四川东南部长宁双河及其附近地区的中奥陶统上部大沙坝组以特殊的岩相和生物相有别于黔北的可比地层十字铺组和鄂西的牯牛潭组，地层中漂浮相化石(笔石)与底栖壳相化石(如腕足动物和三叶虫)多次交互出现，笔石动物属于上层下垂对笔石动物群(Didymograptus artus Fauna)，时代为中奥陶世Darriwilian(相当于Arenig最晚期至Llanvirn)。在所采集的3300余枚化石标本中有近90％是腕足动物化石，经研究共鉴定出19属21种，分属于4目7超科15科，正形贝目和扭月贝目分别有8属，占全部属群的84％。根据属的生物地理分布，该动物群中广布型分子7属、区域性分子6属，仅限于华南的土著性分子占总量的31．6％，显示出此期华南腕足动物群具有较强的地方性色彩，进一步证实中奥陶世海洋生物强分区性的推断。该腕足动物群宜称作Saucrorthis-Glyptorthis动物群。在统计分析的基础上，识别出Saucrorthis-Glyptorthis群落，包括三个以腕足动物占优势的化石群集(association)，即Glyptorthis?simplex群集、Saucrorthis obscura群集和Calyptolepta chengkouensis群集，并结合沉积岩石学以及其它共生化石门类进行群落生态分析，认为该群落所生活的环境以正常浅海砂泥质底域为主(相当于BA2-3)。与上扬子台地贵州遵义十字铺组和重庆城口厚坪组两个相近腕足动物群相比，长宁地区中奥陶世的Saucrorthis-Glyptorthis动物群更接近于厚坪组所产者。与世界上其它一些同期且具相似生态位的动物群相比，除与哈萨克斯坦南部的同期动物群比较接近外，与澳大利亚、北美、英国等地的同期动物群几乎毫无联系，指示华南中奥陶世腕足动物群可能代表一个独特的生物地理小区。

The Middle Ordovician rocks at Shuanghe, Changning, southeastern Sichuan Province and its vicinity are well developed, especially the Dashaba Formation, the upper part of the Middle Ordovician, which is characterized by the intercalations of yellowish green mudstone and brownish yellow silty mudstone with calcareous nodules and limestone lenses. This formation is different both lithologically and palaeontologically from its equivalent rocks, the Shihtzupu Formation in northern Guizhou Province and the Kuniutan Formation in western Hubei Province. In the Dashaba Formation, the beds yielding benthic shelly fossils (such as brachiopods and trilobites) and those graptolitic beds sandwiched each other several times, which are very helpful to determine the age of the shelly faunas. The graptolites belong to the Didymograptus murchisoni Biozone, latest Arenig to Llanvirn in age.

Q915.816

国家重点基础研究发展规划项目 (G2 0 0 0 0 7770 0 )，国家自然科学基金 (No.40 0 0 2 0 0 1,No .4990 2 0 0 1)，中科院创新工程方向性项目 (KZCX2 SW 12 9)联合资助