Fossil Composition And Succession Of Cambrian Series 2 In Eastern Yunnan,and Its Global Correlation Across Continents

During the Cambrian explosion,a large number of exquisitely-preserved fossils provided an excellent window for exploring the early origin and radiation evolution of various metazoans.Discoveries of spectacular soft-bodied animal assemblages from Cambrian Konservat-Lagerst(?)tten around the world have provided incredible insights into the anatomy,behaviour,ecology and early evolution of complex metazoans.South China had a stable tectonic background and continuous sedimentary sequence during the Cambrian,and preserved a series of fossil assemblages.Early Cambrian Konservat-Lagerst(?)tten from South China,such as the Niutitang Fauna,the Chengjiang Biota,the Guanshan Biota,the Kaili Biota and the newly discovered Qingjiang Biota span a wide range of geological time and provide a unique opportunity to map changes in early Cambrian ecological communities over time.Especially,researches have made significant progress on the Cambrian Konservat-Lagerst(?)tten from eastern Yunnan recently.Australian sector was part of East Gondwana during the Cambrian,which well developed Cambrian succession with abundant fossils.The thick carbonate and siliciclastic rocks deposited in South Australia with a huge number of fossils provide excellent conditions for understanding the Cambrian radiation and global correlation in East Gondwana.Furthermore,South China and Australia both belong to Southern Cambrian trilobite biogeographic realm that yielding Redlichiina.Recent progress in biostratigraphy revealed that those two continents share the oldest known brachiopods,small shelly fossils and trilobites during early Cambrian,implied significant relevance between South China and Australia.This study is mainly focusing on Cambrian Series 2 and associated exceptionally preserved fossil assemblages within the deposits from Kunming-Wuding and Malong-Yiliang areas in eastern Yunnan,in addition to the Cambrian Heatherdale Shale Formation and its fossil assemblage from Stansbury Basin,South Australia(East Gondwana).Detailed palaeontological,lithological,sedimentologic and chemostratigraphic studies,integrated with methods such as thin sections and polished slabs,SEM,EDS,?-XRF,etc,providing a better understanding of the detailed process of the Cambrian explosion.1.The brachiopod Palaeobolus shell beds from the Hongjingshao Formation(Cambrian Series 2)is mainly monotaxic concentration which was produced by gregarious behaviour(biogenic)of Palaeobolus.Polished slabs and thin sections revealed that those shell beds were deposited in a quiet environment.The Palaeobolus shell beds play an important role in harding and paving the soft-substrate during the Cambrian explosion.This detailed study also enables an interpretation of impalpable depositional environment changes within similar lithofacies based both on taphonomic and sedimentologic characters of fossils.2.Six major animal groups of the Guanshan Biota were collected in Malong and Wuding areas including brachiopods,arthropods,priapulids,vertulicolians,anomalocaridids and hyoliths,in addition to abundant trace fossils.Brachiopods are both highly diverse and abundant in Malong and Wuding areas.Seven genera of brachiopods were collected in this study including Eoobolus,Neobolus,Westonia,Linnarssonia,Palaeobolus,Nisusia and Kutorgina.Arthropods are also diverse and include Redlichia,Palaeolenus,Isoxys,Tuzoia,etc.Brachiopods and arthropods form the two main components of the Guanshan Biota,while brachiopods vastly outnumber(up to 72.9% on individual abundance)than any other animal groups.Soft-bodied and soft-tissue preservation are common in the Guanshan Biota,such as imprints of the visceral area,the mantle canals including the proximal parts of the vascula lateria,the middle ridge and very rare setal fringes of brachiopods,as well as sclerites of palaeoscolecidans,vetulicolians with the anterior and the posterior body.Unusual digestive structure from Palaeolenus lantenoisi and Redlichia mansuyi are described for the first time,which consist of an expanded stomach region(a crop)under the glabella and paired digestive glands along the alimentary tract.This rare digestive structure has not been observed in trilobites this old.Those soft tissues are mainly preserved by pyritization which is similar to the Chengjiang Biota.3.Detailed studies of lithology and sedimentology revealed that both depositional environments of the Guanshan Biota in Malong and Wuding areas are shallow water(lower shoreface),between fair-weather wave base and storm wave base.There are four types of lithofacies of the Wulongqing Formation in Wuding(the Shijiangjun section),including low–medium bioturbated and interbedded mudstones–siltstones/sandstones,silty mudstones,low to highly bioturbated glauconitic sandstones,mudstones.The relatively shallow depositional environment of the Guanshan Biota is comparable with the Cambrian Emu Bay Shale from Australia and the Ordovician Fezouata Shale from Morocco.4.A detailed study of continuous lithostratigraphy,fossil assemblages,and depositional environments for the first time during Cambrian Epoch 2 in eastern Yunnan,revealing changes of ecological communities both temporally and spatially of the faunal assemblages within three stratigraphic intervals.The community compositions of the Chengjiang Biota and Malong Fauna in Kunming-Wuding and Malong-Yiliang areas are quite different,while the Guanshan Biota is similar on both sides of the Xiaojiang Fault.This spatial variation is probably affected by the lithology,depositional environment and taphonomic process.Generally,the Chengjiang,Malong and Guanshan assemblages share similar types of skeletonized taxa,however,the main trilobite and brachiopod assemblages from each of these deposits are unique.Faunal overturn between the Chengjiang,Malong and Guanshan faunas suggest that sessile benthic members of the assemblages were affected by the same factors that affected mobile trilobites.Furthermore,a detailed palaeontological and stratigraphical investigation found that the brachiopods vastly outnumber any other groups in the Guanshan Biota,and demonstrate that the community transitioned from arthropod(especially trilobites)-to brachiopod-dominated in abundance for the first time at the base of the Wulongqing Formation.Within the brachiopod communities,a lingulate-dominated assemblage transitioned to an acrotheloid-dominated assemblage with the new occurrence of calcareous kutorginides up-section.The detailed study and documentation of this transition provides a better understanding of the differences in faunal composition and overturn between the Malong Fauna and Guanshan Biota.5.A detailed sedimentologic,palaeontological and chemostratigraphic investigation of the upper Fork Tree Limestone to the lower Heatherdale Shale transition in Stansbury Basin,South Australia(East Gondwana)was conducted.Three lithofacies,including the monomict(diagenetic)limestone breccia,the interbedded lime mudstone with calcareous shale and calcareous muddy siltstone and the mudstone,reveals a deepening depositional environment from inner-mid to outer shelf up-section.The fossils from the lower Heatherdale Shale include bradoriids Sinskolutella cuspidata,the rostral spines of Isoxys sp.,indeterminate hyolith conchs,hyolithelminthid tubes,sponge spicules,an indeterminate phasoiid and other indeterminate small shelly fossils.Combined biostratigraphic and chemostratigraphic data suggest a Terreneuvian,Stages 2-3 age for this interval,correlated with the Micrina etheridgei Zone.This study improves regional and global chronostratigraphic correlation and better constrains the depositional setting of this important lower Cambrian package in the Stansbury Basin.Overall,the early Cambrian faunal assemblages and their temporal changes and spatial variation from two adjacent continents have been summarized here.This study highlights the key role of sedimentologic methods in understanding the co-evolution of organisms and environment during the Cambrian explosion,and provides an important referenced significance for the subsequent comprehensive comparison of the communities and depositional environments within and among different continents during the Cambrian explosion.