The Ediacaran Wenghui Biota In Northeast Guizhou, South China And The Environment Before Its "Explosive Prosperity"

The Ediacaran Period witnessed fundamental changes in paleoenvironment,paleoclimate and biotic innovation.It marks the transition between the Cryogenian snowball Earth and the Cambrian Explosion and is one of the most intensively studied intervals in the last two decades.Although the nature and classification of the multicellular organisms in the Ediacaran macrobiotas are still controversial,their diversity and wide distribution are commonly regarded as the prelude of the Cambrian Explosion.The Ediacaran Wenghui biota in northeastern Guizhou,South China,is dominated by macroalgae,with metazoans and ichnofossils.In this dissertation,after re-examination of the fossils and existing publications,14 new genera and 14 new species are identified or amended so that themacroscopic fossils in the Wenghui biota now include 34 genera and 36 species.In the Wenghui biota,the umbrella-shaped metazoan Eoandromeda may be a swimmer in the water column during their early stages of life,but the fully grown Eoandromeda may have mostly stayed on the surface of sediments.The disc-like rhizoid of Discusphyton was regarded as a holdfast organ to attach the macroagla on the water-rich muddy seafloor.The compression Globusphyton was likely a crept macroalga on the seafloor,with zigzag-shaped stolon and pompon-like thallus.The coiled ribbon-like Grypania may have been a benthic macroalga anchored or nestled into soft sediments by its innermost end.In addition,the macroalgal holdfast anchored or nestled into sediments commonly consists of two parts: the rhizome transformed by the base of the stipe and the rhizoid grown on the rhizome.The pith can be observed in the stipe and/or the rhizome.The macroalgal holdfasts in the Ediacaran biotas can be re-divided into 4 rhizome groups and 10 holdfast types.In the Precambrian records,it was a milestone in holdfast development that the segment of thallus had nestled into sediments.The deformed thallus substrate indicates that the macroalgal holdfast has been differentiated and the pith was the stipe,which extended into the rhizome and then sequentially differentiated in the different segments of the rhizome.Lithostratigraphically,the fossiliferous black shales of the upper Doushantuo Formation(Member ?)in the Wenghui area can be correlated with the Doushantuo Member ? in the Three Gorges area,western Hubei Province.The upper Member ? in both the Wenghui and Three Gorges areas can be geochemically correlated.In addition,the Wenghui biota can be paleontologically correlated with the Miaohe biota,sharing many identical species,and can be re-divided into two assemblage biozones.However,there is a fossil-barren interval in the Doushantuo Member ? of the Wenghui section that can be correlated with the dolostone unit underlain the fossiliferous black shales in the Three Gorges sections.In other words,the Miaohe biota in western Hubei may correspond to the middle–upper Wenghui biota above the fossil-barren interval.The explosion of the Wenghui biota was proceeded by a period of prosperous microorganisms in the Yangtze Sea in South China,marked by deposition of ribbon or lamellar carbonate rocks on the platform,carbonate muddy mounds on the upper slope,and stromatolites in the relatively deep-water environments with weak sunlight penetration.The microbial prosperity may have improved the marine environment,especially the oxygen content in water,and set up the paleoecosystem suitable for the development of macrobiota.