tetragraptus approximatus带、Pendeograptus fruticosus带、Didymograptellus bijidus带、Corymbograptus deflexus带、Pseudophyllograptus angustifolius elongatus带、Isograptus victoriae lunatus-Isograptus victoriae victoriae?带及Isograptus victoriae divergens带。根据上述笔石带的划分,在研究区建立了较为完整、连续、分辨率较高的弗洛期—大坪期笔石带序列。综合国内外同期笔石地层资料,对该时期笔石地层进行了国际对比。此外,在弗洛阶顶部识别出Pseudophyllograptus angustifolius elongatus带,将其与上覆Isograptus victoriae lunatus-Isograptus victoriae victoriae?带的界线作为弗洛阶的顶界。通过对南坝剖面弗洛期—大坪期笔石动物群简单分异度、新生数、消失数、新生率、消失率的分析,识别出该时期笔石多样性演变的两个阶段,分别是弗洛早、中期(Tetragraptus approximatus带-Corymbograptus deflexus带)和弗洛末期—大坪期(Pseudophyllograptus angustifolius elongatus带-Isograptus victoriae divergens带)。总体上看,弗洛期—大坪期笔石总体分异度呈增长趋势,均分笔石动物群占绝对优势,但是大坪末期,动物群内部组成发生了重要变化,均分笔石类(Dichograptids)向舌笔石类(Glossograptids)逐步演替。分别以笔石带和化石产出层位为单位进行多样性统计,二者统计结果基本吻合,但以化石层位为单位的统计结果更多地揭示了多样性演变的细节。无论属级或种级多样性曲线均表现为两个峰值(Corymbograptus deflexus带和Isograptus victoriae divergens带)和一个低谷(Pseudophyllograptus angustifolius elongatus带),然而种级多样性变化强度明显大于属级。与华南及其他板块主要地区的对比研究结果表明,南坝剖面笔石多样性在弗洛期—大坪期的首次峰值(Corymbograptus deflexus带)在其他主要地区均有表现,南坝剖面的峰值与华南江南区、阿凡隆尼亚板块笔石辐射的首次峰值大致同期,但晚于华南扬子区、澳大利亚以及波罗的等地区。与华南奥陶纪各主要门类的多样性演化趋势的对比发现,南坝剖面笔石动物群多样性的首次峰值较腕足、牙形类的首次峰值晚,但早于疑源类及三叶虫的首次峰值。研究表明,南坝剖面弗洛期—大坪期均分笔石动物群的多样性演变不仅受区域海平面变化的影响,同时也受控于内部各类群间的演替。更多还原

【Abstract】 Ordovician Radiation is one of the significant macroevolutionary events for marine system during the Phanerozoic era, also regarded as the first massive diversification event of the Paleozoic evolutionary fauna. During the Ordovician, graptolites attained an important development, characterized by sharp increase of diversity, high evolutionary rate and dramatic expansion of ecospace. On a global scale, Floian-Dapingian witnessed the most magnificent diversification of the Dichograptid fauna. A great number of taxa belonging to the Dichograptid fauna originated and subsequently diversified in this period. The Dichograptid fauna played the leading role in this global diversification event, directly affecting the macroevolution of three Ordovician faunas, i.e. Anisograptid fauna, Dichograptid fauna, and Diplograptid fauna.Many studies have been conducted on Ordovician graptolite radiation. However, there are still many unresolved issues. First, few detailed case studies have been documented. Previous studies are normally based on the data published by different authors, of which the reliability may be ignored. Second, the consecutive graptolite sequence has been established in South China, but many graptolite biozones in late Floian to early Dapingian exhibit low-resolution and cannot be correlative globally, resulting in difficulty of comparing the biodiversity patterns in different regions. Third, details of graptolite biodiversification are often covered due to the low-resolution time scale. Lastly, relatively few studies from the central and western Jiangnan Region have been carried out, leading to the incompleteness of the graptolite biodiversity data in the Jiangnan Region (or even South China). In South China, the Ordovician successions are well-developed and distributed widespreadly; graptolite sequences are complete and well-exposed. South China comprises three continuously distributed regions of typical environments in the Ordovician:Yangtze Region (Platform), Jiangnan Region (Slope) and Dongnan Region (Basin). The slope, a crucial environment for the graptolite radiation, is the ideal area to conduct the case studies about Ordovician Radiation in South China. The Nanba section, located near the Nanba Village, Yiyang, Hunan Province, is interpreted palaeogeographically to be part of the Jiangnan Region and characterized by the graptolite shale facies, yielding abundant, diversified and pyritized graptolites. More importantly, the section is consecutive with the Lower-Middle Ordovician (Tremadocian-Dapingian) graptolite strata, thus being an ideal section for studies of Early-Middle Ordovician graptolite taxonomy, biostratigraphy and biodiversification. The present study systematically analyzes the Floian-Dapingian Dichograptid fauna in the Nanba Section, and provides a case for elucidating the graptolite biodiversity pattern.Eighty one species belonging to twenty six genera are identified in this paper, including three genera and three species of the Anisograptid fauna, twenty three