Acritarchs And Some Tubular Microfossils From The Edicaran Doushantuo Formation In South China

The chert nodules and bands in the Ediacaran-early Cambrian strata yield abundant and exquisite microfossils,including acritarchs,tubular microfossils,cyanobacteria,multicellular algae.These microfossils provide extremely important evidences for unveiling the life history of early multicellular eukaryotes and illuminating the evolutionary prelude to the Phanerozoic biosphere.Among them,some acanthomorphic acritarchs show great potentials in stratigraphic subdivision and regional correlation.Taxonomically diverse and morphologically disparate microfossils are found from the chert nodules of the Ediacaran Doushantuo Formation in Songlin area of Zunyin,Guizhou Province.More fossil record and new species increase the biodiversity of Ediacaran acritarchs,facilitating their correlation between globally distributed microbiotas.They extend the palaeogeographic distribution of acanthomorphs in South China beyond the Yangtze Gorges and Weng'an area.Taking the fossil horizons and species composition into account,Liujing microfossil assemblage may be assigned to Tanarium conoideum-Cavaspina basiconica Assemblage Zone,although a precise correlation with the established assemblage zones of acanthomorphs.Microfossils in chert are often three-dimensionally preserved with fine-scale characteristics of morphology and cellular structures.Silicification represents one of the best taphonomic pathways,but its detailed processes and mechanisms are not yet fully understood.We studied microfossils and their embedding matrix minerals in chert nodules of the Doushantuo Formation using Raman spectroscopy as well as transmitted-and polarized light microscopy.The microfossils are composed of organic carbonaceous material with weak structural organization/ordering,which indicates a low degree of thermal maturation and alteration.Raman spectral disparity of carbonaceous material among different portions(vesicle/inclusion)of microfossils reveals some differences in organic composition and structure of their precursor biological matters.The mineral phases of silica matrix that entombs the microfossils are recognized as opal-CT,cryptocrystalline quartz,and their transitional phases.Overall,Raman spectral analyses and petrographic observations document a complete and continuous sequence of silica phase transformation from opal-CT to quartz.When reconstructing the diagenetic history of sediments and mineralization process of microfossils in a localized microenvironment,the silica in pore waters precipitated preferentially at the periphery of microorganisms in the form of opal,gradually decreasing in abundance as it transformed into more stable cryptocrystalline quartz.The recrystallization took place in cryptocrystalline quartz to form grains.Authigenic fluorapatite readily precipitated in the vicinity of some microfossils and within their degraded organic fragments.While apatite was crystallizing around organisms and their biostructures,silica precipitated in the surrounding voids,inhibiting the space of potential apatite crystallization.The silicification and phosphatization of microfossils occurred in the microbial sulfate reduction zone of the sediment column,and both were driven by the localized pH change resulting from sulfate reduction and pyrite precipitation in the microenvironment,thus facilitating the exceptional fossil preservation in the Doushantuo chert nodules.The tubular microfossil Sinocyclocyclicus guizhouensis in the Ediacaran Doushantuo Formation were previously considered as a stem-group cnidarian,a filamentous cyanobacterium,or other alga.Its phylogenetic affinity affects our understandings into the evolutionary history of early eumetazoans.Abundant,exquisitely preserved tubular microfossil Megathrix longus were found from the early Cambrian Yanjiahe Formation in the Yangtze Gorges area,whose morphology is extremely similar with Sinocyclocyclicus guizhouensis in the Ediacaran Doushantuo Formation.Studies of the morphology and taphonomy revealed the inner cross-wall of Megathrix longus to be topographically flat,the trichome terminus to be blunt and closed,and incomplete cross-walls regularly intercalated between complete ones.The deformation of trichomes,as well as the corrugation of its cross-walls,is a postmortem feature caused by compaction rather than a biological feature as previously interpreted.Statistical analysis indicated that chamber division is accomplished by the closure of incomplete cross-walls.This process which contributed to trichome growth is extremely similar to that of modern Oscillatoriaceae.Trichome fragmentation was also evidenced by a series of specimens,which highly resembles that of cyanobacteria particularly the Oscillatoriaceae.Meanwhile,the termini of Megathrix longus suggest that it was planktonic.In general,Megathrix longus bears a close affinity to the extant Oscillatoriaceae(Cyanophyta).Some Ediacaran-early Cambrian tubular microfossils have great similarities on their morphology,ecology,individual development and reproduction.The study on their phylogenetic affinities tend to support the opinion that some Ediacaran Doushantuo tubes are large filamentous cyanobacteria,rather than eumetazoans(cnidarians).