A Late Ediacaran Problematic Microfossil Biota And Its Depositional Environment From Nanjiang, Sichuan Province

The Ediacaran biotas are of great significance in our understanding of the evolutionary history of early animals at the Precambrian-Cambrian transition. Here we report a morphologically diversified microfossil biota from the late Ediacaran Beiwan Member of the Dengying Formation at the Zhiziyuan section in Shangliang area, Nanjiang County of Sichuan Province. Fossils were isolated from the hosting rock using acetic acid maceration method. Abundant specimens were scanned using Scanning Electron Microscope and their chemical composition were investigated by Energy Dispersive Spectrum analysis. A small fraction of the fossils and the hosting rocks were studied in thin sections under polarized light microscope. Four types of fossils are reported here:(1) filamentous fossils. Two types (Type A and Type B) filamentous fossils are identified based on their size and morphology. Type A falls in a diameter size range of 2-2.5 ?m and filamentous individuals are randomly overlapped, forming a massive three-dimensional aggregation. Type B shares a similar complicated preserved morphology, with a diameter range of 1.5-5 ?m. Nonetheless, unlike those seen in Type A fossils, a number of circular cavities occurred in the substrate of Type fossil aggregations, suggesting a different taxonomic position. (2) spherical fossils. The Nanjiang sphere is characterized by its pretty smaller size (25-40 ?m). The great majority of the spheres are globular, with rare cases are axiolitic. Spheres are primarily preserved as clay minerals. Fractured specimens reveal that the sphere has a hollow central cavity and the wall is extremely thin (5-10 ?m). (3) agglutinated fossils. These fossils consist of densely packed dolomite crystals, with a variety of morphologies including tubular, kidney-, and comb-shaped forms. Abundant spherical cavities occur between dolomite crystals, which may have been the abandoned living house of animals with unknown affinities. (4) hat-shaped fossils. Completely preserved fossil specimens indicate that they are a number of hats grew from the same substrate. The substrate were served as a apparatus of conneting these hats. Both the hats and the substrate are replicated by needle-shaped clay minerals, with the long axis of the needles parallelly arranged. This may present that the original ecological strategies during life time of the organisms. In addition, this study conduct a sedimentological investigation on the fossiliferous Dengying Formation. The thick-bedded dolostone sequence of the Algal Dolomite Member was deposited in an intertidal environment, whereas the siliciclastics-dominated Gaojiashan Member represents a transgression interval following the tectonic uplift occurring at the Algal Dolomite and the Gaojiashan members. The Beiwan Member, in turn, recorded a suite of thick-bedded dolostone in similar facies with the Algal Dolomite Member. In summary, the three members of the Dengying Formation at the Zhiziyuan section respectively reflects an Algal Dolomite regression, a Gaojiashan transgression, and a Beiwan regression.