Origins of biodiversity in eastern Tibet revealed by Eocene mammals and fossil tree rings

Asia’s exceptional biodiversity is reflected in the Hengduan Mountains fossil record in eastern Tibet, yet the timing and drivers of mammalian diversification remain enigmatic. Here we combine U–Pb dating of mammalian bone-cavity calcite and palaeosol nodules from the Relu Basin with stable-isotope palaeoaltimetry of fossil tooth enamel. We show that the Relu mammal assemblage was buried at 39.2–38.1 Ma, making it the oldest directly dated Cenozoic mammal fauna reported from the Tibetan Plateau, and that herbivores inhabited near-modern habitat elevations of 3463 m (+819/−869). This directly dated Eocene mammalian assemblage coincided with diversification of high-elevation forests, implying that a montane biodiversity hotspot had emerged by ~39 Ma. Mediterranean-type false-ring signals in 41.5-Ma fossil wood and climate-model simulations are consistent with Eocene surface uplift contributing to a strongly seasonal, bimodal-rainfall regime that predated the modern Asian monsoon and provided environmental conditions favourable for early high-elevation biotic diversification. These results refine Tibetan mammalian chronology and provide a benchmark linking mountain-building, rainfall seasonality and biodiversity.