Miocene Ulmus fossil fruits from Southwest China and their evolutionary and biogeographic implications

The genus Ulmus L. (Ulmaceae) has a rich Cenozoic fossil record from the Northern Hemisphere, which provides essential information for evolutionary and biogeographic histories. However, compared to its abundant leaf fossils, fruit fossils of Ulmus, which allow accurate identification, are still scarce. In this study, we report two new species of the section Ulmus, Ulmus prelanceaefolia Q.Y. Zhang et Y.W. Xing sp. nov. and U. maguanensis Q.Y. Zhang et Y.W. Xing sp. nov. from the Miocene Huazhige Formation in Wenshan and Maguan basins, southeastern Yunnan, southwestern China. These two species are established based on several well-preserved fossil samaras. Ulmus prelanceaefolia represents the only unequivocal record of the ser. Lanceaefolia in the world and U. maguanensis represents the earliest unequivocal record of the ser. Nitentes in East Asia. The evolutionary and biogeographic histories of Ulmus are discussed by summarizing and revising the fossil fruit records. We deduce that the narrow-winged fruit may be primitive and the two narrow-winged sections, sect. Chaetoptelea and Trichoptelea are the early evolved lineages in Ulmus and have diversified since the Eocene. The Oligocene was an important epoch for the rapid diversification of broad-winged lineages of Ulmus, which was likely due to dispersal advantage in the more open forests originating as the global climate began to cool. We also found frequent exchanges/dispersals and extinction events between or within North America, Asia, and Europe since the Eocene. The North Atlantic and Bering Land Bridges served as important dispersal corridors for Ulmus during the Cenozoic.