道虎沟生物群是我国近年来古生物学研究的热点之一。虽然绝大多数学者认同其时代属于中侏罗世,但它的产出地层的划分和对比仍存争议。一些学者认为道虎沟化石层属于中侏罗统九龙山组或海房沟组,但另一些学者却认为它属于中侏罗统或中上侏罗统髫髻山组。一些学者将化石层底部的砾岩层划归九龙山组,而将化石层主体连同其上的大套火山岩划归髫髻山组。通过近些年的野外考察,笔者在道虎沟村东的道虎沟化石层底部砾岩中找到了与其他层位类似的凝灰质页岩夹层,内含典型的道虎沟生物群动植物化石。因此,道虎沟底部的砾岩无疑是道虎沟化石层的一部分。结合目前地层及同位素年代学证据,道虎沟化石层归人中侏罗统上部的海房沟组较为合适。随着燕山运动的发生,道虎沟生物群的代表分子在宁城盆地中侏罗世沉积开始之前就已经产生,其起源需要到更古老的地层中寻找。
In recent years, the Daohugou biota has become one of the highlights of palaeontological research in China. Although most authors agree that it is Middle Jurassic, the detailed stratigraphical division and correlation remain controversial. Some suggest that the Daohugou beds belong to the Middle Jurassic Jiulongshan/Haifanggou Formation, but others propose that they belong to the Middle Jurassic or Middle/Lower Jurassic Tiaojishan Formation. Some authors have suggested that the conglomerates of the lower beds belong to the Jiulongshan Formation, whereas the fossiliferous layers and a large part of the overlying volcanic rocks are assigned to the Tiaojishan Formation. During recent fieldwork, we found tuffaceous shales embedded in the bottom conglomerates of the Daohugou beds which are similar to typical Daohugou fossiliferous layers, containing typical elements of the Daohugou biota such as insects (Yanliaocorixa chinensis), branchiopods (Anostraca, species undetermined), bivalves (Ferganoconcha sibirica ) and plants (Anomozamites spp. ). Therefore, thelower conglomerates are undoubtedly part of the Daohugou beds. Considering the stratigraphical and radiometric evidence, it is plausible that the Daohugou beds belong to the Middle Jurassic Haifanggou Formation. Some authors constrain the absolute age of the Daohugou beds to 168-152 Ma (Liu et al. , 2006) or 166-159 Ma (Sullivan et al. , 2014), causing some confusion of the Daohugou, Daxishan (at Linglongta Township, Jianchang County, Liaoning Province) and Bawanggou ( at Mutoudeng Township, Qinglong County, Hebei Province) beds (Sullivan et al. , 2014). Recently, abundant fossil vertebrates have been discovered in Daxishan Village, including feathered dinosaurs and early mammals, which are of palaeontological significance. The conchostracans from Daxishan and Bawanggou are distinctly different and potentially related to those from Daohugou (Euestheria luanpingensis). Therefore, the Jiulongshan/Haifanggou Formation and the