Improvement of the Chinese Neogene Terrestrial Biochronological Framework Based on the Linxia Basin

The Chinese Neogene terrestrial biochronological framework has been established based on large and small mammalian faunas, including seven units, namely the Xiejian, Shanwangian, Tunggurian, Bahean, Baodean, Gaozhuangian, and Mazegouan ages. In the past, the positions of the Chinese mammalian faunas in the chronological chart and their correlations to European and North American faunas were determined mainly by the evolutionary levels of mammals because of the lack of accurate absolute ages. In recent years, however, great progress has been made in Chinese Neogene terrestrial biostratigraphic and chronostratigraphic studies. Most of the classic regions have been revisited, and new discoveries have been made in well-exposed fossiliferous regions, especially in the Linxia Basin in Gansu, northwestern China. The late Cenozoic deposits are well exposed and mostly flat-lying in the Linxia Basin with a maximum thickness of more than 1600 m, and their exposures in various parts of this basin produce abundant vertebrate fossils, especially mammalian fossils. Up to now, over one hundred known localities produced more than forty thousand mammalian specimens, mainly including skulls, mandibles, and some skeletons. The late Cenozoic deposits of the Linxia Basin are divided, in chronological order, into nine lithologic units: the Tala, Jiaozigou, Shangzhuang, Dongxiang, Hujialiang, Liushu, Hewangjia, Jishi, and Wucheng formations, and mammalian fossils are found in all the rest of the formations. The Platybelodon fauna is collected from sandstones of the Middle Miocene Hujialiang Formation, and the Hipparion fauna is from red clays of the Late Miocene Liushu Formation and the Early Pliocene Hewangjia Formation. Magnetostratigraphic work has also been conducted in several sections of the Linxia Basin. The land mammal ages for China have now been better constrained by data from new localities and new taxa of the Linxia Basin. The significance of these new data is that they are improving our understanding of mammal turnovers and boundary calibrations. The faunal components are thought to be stable throughout each biochronological unit, without large changes occurring. For each unit, series of species or genera are regarded as characteristic forms on the basis of correlation with the European and North American taxa. Generally, the biochronological units deal with the first appearance datum of mammals at the generic or species level. Some of these genera are immigrants, and their entry into China often marks the beginning of a unit. The exact correlations between the Chinese and other continental mammalian ages based on more accurate dating will increase our ability to recognize mammal intercontinental migrations. The increasing number of paleomagnetic measurements has enabled a good calibration to be made for the correlation of Chinese Neogene mammalian faunas with their European and North American counterparts. This will provide a foundation for the improvement of the Chinese Neogene terrestrial biochronological framework.