The Study Of Ediacaran Phosphatized "Embryo-like" Microfossils From Zhenba,Southern Shaan'xi

Ediacaran Period represents a crucial time interval for evolution of Metazoans and marine ecosystem in life history.During the Ediacaran,the Basal metazoans arose,coupled with marine ecosystem transformation from algae-dominated type to metazoan-dominated type.Despite of the importance of the Ediacaran palaeontology,numerous questions and debates exit.Many Ediacaran fossils are of uncertain or debated in affinity,especially for those fossils which had been interpreted as metazoans.Different from the Cambrian problematic metazoan fossils whose phylogenetic debates exist among different phyla or lower rank taxonomy,many Ediacaran problematic fossils can not be determined whether they belong to metaozoans or other Kingdoms.Therefore,the investigation of Ediacaran fossils and determination of their affinities may provide key evidence for a better understanding of Ediacaran biodiversity,as well as origin and evolution of early metazoans.The Zhenba Microfossils Assemblage from the basal part of the Dengying Formation in Zhenba County,Southern Shaanxi province,was preserved as phosphatized 3-D bodies,and therefore represents a new locality of “Weng'an”-type Lagerst?tte in the Yangtze Block.Three morphological gene and Four species of spherioid fossils including Megasphaera inornata ? M.ornata ? Spiralicellula bulbifera and Bacatisphaera cf.baokangensis are identified from the Zhenba Microfossil Assemblage.The common Ediacaran Large Acanthomorphic Acritarchs(LAAs)are lacking from the Zhenba Assemblage.Adding its occurrence in the upper Ediacaran Dengying Formation,the assemblage is likely late Ediacaran in age.While considering that the lacking of LAAs might due to limited excavation,it's more safe to constrain the age of the Zhenba Microfossils Assembalge in Ediacaran,and more precise constrains are expected.The Zhenba Microfossils Assemblage also contains abundant “embryo-like” microfossils.The problematic “embryo-like” microfossils are emphatically investigated towards their possible biological affinities.The results are summarized as follows:The Zhenba “embryo-like” microfossils reveals different types of cell division.Four types of life cycle including “equal binary fission”,“unequal binary fission”,“successive multiple fission” and “simultaneous multiple fission”are reconstructed from Zhenba “embryo-like”microfossils.The Four life cycles almost cover all types of the Zhenba “embryo-like”microfossils.As currently understood,a protistan interpretation is more consistent with fossil evidence than the metazoan-embryo hypothesis.And the multicellular-algae hypothesis is barely suitable to interpret the “embryo-like” microfossils.Although the metazoan interpretation can't be completely excluded,it should be verified only by the finding of undoubted post-blastulation specimens.Two different patterns of vesicle ornamentation including polygonal and tubercular types are recognized in specimens of M.ornata.The different ornamentations probably results from precisely regulated growth of vesicle,with polygonal type bifurcating into tubercular type.The vesicle-growth phenomenon is comparable with “Morphogenesis”among protists.While such growth pattern have not yet been found among metazoan embryogenesis.Therefore it suggests a protistan origin.Combined with materials reported from the Weng'an Biota and the Doushantuo silicified acritarch assemblage in Three Gorge area,different mechanisms of “Morphogenesis” among Ediacaran Large Acanthomorphic Acritarchs(ELAAs)are speculated.The speculation implies that morphological diversity of Ediacaran aritarchs may greater than previously thought.