Terminal Ediacaran Tubular Fossil Biotas From Southern Shaanxi Province And The Three Gorges Area Of The Yangtze Platform And Their Biostratigraphy

Terminal Ediacaran fossil biotas are of pivotal paleobiological and paleoecological importance to help understand evolutionary history at the Precambrian–Cambrian transition.The scarcity of terminal Ediacaran fossils,however,profoundly place a hinder for our understanding of the evolutionary history at the Ediacaran–Cambrian boudary.This project is focused on the paleobiology and taphonomy of three fossil assemblages from the terminal Ediacaran Dengying Formation in southern Shaanxi Province and the Three Gorges areas,South China.Fossil specimens were recovered from two representative Ediacaran–Cambrian boundary sections(Shengchangba and Baimatuo).Furthermore,the stratigraphic and geochronologic framework of the Ediacaran System at the Zuojiawan section of Southern Shaanxi Province are studied.The key points of this dissertation are summarized as following:1.Ediacaran index fossil Cloudina is discovered for the first time from the Dengying Formation at the Baimatuo section,Three Gorges area,western Hubei Province,South China.This broads the global paleogeographic distribution of the Cloudina–Sinotubulites Assemblage and sheds news insights into the biostratigraphic correlation of the terminal Ediacaran successions.2.A new species of Sinotubulites(S.quadratus n.sp.)and a tubular fossil Bambootubus nodus gen.et sp.nov.are reported from the terminal Ediacaran Baimatuo Member of the Dengying Formation at the Baimatuo section,Three Gorges area of the Yangtze platform,South China.S.quadratus n.sp.is tetraradial in cross section and four longitudinal ridges occur in the exterior surface of the outer wall.The tetraradial symmetry seen in S.quadratus n.sp.differs from the triradial symmetry(S.triangularis),pentaradial symmetry(S.pentacarinalis)and hexaradial symmetry(S.hexagonus)of other known Sinotubulites species.Bambootubus nodus gen.et sp.nov.is a cylindrical tubular fossil with diagnostic evenly spaced transverse ridges on the outer surface of the tube.3.Ediacaran index fossils Cloudina and Sinotubulites are found from the Ediacaran Beiwan Member of the Dengying Formation at the Shengchangba section,southern Shaanxi Province.These findings broad the paleogeographic distribution of the Cloudina–Sinotubulites Assemblage in southern Shaanxi Province(previously only found at the Lijiagou section).4.New tubular fossil taxa(Unicyliatum shengchangbaensis gen.et sp.nov.and an unnamed tubular fossil)from the Ediacaran Beiwan Member of the Dengying Formation at the Shengchangba section,South Shaanxi Province,are documented in this study.U.shengchangbaensis gen.et sp.nov.is a cylindrical tube withboth ends open.Regular and dense transverse ridges develop on the outer surface of the tube,sharing similarities with coeval fossils(e.g.,Corumbella)in ornamental patterns.5.A number of conical tubular fossils,similar to some early Cambrian tubular fossils(e.g.Conotheca),are also found from the Ediacaran Beiwan Member of the Dengying Formation at the Shengchangba section,South Shaanxi Province.These fossils shed insights into the study of evolutionary continuity of the conical morphology and provide important clues for the study of the evolutionary relationship between the conical tube fossils at the Ediacaran–Cambrian transition.6.The secondarily silicified fossil assemblage at the Shengchangba section and the Baimatuo section were transported.The great majority of the fossils are fragmented and the rose diagram shows a weak preferred orientation of the long axis of tubular fossils.7.Stratigraphic framework of the Ediacaran Doushantuo Formation at the Zuojiawan section in southern Shaanxi is revised.Typical early Cambrian tubular fossils(e.g.,Conotheca,Siphogonuchites,and Cambrotubulus)are found in intervals which traditionally interpreted as the Doushantuo Formation.Combined with regional stratigraphic lithological correlation,the previously assigned "Doushantuo Formation" at the Zuojiawan section is moved to the basal Cambrian.8.Geochronological investigation suggests that the Doushantuo Formation in southern Shaanxi have been deposited between 633–551 Ma.The youngest zircon age obtained from the bottom of the Doushantuo Formation at Liping section is 632.9 ± 6.96 Ma,and the youngest zircon age obtained from the upper part of the Doushantuo Formationis 558.5 ±5.94 Ma.