Geobiological Study On The Biota From The Chiungchussuan Shuijingtuo Formation (Cambrian Series 2),Western Hubei Province, South China

South China stands as one of the places around the globe that possesses the most well-preserved and complete Precambrian to Cambrian marine sedimentary sequences,recording all the relevant geological processes within that critical time interval.Several Ediacaran and Burgess Shale-type biotas have been discovered from the shallowand/or deep-water environments.Early Cambrian represents one of the most critical time intervals in early life evolution and that of oceanic chemistry.Most striking is the rapid appearance and subsequent radiation of metazoan,which set in place a metazoanpredominated marine ecosystem replacing the microbe-featured one in the Precambrian.Accompanying significant biotic events include increase in body size,skeletonization,complication of morphology and first appearance of a complex food-web structure that contained nearly all the modern marine faunas and that was quite similar with the modern one.Combined all together,these historic biotic events are what geologists called the “Cambrian explosion”.With ongoing efforts,the nature of the “Cambrian explosion” and the evolutionary processes of biota during the Cambrian period are getting close to be understood.Yet,details remain to be unraveled,among which are the originations of the originations of radiolarians and sponges.At the present,inconsistency occurs in the report of the earliest radiolarian fossils with the classic opinion insisting that the earliest radiolarian can only be traced back to the middle-late Cambrian while radical researchers believe that the ancestor of radiolarians could have appeared as early in the earliest Cambrian or even latest Precambrian.Despite difficulties due to the rare occurrence and poor preservation of fossils in the sedimentary sequences,more investigations into the fossil record from the Precambrian-early Cambrian sequences are necessary in order to put such origin issues to rest.As for now,most fossil studies of the Precambrian and Early Cambrian have mainly focused on the siliciclastic rocks in shallow-water environment and deep-water siliceous-muddy rocks while systematic reports from the carbonaceous shales in medium deep-water environments are rare.In this light,this study is designed to examine the fossil materials from the Lower Cambrian medium deep-water Shuijingtuo Formation to reveal the evolution of biotas in the relevant water depth during the Early Cambrian and,with the integration of geochemical data,its co-evolution with the contemporaneous oceanic environmental conditions.By the compilation of study materials from 3 medium deep-water continuous and well preserved sections(Luojiacun,Jiuqunao and Gunzi’ao sections)in Zigui area,this study aims to conduct systematic analyses on biotic taxonomy and to investigate into the early evolution of radiolarians.Based on the rich fossil materials,this study also performed quantitative paleo-community structure analysis,described the composition as well as distribution patterns of the medium deep-water planktonic organisms and discussed the planktonic ecosystems.With that,this study provides new materials for studies of the Shuijingtuo biota and for further geobiological studies.Main conclusions are as follows:1.On the basis of the characterization and taxonomic study of the radiolarian fossils from the Chiungchussuan Shuijingtuo Formation in Zigui area,western Hubei,the following conclusions are reached:(1)Radiolarians from the Chiungchussuan Shuijingtuo Formation in Zigui area of western Hubei are relatively common,they are all spherical polycysticerca and can be classified into four types: Paraantygopora porosa gen.et sp.nov.,Braunosphaera sinensis gen.et sp.nov.,Tetrasphaera? sp.and spherical radiolarians.Paraantygopora porosa,Braunosphaera sinensis and spherical radiolarians can be assigned into Order Spumellarina.Order and family of Tetrasphaera? sp.is undetermined.(2)The shell structures of the radiolarian fauna from the Shuijingtuo Formation are complex,including both incompletely separated or completely separated concentric layer structures.Most of the specimens are with meshed shells formed by the fusion of bandage plates and the shell is also named as perforated plate structure.Shells bear raised rims of pores.Length of the raised rim varies.Adjacent shells are connected by the raised rims.These characteristics are similar to some Early Ordovician species.A few radiolarian specimens have latticed shells,which have been reported from the early Cambrian in South China.In structure,their outer shell is similar to some radiolarian fossils from the Late Paleozoic and Mesozoic,but can be easily distinguished from other Early Paleozoic species.(3)So far,the radiolarian fauna from the Shuijingtuo Formation contains six taxa in genus,including two genera reported previously,suggesting relatively diversification of radiolarian fauna during the early Cambrian,which may be synchronous with the macrobiota explosion and prosperity in shallow water strata.2.Quantitative paleo-community analysis based on the rich fossil materials from the Luojiacun section has revealed 4 paleo-ecological associations upsection,namely the A,B,C,D associations.Biotic community and their association with environment conditions were discussed within each association,the co-evolution of which provides environmental constraints on the diversification and complication of biota during the Early Cambrian.Association A characterizes a planktonic ecosystem predominated by Isoxys,with a total of 3538 fossil specimens grouping into 10 species and featuring the highest fossil abundance and lowest diversity.Environmentally,anoxia seems to be the major control of association A with U/Th ratio showing anoxia in the bottom water.High Al values indicating abundant input of terrestrial materials and nutrients that stimulated primary productivity pose as another factor of the flourish and predominance of planktonic organisms,which comprise of 2760 individual fossil specimen and accounting for 78.01% of the total specimen recovered for this association.In comparison,benthic species dominate association B in both biotic diversity and fossil abundance,among which Bradoriid(27.43% in relative abundance,similarly hereinafter),Protoconodonts(10.55%)and spicule sponges(9.78%)are the top 3 abundant out of the 2475 fossil specimens in 36 species recovered.Association B occurs in marl of shallow water environment,which partly accounts for the increase of bio-diversity and appearance of shallow-water adapted benthic species.Geochemical data,e.g.,the U/Th ratio suggest low-oxygen conditions that clearly showed amelioration from association A.Both a change towards shallower waters and amelioration of redox condition accounted for the transformation from a planktondominated community to a bentho-dominated one.Upsection in association C,2541 specimens assigning to 34 species were recovered,of which spicule sponges takes the predominant place and accounts for 44.82% in abundance of the total.Follows next is planktonic algae which constitutes 26.99% in abundance.Aluminum abundance points to a steadily fluctuating influx of terrestrial materials that provided sufficient nutrients for the flourish of algae,which could also be the main control of the change from benthic community in association B towards a plankton-dominated community in association C.Last association D covers 505 fossil specimens distributed in 13 species,mostly planktonic algae,and features the least bio-diversity throughout the section.U/Th ratios indicating a change towards oxygenating conditions and relatively high Al content suggest an overall amelioration of environment with abundant and yet notoverwhelmed nutrient supply that favors the blooming of planktonic algae.In sum,by the integration with geochemical data and sedimentary features,the Shuijingtuo biotic associations are mainly controlled by redox conditions,terrestrial material and nutrient supply as well as water depth,which are also assumed to be of great significance for the biotic evolution in Early Cambrian deep-water basins.This study stands as a support for the importance of oxygen contents to the evolution of life in Early Cambrian.