The Great Ordovician Biodiversification in South China:A Synopsis

摘要

Based on intensive case studies conducted in South China over the past 10 years,the great Ordovician biodiversification event (GOBE),i.e.,the Ordovician radiation,for the first time,can be calibrated against graptolitic biozones,much more refined than ever before.Taxonomically,the Ordovician brachiopod radiation commenced at the beginning of the period,started to accelerate in the earliest Floian (Tetragraptus approximatus Biozone),and reached its first acme in the mid Floian (Didymograptellus eobifidus Biozone) which was 4-5 graptolitic biozones earlier than the global trend.The taxonomic diversity of trilobites dropped during the Tremadoc,and continued to rise from the beginning of Floian and reached its first acme in the Nemagraptus gracilis Biozone (early Sandbian,Late Ordovician) in South China.The diversity change in graptolites shows different patterns on the Yangtze Platform and the Jiangnan Slope.Paleoecologically,the Ordovician brachiopod radiation of South China first occurred in shallow marine environments (middle part of the Upper Yangtze Platform) corresponding to lower BA 2 to BA 3,and then gradually expanded their ecospace both to more offshore and nearer shore benthic regimes.Trilobite biofacies macroevolutionary trends in South China have been discussed in detail by Zhou Zhiyi and his colleagues in several papers.Cladistic analysis on three major Ordovician graptolite faunas of South China shows that the lower part of the Jiangnan Slope was the center of graptolite evolutionary novelties where the anisograptid fauna originated,and then expanded to the upper part of the slope (the dichograptid fauna) and across the vast area of the Yangtze Platform (the diplograptid fauna).Faunal analyses indicate that the great Ordovician biodiversification in South China was also characterized by faunal turnovers in all major groups.The potential mechanisms and/or the environmental background of all these macroevolution-ary processes are still controversial.More and more researchers now accept that there were multiple causative factors combining to generate 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 the physical environments.