Abstract: The Hengduan Mountains region (HMR) on the southeastern Tibetan Plateau, supports a high diversity of herbs, particularly in its subalpine to alpine ecosystems, due to high altitude and cool temperate climate. Current understanding on the formation of such herbaceous richness is based chiefly on molecular phylogenies; however, direct geological evidence is lacking because herbs are rarely preserved as macroscopic fossils. In this study, we present abundant fossil fruits and seeds of herbs from the late Pliocene Heqing Basin in the southern HMR. Our systematic analysis shows the presence of at least 18 species belonging to 11 genera, that is, Ranunculus, Corydalis, Rumex, Polygonum, Chenopodium, Stellaria, Fragaria, Astragalus, Aster, Carex, and Schoenoplectus, of which Polygonum is most abundant followed by Astragalus. This finding throws the first light from fossil evidence on the rise of herbaceous diversity in the region. We interpret the local assembly of these herbs as resulting from rapid pre-Pliocene species diversifications of many herbaceous groups in the HMR. As nowadays most of these herbs grow primarily in meadows and a few occur as subaquatic plants, we suggest an open meadow hosting some scattered shrubs in the vicinity of a vegetated wetland in the Heqing Basin during the late Pliocene. This provides the first direct evidence of past treeless open vegetation within the HMR and thus improves our knowledge of vegetation evolution in the region. We suggest that the uplift-induced climate cooling and monsoon-associated precipitation seasonality are potentially the key driving forces for the opening of meadow vegetation in the HMR.

Key words: fossil carpology, Hengduan Mountains, late Pliocene, meadow, monsoon, Tibetan Plateau