Re-study Of The Recommended Stratotype Section Of The Terminal Stage Of The Ediacaran System In South China-The Gaojiashan Section

The Ediacaran period(635.2?538.8Ma)is a crucial period in the evolution of life.Undrestanding the complex interaction between geological and biological events in the period of geological history requires a sophisticated chronostratigraphic system.Therefore the internal division of the Ediacaran has become an important theoretical problem in stratigraphic division.And it is still an urgent problem to establish a global standard for high resolution division and correlation within the Ediacaran system.The presence of several successive Ediacaran fossil Lagerst?tten and important sections on the Yangtze platform has proved that South China is an ideal area for the study of the Ediacaran System.The latest chronological studies divided the Ediacaran System into2 series and 6 stages,but the criteria for the stage division are still in discussion.The mineralized skeletal fossil assemblage,represented by the Gaojiashan biota,is an important window for the study of the biodiversity of early life on Earth and a key link for the establishment of the transition from Ediacaran soft-bodied organism assemblage to small-shell fauna,as well as an important section for the further division of the Ediacaran.This section has been studied for more than 30 years and has yielded abundant fossil documents,which has been regarded as a favorable place for the construction of the stage.Aiming at the end of the order of the brand and the boundary position,choose Gaojiashan section as the research object,from a biological stratigraphy,chemical stratigraphy,paleontology,comprehensively discusses the Ediacaran system at the end of the order of the profile of the candidate profile with the superiority,microfossil groups and the study on system vase-shaped and globular microfossils.Among the alternatives for establishing the Terminal Stage of the Ediacaran,the the first appearance datum(FAD)of three index fossils named Shaanxilithes ningqiangensis,Conotubus hemiannulatus and Cloudina hartmanae is widely discussed.We here proposed the FAD of C.hartmanae as the basal boundary of the Terminal Stage owing to its wide distribution and easy identification.The position recommended in this paper is 42.6m from the bottom of the Gaojiashan section of the Dengying Formation,which is the FAD of C.hartmanae.The?¹³C of the Gaojiashan profile has a positive drift of up to 5.8‰at the FAD of C.hartmanae.This feature can be compared with multiple cotemporaneous sections of the Yangtze Platform and even the whole world.The Gaojiashan section in Ningqiang area of Southern Shaanxi has the conditions to establish the Stratatype of the Terminal Stage of the Ediacaran(TES).The section has complete exposure,moderate thickness,simple structure,continuous sedimentation and biostratigraphy,and abundant fossil types.Apart from the three types of fossils above,the presence of Gaojiashania,vase-shaped microfossils and spherical fossils leads the Gaojiashan section have great competitive advantages in continuous evolution from bottom to top.Additionally,the biostratigraphy and chemical stratigraphy of the Gaojiashan section have a good coupling relationship,which is the best candidate stratotype section for the Terminal Stage of the Ediacaran at present,and provides the most valuable comparison standard for the evaluation of the GSSP of the Terminal Stage of the Ediacaran.Vase-shaped Microfossils(VSMs),the oldest heterotrophic protozoa that has been extinct after the Cryogenian,are large in size(700?m to 2500?m)and bear oral pole with simgle form.They are limited to pre-Sturtian strara in Neoproterozoic marine rocks and extensively used in biostratigraphy correlation.Based on detailed investigation and comparison on more than 400 VSM specimens from the Gaojiashan biota,2 genera and 5 species are systematically described,including Protolagena limbata,P.papillata,P.gaojiashanensis,Sicylagena formosa and S.latistoma.Morphological evidences show that the present VSMs are quite distinguished from Tonian VSMs assemblages in the lacking of polygonal oral pole and ornamentation to the testate amoebae.Moreover,the results of scanning electron microscope,epoxy embedding section and EPMA show that VSMs in the Gaojiashan section are founded in siltstone and preserved through various ways,including calcification,silicification and pyritization.The composition of the test wall can be silica,iron oxide or carbonate.Although the secondary replacement can explain some functions of these walls,the VSMs may have primitive biomineralization.We conclude that the original wall is mainly organic,the subsequent decomposition of organic matter and template induction lead to the formation of mineralized tests.The VSMs of the Lijiagou section are found in clastic dolomite,preserved as three dimensional phosphatization,which represents a new window of taphonomy.On the basis of the size,morphology and test wall ultrastructure of VSMs,we interpret them as foraminifera.Thus they may represent the oldest foraminiferal fossil record in geological history and obviously novel supplement to the diversity of Neoproterozoic VSMs worldwide.More importantly,it provides the earliest morphological evidence for heterotrophic eukaryotes in marine ecosystems.On the basis of more than 100 spherical fossils,we here describe 2 new genera and 5 new species,including Pernulosphaera laevis,P.papillata,P.Tuberculata,Aggregatusbulbosus dicho and A.tridens.The result of micro-CT shows that the compartments and septal walls radiating from the inside to the outside in three dimensions.We interpreted them as double-layer cysts with vesicle structure of complex decoration and ontogeny.The new explanation of their functional morphology,together with the cell ultrastructure,indicates that they may represent an encystment stage of green algae.