陕南末元古代高家山生物群主要化石类群及其特征

FOSSIL EVIDENCES OF LATEST NEOPROTEROZOIC GAOJIASHAN BIOTA AND THEIR CHARACTERISTICS

末元古代高家山生物群是以骨骼化石为主 ,兼有软躯体后生动物及宏观藻类化石的一个多门类化石组合 ,是目前已知最早且最为多样化的骨骼化石生物群。化石主要产于上震旦统灯影组高家山段中上部 ,时限约5 6 5— 5 43Ma。目前虽然尚难对其进行系统的生物学分类 ,但根据化石的形态特征可以划分为以下几个类群 ,即具矿化壁的骨骼类群 ,包括管状化石、锥管状化石、瓶状化石、球状化石、杯状化石和疑难化石等 ,以及蠕形动物化石类群 ,遗迹化石类群和宏观藻类化石类群等。还选取与其同时代且特征较为相似的峡东西陵峡生物群和南非纳米比亚纳玛生物组合进行比较。

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