genera and seventy eight species of the Dichograptid fauna. Among the Dichograptid fauna, key groups in biostratigraphic divisions and correlations are described and discussed here. On this basis, together with data from other regions, Didymograptus M’Coy,1851is revised according to the features of the proximal development. It is considered that Acrograptus, Baltograptus, Corymbograptus, Didymograptellus and Expansograptus are completely independent from Didymograptus M’Coy. Beside the five genera, the rest of Didymograptus M’Coy are assigned to Didymograptus s. str., including four different types, one of which is named as Norvegiograptus gen. nov. herein. The new genus is a didymograptid with stipes declined or deflexed, characterized by proximal development of artus type with th11as the dicalycal theca, and th11budding from the middle of the metasicula. Furthermore, a new subspecies is established as Corymbograptus v-deflexus cruciatus subsp. nov.Seven graptolite biozones are identified in the Floian-Dapingian in ascending order: Tetragraptus approximatus biozone, Pendeograptus fruticosus biozone, Didymograptellus bifidus biozone, Corymbograptus deflexus biozone, Pseudophyllograptus angustifolius elongatus biozone, Isograptus victoriae lunatus-Isograptus victoriae victoriae? biozone and Isograptus victoriae divergens biozone. A complete and consecutive graptolite succession that can be globally correlated in high resolution is set up for the first time in the Floian-Dapingian in the Nanba Section. In addition, the graptolite biozone of the topmost Floian-Pseudophy/lograptus angustifolius elongatus biozone is identified beneath the base of the Dapingian in the Nanba section. This shows that near the boundary of the Floian and Dapingian, the graptolite succession is consecutive. The base of the Dapingian is confined to the base of Isograptus victor iae lunatus-Isograptus victoriae victoriae biozone.Based on the biodiversity analysis of graptolites in the Nanba Section, two phases of graptolite diversification are identified in the Floian-Dapingian. The first phase occurred in early-middle Floian(Tetragraptus approximatus biozone-Corymbograptus deflexus biozone); the second phase ranged from late Floian to Dapingian (Pseudophyllograptus angustifolius elongatus biozone-Isograptus victoriae divergens biozone). Overall, the graptolite diversity shows a spectacular, rapid and progressive increasing trend during the Floian-Dapingian, which is driven by the expansion of the Dichograptid fauna. However, in the late Dapingian, major changes occurred within the Dichograptid fauna, characterized by the significant appearance of Glossograptids and the replacement of Dichograptids by Glossograptids.To estimate the taxonomic diversity for each graptolite biozone and bed-by-bed respectively, the results indicate that the two diversity curves agree well with each other, whereas the latter depict diversity changes in more detail. Both generic and species diversities exhibit two peaks (Corymbograptus deflexus biozone and Isograptus victoriae divergens biozone) and one trough(Pseudophyllograptus angustifolius elongatus biozone), but the graptolite diversification is much stronger in the low-hierarchy graptolite groups.A comparison of Floian-Dapingian graptolite diversity between the Nanba Section and other main regions including South China, Australasia, Baltica and Avalonia is conducted in the dissertation. As the results show, the first peak of the Floian-Dapingian graptolite diversity in the Nanba Section is rather obvious in other regions, in accordance with that in the Jiangnan Region of South China and Avalonia, but later than that in the Yangtze Region of South China, Australasia and Baltica. Comparative study on graptolites in the Nanba Section and brachiopods, acritarchs, trilobites and conodonts in South China during the Floian-Dapingian shows that the first peak of graptolite diversity is later than that of brachiopods and conodonts, but earlier than that of acritarchs and trilobites. It is here indicated that the diversification of the Dichograptid fauna in the Nanba Section was not only affected by the local sea-level changes occurring in the Floian-Dapingian, but also the replacement between different groups within the Dichograptid fauna.更多还原