Miocene Equisetumtubers from the Wulan Basin, Northeast Qinghai-Tibetan Plateau and their paleoecological significance

Equisetumfossils are often reported from the Cenozoic strata in the Northern Hemisphere, but their occurrence in the Qinghai-Tibetan Plateau (QTP) region is scarce. In this investigation, two new species of tuber fossils of Equisetum— Equisetum wulanenseZhang and Xie, n. sp. and Equisetum kekeenseZhang and Xie, n. sp. from the Miocene Youshashan Formation of the Wulan Basin, Qinghai Province are reported. The extant Equisetumplants are usually living in wet habitats. Therefore, fossil species, E. wulanenseand E. kekeense, were likely to live in moist environments during Miocene too. The discovery of E. wulanenseand E. kekeensein the late Middle Miocene (∼13.4 Ma) indicates that the local climate of the Wulan Basin was more favorable during this period. This new finding confirmed the previous view that the Neogene aridification in the northern QTP was varying: the climate in the Early Miocene and the Middle Miocene were relatively warm and humid and then the climate gradually became colder and drier since the Late Miocene. It is widely accepted that the uplift of the QTP intensified the trend of cooling and drying since the Late Miocene, and eventually formed a cold and dry climate pattern in the northeast margin of the QTP. The discovery of Equisetumfossils in this investigation improved our understanding on the Cenozoic diversity of Equisetumplants in the northern QTP, as well as on the paleoecological environments of the Wulan Basin in Miocene.