Continuous existence of Zanthoxylum (Rutaceae) in Southwest China since the Miocene

Fossil seeds of Zanthoxylum L. (Rutaceae) were studied from three fossil floras in Yunnan Province, Southwest China, the late Miocene Shuitangba, late Pliocene Fudong, and early Pleistocene Nanbanbang. Based on seed morphological characters, the Shuitangba seeds were assigned to a new fossil species, Zanthoxylum trachyspermum sp. nov. H. Zhu et Z.K. Zhou, the Fudong seeds were determined only at the generic level, and the Nanbanbang seeds were designated to a modern species, Zanthoxylum avicennae (Lam.) DC. These three new fossil records, together with Z. tertiarium (Heer) Gregor et Hantke from the early to middle Miocene Mangdan flora and modern distributions of Zanthoxylum in Southwest China, imply that Zanthoxylum has continuously existed in this region at least since the early to middle Miocene. Quaternary glaciations have not caused the disappearance of Zanthoxylum from Southwest China, differing from the situation in Europe where the genus was well represented prior to the Pleistocene but became extinct thereafter. The severe post-Miocene environmental changes caused by the continuous uplift of the Qinghai-Tibet Plateau and adjacent areas might have promoted the establishment of its modern high diversity in Southwest China through enlarging environmental heterogeneity.