Abstract: Based on the progress of the recent study on marine strata and correlation of terrestrial with marine strata in the world, and on the new dating of the Eocene—Oligocene boundary (34 Ma), the Mongolian Hsanda Gol Formation should be of Early Oligocene in age rather than Middle Oligocene thought in the past The Ulantatal Fauna of China has been widely used since its discovery, and has been assigned as Chinese Middle Oligocene Land Mammal Age (Tong et al., 1995). This fauna, however, shows great similarities with Hsanda Gol Fauna. The former has 1 7 of 21 genera studied and 14 of 33 species in common with the latter, which indicates that the Ulantatal Fauna is of Early Oligocene instead of Middle Oligocene in age. A new division of the Chinese Oligocene is also proposed in the present paper.

摘要： 根据近年来国际上海相第三纪地层及所含生物研究的进展、海陆相地层对比的精确、划分方案的变化，蒙古三达河组的地质时代应为早渐新世，而不是原来认为的中渐新世。中国乌兰塔塔尔动物群中已研究过的21属哺乳动物化石，有17属与三达河动物群共有，占乌兰塔塔尔动物群总数的81%>;33个种中与后者相同或相似的有14个，占43%。这种相似性表明乌兰塔塔尔动物群的时代也应为早渐新世，而不是原来认为的中渐新世。