| The Ediacaran-Cambrian(E-C)transition was a critical stage for the evolution of early complex life on Earth,marked by the disappearance of ‘Ediacaran Biota’ and the subsequent ‘Cambrian Explosion’ of metazoan life.The traditional view holds that the E-C boundary marks an abrupt revolution in life,including the mass extinction of Ediacaran biota and a fast and explosive radiation of many animal clades in the early Cambrian,occupying empty ecospace after the end-Ediacaran extinction.However,recent studies have inferred that the E-C biological revolution was not just a simple or abrupt biotic replacement event but potentially involved more complex and more gradual evolutionary processes than previously recognized.Compared with those of shallow-water carbonate and siliciclastic successions,relatively few studies of fossils in deep-water cherty facies have been undertaken,especially in the E-C continuous sequences.In this study,detailed investigations were conducted on deep-water E-C successions at Longbizui and Chaluo sections of western Hunan,and also shallow-water E-C strata at Fengziqing and Xiaomi sections of northeastern Yunnan,South China.The numerous fossils recovered from the Liuchapo Formation at Longbizui were assigned to two assemblages,the Lower Liuchapo Assemblage(LLA)and the Upper Liuchapo Assemblage(ULA).The LLA is mainly composed of a series of soft-bodied fossils(4genera and 6 species),including Longbizuiella hunanensis gen.et sp.nov.,Nenoxites jishouensis sp.nov.,Nenoxites irregularis sp.nov.,Nenoxites sp.,Helanoichnus sp.,Shaanxilithes? sp.and tiny-sized spherical fossils.The previously reported Chinese“Horodyskia” fossils and “Palaeopascichnus” fossils in the E-C strata were revised,and respectively reassigned to the genera Longbizuiella and Nenoxites in this study.The ULA(7 genera and 8 species)consists of skeletal fossils dominated by protoconodonts,hyoliths and Cambroscleritida,abundant sponge spicules,Poratusiramus xiangxiensis gen.et sp.nov.,and other undetermined taxa.Biostratigraphic correlation support a latest Ediacaran age for the LLA,whereas the majority of the ULA belongs to early Cambrian—thus the change from the LLA to the ULA coincided with the E-C transition.Meanwhile,abundant fossils were also recovered from the Liuchapo Formation at Chaluo section,including skeletal fossils dominated by hyoliths,sponge spicules,Poratusiramus xiangxiensis,and other undetermined taxa.These fossils are similar with those of Longbizui in the general feature.Apart from deep-water sections,numerous fossils were also recovered from the Zhujiaqing Formation at Fengziqing and Xiaomi shallow-water sections.Abundant Small Shelly Fossils(SSFs)and sponge spicules were found in the Zhongyicun and Dahai members of Fengziqing section.These SSFs are mainly composed of 8 genera and 12 species,including Paragloborilus aff.subglobosus,Purella aff.antique,Purella sp.,Xianfengella sp.,Xianfengella? sp.,Turcutheca sp.,Conotheca subcurvata,Conotheca obesa,Conotheca sp.,Halkieria sp.,Olivooides sp.,and Archaeooides sp.According to their feature,the upper Zhongyicun Member can be correlated to the second shallow-water SSF Zone,namely Paragloborilus subglobosus-Purella squamulosa Assemblage Zone.Similar SSFs and sponge spicules were also recovered from the Zhujiaqing Formation at Xiaomi section,with few possible soft-bodied fossils of the Dengying Formation.The data of present study indicate that the biological revolution in deep-water facies is likely to have been synchronous with that of shallow-water environments both in timing and feature.In the shallow-water facies,the late Ediacaran organisms were in low abundance and diversity,whereas the early Cambrian biotas were more abundant and more complex.The shallow-water fauna,including both benthic and pelagic members,developed a biomineralization ability during the E-C transition.Thus,the calcareous and phosphatic skeletal organisms began to flourish in the shallow-water environments.Similarily,at Longbizui and Chaluo,fossil abundance and diversity decreased gradually during the latest Ediacaran and recovered stepwise after the E-C transition.The deep-water fauna also developed a biomineralization ability during the E-C transition,including both benthic and pelagic members.Siliceous skeleton-forming organisms,represented by the spicule-bearing sponges,colonized the deep seafloor in great abundance.The studied fossils herein include the main E-C fossils types in the deep-water strata with new genera and species,which is the representative of the deep-water fauna.Our results suggest that,during the E-C transition,a biological revolution may have occurred in deep-water settings synchronously with the well-studied faunal turnover in shallow-water facies.Fossil diversity and abundance decreased gradually in approaching the E-C transition,which was then followed by a gradual recovery.Both benthic and planktic organisms developed the ability of biomineralization.Sponges are one of the most representative organisms in the early Cambrian,and also one of the earliest-branching metazoan taxa.However,the timing of origin of sponges remains controversial.Based on the sponge spicules of Longbizui,it is indicated that sponge spiclues evolved rapidly in complexity(from monaxon to diaxon-traxon and then to tetraxon and polyactinal forms),abundance and size during the first SSF zone of the early Cambrian.The similar sequence was observed in the first SSF Zone of Fengziqing section.In addition,the evolutionary sequence of sponge spicule types can potentially serve as an alternative biostratigraphic index when SSFs or other fossils are not well-preserved in deep-water facies of early Cambrian age. |
