高家山生物群化石组合面貌及其特征

ASSEMBLAGE ZONES OF GAOJIASHAN BIOTA AND THEIR CHARACTERISTICS

新元古代末期的高家山生物群 ,经系统研究自下而上可以划分为 3个化石组合带 ,即 :以蠕形动物爬迹为代表的 Shaanxilithes- H elminthopsis组合带 ;以锥管虫类和瓶状化石为主 ,并见大量软躯体蠕形动物和宏观藻类的Conotubus- Gaojiashania- Protolagena组合带 ;以多种管状骨骼化石共生为特征 ,并伴生有杯 (钵 )状、分枝管状及球状化石等的 Sinotubulites- Cloudina组合带 ,代表了高家山生物群鼎盛时期的产物。进一步的研究揭示高家山生物群是迄今为止地球上所发现的最早且最为多样化的骨骼化石组合。

The Gaojiashan (GJS) Biota comprises the most diversified Precambrian skeleton fossil assemblages ever known, and consists of tubular fossils, conotubular fossils, vase shaped microfossils, globular fossils and cup like fossils, as well as several other forms of problematics. The pyritized worm like fossils are also distinct members of the biota, quite different from other Precambrian ones. They are body fossils and not mould, some of which are as large as 10mm in diameter and 100mm in length. Four main preservation states can be recognized, namely flattened organic or glauconitic structures, and three dimensionally calcified or pyritized cylinder structures. There are 12 ichnogenus and 15 ichnospecies in the biota, which comprise not only grazing traces on the bedding planes of the worm like animals but also vertical burrows such as Skolithos and Intrites . Three types of carbonaceous megafossils have been detected in the grayish brown siltstone in the middle and upper parts of the Gaojiashan Member, namely circular or elliptical carbonaceous film without ornamentation, straight or curved ribbon like structure with tiny branches, and foliated structure having longitudinal sculptures . The GJS Biota consists of three fossil assemblages in ascending orders as follows: 1. Shaanxilithes Helminthopsis trace fossil assemblage appears at the base of the Gaojiashan Member, and may represent the earliest activity of soft bodied animals in the biota. It is widely distributed geographically ranging Xuanjiangping and Kuanchuanpu eastwards into Bogedong and Yuandunzi, and therefore is of a valuable guide for regional correlation. 2. Conotubus Gaojiashania Protolagena assemblage occurs in grayish brown and yellowish green siltstone interbedded with thin carbonate rocks of the middle part of the Gaojiashan Member, and is the oldest mineralized skeleton fossil assemblage ever known. It is the primary flourishing stage of the biota and is principally made up of conotubular fossil Conotubus and vase shaped microfossils such as Protolagena and Sicylagana, accompanied with large quantity of soft bodied worm like fossils including Gaojiashania, Ningqiangella and Yudaiella and carbonaceous megafossils such as Vendotaenia, ?Chauria, and ?Shouhsiania, as well as trace fossils. They usually occur in profusion and more than 100 individuals may be found on a single bedding plane with an area of 750cm 2. 3. Sinotubulites Cloudina assemblage is found in grayish dark carbonate rock of Bed 9 in Niuluokenggou section, Gaojiashan and grayish white calcarenite of Bed 6, in Dahezigou and Lijiagou sections, Kuanchuanpu, in the upper part of the Gaojiashan Member. The mineralized skeleton fossils attain their greatest development, and flourish both in number of individuals and variety of species. Apart from Sinotubulites, Qinella and Cloudina , cup like fossils and problematic fossils also richly occur in the assemblage. Large worm like fossils with a diameter over 1cm in association with trace fossil sequences 5 and 6 are also the found in the Gaojiashan′s Niuluokenggou section.