陕西宁强梅树村阶瓶状微化石的发现及其意义

DISCOVERY OF MEISHUCUNIAN VASE-SHAPED MICROFOSSILS IN NINGQIANG, SHAANXI AND ITS SIGNIFICANCES

本文报道的瓶状微化石,发现于陕西宁强宽川铺袁家坪剖面,化石产出层位为下寒武统梅树村阶下部小壳化石Anabarites-Protohertzina带。所获微化石数量颇为丰富,囊壳甚小并业经碳化。袁家坪瓶状微化石群的发现,填补了梅树村阶下部此类化石的空白,进一步表明发生于晚前寒武纪的该类具瓶状囊壳的微动物群,曾延入寒武纪早期的事实。现有微化石以碳质保存,为探讨其囊壳原始物质组成提供了新的证据。

The VSMs (vase-shaped microfossils) under discussion in this paper were found in the Yuanjiaping section, 3 km southwest of Kuanchuanpu Village, Ningqiang County, Shaanxi Province. Stratigraphically, these VSMs belong to the small shelly fossil zone Anabarites-Proto-hertzina. The fossiliferous bed occurs as a thin chert interbed within the lower Meishucunian limestone, which abounds with organic matter and contains a few pebble-shaped siliceous intra-clasts, in which there are also many VSMs.The VSMs collected were recognized both in thin sections and from acid-resistant residues of samples. About 30 fossil individuals per square milimetre of thin section were observed under the microscope. Their lengths range between 15 and 60 μm, commonly between 20 and 40 fan. The tests of VSMs are all preserved as organic carbon, and those occurred in the intraclasts were mostly replaced by siliceous materials. The organic carbon of tests would be only the remainder of a certain complex organic components, which had been carbonized in the diagenetic stage. In thin section, it can be seen that such carbonic tests have opaque walls, both interior and exterior surfaces of which are covered with a calcareous membrance. Such membrances are very clear when the matrix and filling materials of the tests are black organic matter. Obviously, these calcareous membrances are only the external things for microfossils, and should be the homoloque in origin with the veined calcites filling in cracks of the chert bed.Examination of most specimens indicates that the two communities of VSMs yielded in the chert bed itself and the intraclasts are similar in general features. This only lies in that they may represent separately the contents of the individuals (assemblages) of distinct generations in the same microfauna. In considering the characteristics of the tests, the above-mentioned VSMs assemblage is divided into 5 types.