Late Miocene fossils from Nainnainxuonla, Nyalam, Xizang,and their implications for paleoenvironment

The southern slope of the Nainnainxuonla Mesa, Nyalam, Xizang (Tibet)preserves thick Neogene sediments of the Dati Paleolake Basin, which is considered crucial for studying the Neogene uplift and environmental changes in the Himalayan region. Remains of the late Late Miocene Hipparion (Cremohipparion)forstenae have been found in the Nainnainxuonla section. Although significant work has been done on the stratigraphy and vertebrate fossils of representative Neogene sedimentary basins in the Himalayas, with emphasis on the Zanda and Gyirong basins, research on the Dati Paleolake Basin remains limited. In 2020, the paleontology team of the Second Comprehensive Scientific Expedition to the Qinghai.Tibet Plateau (QTP), organized by the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, investigated the Mesozoic.Cenozoic strata and paleontology in the Nyalam region. A small number of vertebrate fossils were discovered from the Nainnainxuonla section, including cyprinid pharyngeal teeth, limb bones of rhinoceroses and hipparion limb bones, and lagomorph cheek teeth. Among these fossils, the lagomorph fossils belong to Bellatonoides eroli, representing the first discovery of this genus in the QTP and indicating an age no later than the early Late Miocene. Two distinct stratigraphic levels and ages have been identified in the Nainnainxuonla section based on mammalian fossils. The upper fluvial facies strata yield H. (C.)forstenae of late Late Miocene (Baodean)age, while the lower lacustrine facies strata yield B. eroli, possibly of early Late Miocene (Bahean)age. During the late Middle to early Late Miocene, B. eroli was typically found at the strata in Ankara, Turkey and central Inner Mongolia, China. It was often associated with primates and other forest.dwelling mammals. H. (C.)forstenae, which is believed to have adapted to open habitats, is distributed in the late Late Miocene strata of Gyirong, Tibet, and Baode, Shanxi. Its altitudinal distribution does not exceed 2900 meters. Based on these fossil evidence, it is reasonable to infer that the Nyalam Dati Paleolake Basin had a paleo.altitude nearly in 2000 meters lower than its current elevation during the early to late Late Miocene. From the early to late Late Mionce, the paleovegetation type changed from the forest environment to the forest.grassland with quite open environment. In general, the paleoenvironment of Nainnainxuonla during the Late Miocene was much more favorable than it is today