Yanjiahe Biota From The Early Cambrian Of Yichang, Hubei, China

Some fossil-collecting campaigns have led to finding an abundant variety of fossils from the Yanjiahe Formation of Lower Cambrian in Yichang,Hubei Province.Besides a number of small shelly fossils and Acritachis,abundant fossils including Cyanobacteria,Macroalgae, Protoconites and Yanjiahella(gen.nov.) have been discovered.So as to differentiate from the contemporary Meishucunian biotas of Early Cambrian,this fossil assemblage comprising distinct macroscopic animals and algae as well as abundant SSFs tentatively is termed herein as "Yanjiahe biota".For a long time,our previous understandings of the palaeobiology Meishucun Age of the early Cambrian were confined to a combination of SSFs and Acritarchs. The Yanjiahe biota first systematically outlines the palaeobiological community of shelf-sea carbonate platform facies or a local depression in the platform in the Meishucun Age.The macroscopic fossil assemblage provides significant fossil evidence on life evolution from the late Precambrian to "Cambrian explosion".1.LithostratigraphyTwo drilling cores and eight stratigraphical sections of Yanjiahe Formation,Lower Cambrian are described.Based on the features of development,distribution,petrology and lithofacies of strata,The Yanjiahe Formation is subdivided and correlated in details.2.The study of Yanjiahe BiotaFeatures,distribution and assemblage of fossils from many of the ten geological sections, including Cyanobacteria,Acritachis,SSFs,Globular fossils,Macroalgae,soft-bodied metazoans,have been comprehensively discussed.The phylogenetic relationship of Macrofossils are discussed.2.1 Through the study of the SSFs of Lower Assemblage of "Circotheca -Anabarites -Protohertzina",Upper Assemblage of "Lophotheca-Aldanella-Maidipingoconus" and SSFs preserved in nodules,the SSFs preserved in nodules are less abundant and diversified than those of Upper Assemblage,more abundant and diversified than those in Lower Assemblage, which may indicate their evolutionary interim from SSFs Zoneâ… to Zoneâ…¡.2.2 The macroscopic algae from the Yanjiahe biota,in terms of both preservation and morphology,possess considerable earmarks of the Miaohe biota.In addition,this biota also has potential affiliations with the Niutitang biota and Chengjiang biota.Conclusively,It will definitely provide fossil evidence for the evolution of macroscopic algae.2.3 The comparison study on size,morphology and life style in Protoconites with Cambrorhytium major and Cambrorhytium(or Archotuba conoidalis) shows right smart comparability,it possibly reflect the correlation of them in phylogenetic relationship.The discovery of Protoconites in Yanjiahe Biota provides important fossils evidence for study the position of cone-tubular fossils from Ediacaran to the middle Cambrian.2.4 The comparison study on soft-bodied macroscopic metazoans"Yanjiahella(gen. nov.)" with some kinds of Echinodermata shows some sort of similarity on morphology,it possibly reflect the correlation of them in phylogenetic relationship.It provides important fossils evidence for study of the radiation and evolution of early metazoans or Deuterostomes.3.Sedimentary Environments of Yanjiahe FormationBased on the features of development,distribution,petrology and lithofacies of strata, considering the limited distribution of the Yanjiahe Formation,we tend to propose the sedimentary facies of the research area to be a locally depressed shelf-sea carbonate platform or offshore basin during the Meishucunian Stage of the Early Cambrian.The lithofacies' changes in Yajiahe Formation make up of two sedimentary cycles.They reflect the seawater evolutional process:transgressive-regressive- transgressive-regressive.4.ChronostratigraphyZircons were obtained and analysed from the tuff layer of Gunziao section between Yanjiahe Formation and Shuijingtuo Formation,Yichang,Hubei Province.They yielded weighted mean ²⁰⁶pb/²³⁸U ages of 511.0±9.1Ma(MSWD=0.40).The Age determination provides an accurate chronostratigraphic evidence for the boundary between Yanjiahe Formation and Shuijingtuo Formation in Yichang.