FOSSIL EVIDENCES OF LATEST NEOPROTEROZOIC GAOJIASHAN BIOTA AND THEIR CHARACTERISTICS

A divere fossil assemblage from the approximately 550Ma latest Neoproterozoic Dengying Formation of South Shaanxi, China is described in this paper. It contains tubular skeletonal fossils, conotubular fossils, vase shaped microfossil, as well as several other forms of problematic microfossils, worm like structures, trace fossils and carbonaceous megafossils. The fossil assemblage not only changes conspicuously the picture of soft bodied animal dominant marine life in Neoproterozoic, but also encourages us to search for skeletal fossils in Vendian and old rocks. The fossils herein described come from two well exposed sections of the Dengying Formation, namely the Lijiagou section, about 3km south of the village Kuanchuanpu, and the Gaojiashan section, about 3.5 km southeast the township Hujiaba, Ningqiang Country, Shaanxi Province, China respectively. In the former section, the fossils occur in light gray dolomites of the upper part of the Gaojiashan member, and overlain by about 35m of interbedded cherts and limestones which contain the first assemblage of Cambrian skeletal fossils in its upper part. In the latter section, however, the fossils are from siltstone and carbonate rocks of the Gaojiashan member, and overlain by over 300m of dolostones of Beiwan member, which in turn, is covered by about 13m of calcarenite bearing Cambrian small shelly fossils. Fossils from the clastic rocks of the Gaojiashan member are generally pyritized, with the richest ones occurred in the siltstone bearing horizontal or rythmitic beddings which represent an undisturbed setting below the wave base. It is well known that pyrite forms, as a result of the activity of sulfate reducing bacteria, are usually in fine grained marine sediments, but pyritization of soft tissues is rare. It requires the rapid burial of carcasses to form isolated concentrations of organic matter. Tubular skeletal fossils from Lijiagou section are well preserved through secondary phosphatization, and can be chemically extracted from the rocks, which made it possible to reconstruction their morphology and fine structures.FOSSIL EVIDENCE OF DISPARITY 1 Mineralized skeleton fossils The Gaojiashan Biota comprises the most diversified skeleton fossil assemblage ever known, and consists of tubular fossils, conotubular fossils, vase shaped microfossils, globular fossils and goblet shaped fossils as well as several other forms of problematics. Tubular fossils Tube like shells, generally straight or slightly curved, cylindrical to weakly tapering. The largest persevered length 25mm, the diameter is 3.5mm. Two types can be distinguished in morphology, namely, the single layered tubular shells, represented by Chenella, and multi layered column in column structure, by Sinotubulites and Qinella. Conotubular fossils Straight to sinuous conical tubes of possible cone in cone structure, with the initial part closed and terminal part opened multiple walls nearly parallel to each other, and somewhat oblique to the long axis intersecting with the inner wall to form a common wall. Conotubular fossils in this biota are quite diversified, and only Cloudina and Conotubus are discussed in the paper. Vase shaped microfossils Though morphologically similar to the vase shaped microfossils with worldwide distribution, the fossils in the biota are distinct in their large size (1.5-2.5mm in diameter), thick calcareous wall and rigid deformation. Globular fossils spherical or elliptical structures, with a diameter range 0.05-1mm, single or mulitiplied layer wall, occur in isolation or in string. Cup Like fossils only found in thin sections of carbonate rocks, they are of cup like or bowl like structures bearing irregular dental wall, with the initial part convergent and closed, forming a base like structure, and divergent by degrees towards terminal part. Problematic fossils The problematic fossils in this assemblage are rather varied, comprise interwoven branched tubular fossils of possible calcified algae, hollow and filled spherical s