Oct. 24-26, 2009, Yingjie Overseas Exchange Center, Peking University, Beijing, China中文版

The great Ordovician biodiversification of South China: A synopsis

Renbin Zhan1 and Jianbo Liu2

1 State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China (rbzhan@nigpas.ac.cn)
2 Department of Geology, Peking University, Beijing 100871, China (jbliu@pku.edu.cn)

Abstract

About 10 years’ intensive case studies in South China enable us to open a “window” to view the great Ordovician biodiversification in great detail. There are several advantages of conducting such investigations in South China, including: 1) the Ordovician here is completely developed and well exposed at many localities; 2) various kinds of lithofacies and biofacies are differentiated from the Yangtze Platform through the Jiangnan Slope to the Zhujiang Basin; 3) fossils of different ecotypes (such as planktonic graptolites, vagile trilobites, nektonic nautiloids and benthic brachiopods) are preserved together or sandwiched at most sections on the Yangtze Platform, providing a solid basis for chronostratigraphical correlation and macroevolutionary analysis; 4) many complete sections have a long history of stratigraphical and paleontological research, particularly on those major Ordovician fossil groups and their biostratigraphy. Based on tens of thousands newly collected fossil specimens, preliminary systematic revision has been finished for the major fossil groups including brachiopods, graptolites, trilobites, acritarchs and bivalves. Hence, a high resolution chronostratigraphical framework has been established in South China with different biozonations for the Platform, Slope and Basin areas, and the precise correlation on the regional and global scales has also been suggested. For the first time, the great Ordovician biodiversification could be discussed against biozones, much more precise than ever before.

Taxonomically, the brachiopod commenced it Ordovician radiation at the beginning of this Period, started to accelerate in the earliest Floian (Tetragraptus approximates Biozone, Early Ordovician), and got its first acme in the mid Floian (Didymograptellus eobifidus Biozone) which was 4-5 graptolitic biozones earlier than the global trend recognized by Webby (2000) through compiling the data of all major fossil groups (Undulograptus austrodentatus Biozone, early Darriwilian, late Mid Ordovician). The taxonomic diversity of trilobites keeps growing since the beginning of the Ordovician and got its first acme in the Nemagraptus gracilis Biozone (early Sandbian, Late Ordovician) in South China. The diversity change of graptolites shows different patterns on the Yangtze Platform and the Jiangnan Slope, where they were generally fewer components of the fauna on the platform and much more abundant on the slope. A small peak also occurs on the Yangtze Platform in the early to mid Floian, and a much bigger diversification happened on the Jiangnan Slope in the Undulograptus austrodentatus Biozone.

Paleoecologically, the Ordovician brachiopod radiation of South China first occurred in the normal marine environments corresponding to lower BA 2 to BA 3 benthic regimes, and then gradually expanded their ecospace both to more offshore deeper water and more nearshore shallower water benthic regimes. They got their widest distribution in the Expansograptus hirundo Biozone, three biozones later than the taxonomical diversification showing an apparent ecological lay-behind during the Ordovician radiation. Similar phenomenon was also recognized in Great Basin North America (Droser and Sheehan, 1997). Trilobite biofacies macroevolutionary trends of South China have been discussed in detail by Zhou Zhiyi and his colleagues in several relevant papers. They found that the Ordovician trilobite radiation could be attributed to their diversification in the offshore deep shelf environments in the early Late Ordovician. Cladistic analysis on the Ordovician graptolites of South China shows that the downslope of the Jiangnan Slope was the originating place of graptolite novelties, and then expanded to the shallower part of the slope and the vast area of the Yangtze Platform. So, none of the case studies conducted in South China cater for Jablonski et al. (1983) ’s “onshore origination and offshore expansion” hypothesis. Most recent investigations document some more cases that originate in deeper water environments and expand to the nearer shore shallower localities, such as the famous Ordovician brachiopod Yangtzeella, and the Nereidella-Protoskenidioides Association from the upper Jiangnan Slope (Sandu, southeastern Guizhou Province, SW China).

