Taxonomy, Biostratigraphy And Diversification Of The Dichograptid Fauna From Floian And Dapingian (Early-Middle Ordovician), Yiyang,Hunan

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.