Darriwilian (Middle Ordovician) Trilobites Of The Upper Yangtze Region,South China

The Great Ordovician Biodiversification Event(GOBE)was represented by an important increase in diversity during the biological evolution.During this period,multiple radiation climaxes finally formed a highly complex marine ecosystem dominated by the Paleozoic Evolutionary Fauna.Darriwilian(Middle Ordovician)was a key interval for trilobite macroevolution.The global trilobite diversity reached the peak during this interval,the Ibex Fauna representing the Cambrian Evolutionary Fauna declined sharply in diversity,while the Whiterock Fauna representing the Paleozoic Evolutionary Fauna flourished rapidly and occupied the advantage in both richness and diversity.However,the specific manifestation and dynamics of trilobite macroevolution in South China and even the world are not clear,and there is a lack of case studies on the palaeoecology and biogeography of trilobite,so many works need to be strengthened and improved.Based on the previous works,eight Middle Ordovician Darriwilian sections in different palaeogeography have been selected for this study from the upper Yangtze region of South China.Altogether,2182 trilobite specimens have been collected for systematic,palaeoecological,biogeographical and macroevolution study under the high-resolution chronostratigraphic framework.Identified trilobites 66 species of 42 genera or subgenera,20 families and 6 orders,including 2 new genera and 7 new species.In terms of palaeoecology,trilobite ecological associations are recognized from eight sections of five regions in South China,and their vertical replacement reflects the trend of sea level change during the Darriwilian.Three macroevolution phases in the Middle-early Late Ordovician South China could be recognized,including the radiation phase,the decline phase and the re-radiation phase.The replacement of the Middle Ordovician ecological trilobite association in South China is mainly affected by the change of sea level and bottom environment.The distribution,dominant groups and diversity changes of the Whiterock and Ibex faunas are analyzed in the different space-time and environment,meanwhile,the pattern and environmental background of the second radiation of the Ordovician trilobites in South China are discussed.The global pattern of trilobite radiation during the Ordovician is analyzed systematically:the differences in the starting time,the starting environment,the dominant groups and the expansion direction of the Whiterock Fauna lead to variable radiation trajectories and the diversity change trend,however,sea level rise events,as the environmental background of the Whiterock Fauna radiation in most of the global plates and regions,may have triggered the Ordovician trilobite radiation directly.Based on the Network Analysis to trilobite data from the late Dapingian of the Middle Ordovician to the early Sandbian of the Late Ordovician,the trilobite biogeography could be divided into four regions:Low Latitude Region,Middle Latitude Region,West Gondwana Region and East Gondwana Region.