Faunal analysis indicates that the great Ordovician biodiversification of South China was also manifested by the faunal turnover in all major groups. It has been internationally recognized that the trilobite macroevolution in Ordovician is actually the Whiterock fauna (represent of the Paleozoic Evolutionary Fauna) gradually took over the Ibex fauna (relict of the Cambrian Evolutionary Fauna). The replacement had been completely finished by the end of Ordovician in South China. The Ordovician brachiopod radiation of South China was owing to the originating and flourishing of the Sinorthis fauna. It took over its predecessor, the Finkelnburgia fauna or the Tritoechia fauna, within a very short time rapidly, first in the middle shelf and then in much more offshore as well as nearer shore areas of the Yangtze Platform. The Sinorthis fauna was replaced by the Saucrorthis fauna, a typical regional brachiopod fauna in South China, when the second acme of the biodiversification occurred in the Darriwilian (late Mid Ordovician). The Ordovician graptolites could also be recognized as a series of macroevolutionary faunas, such as the isograptid, anisograptida, the dichograptid, and the diplograptid faunas. The isograptid fauna originated in the lower Jiangnan Slope, and gradually evolved and expanded to the widely flourishing diplograptid fauna (e.g. Undulograptus austrodentatus) in the entire South China.

All what were mentioned above are macroevolutionary phenomena we recognized in South China from our case studies implies some essence and regularities of the great Ordovician biodiversification. However, the potential causeal mechanisms and/or the environmental background of all these macroevolutionary processes are still controversial. More and more researchers now accept that there were multiple causative factors combining to induce the Ordovician biodiversification, e.g., intrinsic biological factors, global and/or regional extrinsic physical/chemical factors, as well as feedback of the biodiversification event on physical environments. The uniqueness of the Ordovician biodiversification of South China provides a compelling case for a regional factors (e.g., sea level fluctuation), rather than global factors, controlling the onset and development of the bioevent in South China. Furthermore, the sedimentary systems in South China have been changed markedly as a consequences of the first major phase of the Ordovician biodiversification. More detailed multidisciplinary approaches and international collaborations on this particular topic are on-going.

Renbin Zhan

State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China (rbzhan@nigpas.ac.cn)

Biography

Zhan Renbin, male, born in November 1965, research professor. Finished his undergraduate in the Geology Department of Nanjing University in 1986, he had got his M.Sc. and Ph.D. degrees at the Nanjing Institute of Geology and Palaeontology (NIGP), Chinese Academy of Sciences (CAS) in 1989 and 1994, respectively. His major research interest is on the Early Palaeozoic stratigraphy, brachiopods and those related areas. For about 20 years, he has published more than 80 peer-reviewed academic papers and seven monographs by himself or coauthored with his international colleagues. He was doing post-doctoral study in the Natural History Museum London from December 1996 to December 1997 on the Mid-Late Ordovician brachiopods of South China and Myanmar and their palaeobiogeographic implications. He was also invited to be a research professor at the Department of Earth Sciences of the University of Western Ontario Canada for four times (1999, 2004, 2006 and 2008, altogether about 28 months) collaborating with Profs. Jisuo Jin and Paul Copper on the Ordovician brachiopods of Manitoba and Anticosti Island. Having got his job at NIGP immediately after his M.Sc. study, he had got promoted several times as an associate research professor (1996), research professor (1998), and research professor of the Innovation Project of CAS (1999, 2001, 2005). He is currently the Head of the Palaeoinvertebrate Section at NIGP, the associate editor of Acta Palaeontologica Sinica, and a titular member of the International Subcommission on Silurian System. He was awarded as an “Outstanding Post-doctoral Researcher” by the People’s Government of Jiangsu Province in November 2005, and the “Outstanding Young Scientist Research Fund” by the National Natural Science Foundation of China in 2008