Bauhinia is a large and taxonomically complex group in the Cercidoideae (a subfamily of the Fabaceae), with nearly 380 living species, including trees, shrubs, lianas and herbs, distributed mainly in the pantropical regions. However, its evolutionary and biogeographical history is still unclear due to the scarcity of fossil records.  

During a field research in the Tibetan Plateau, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences collected fossil leaves of Bauhinia from the upper Paleocene Liuqu Formation, Lazi County, southern Tibet. 

After detailed morphological comparison with living and previously reported fossil species, the researchers classified the fossil leaves as a new species, namely Bauhinia tibetensis.  

With the new fossil evidence, the researchers reconstructed the evolutionary and biogeographic history of Bauhinia. Together with phylogenetic analyses, the results suggest that Bauhinia originated in Africa and spread to what is now the southern Tibetan Plateau by the late Paleocene, after which the Asian clade began to diverge. 

With the assignment of 66.0 Ma as the stem node, the divergence age of Bauhinia was in the earliest Paleocene. The analysis showed that Bauhinia tibetensis is from the latest Paleocene (about 56 million years ago) of the southern Tibetan region. Bauhinia tibetensis is the earliest fossil record and a representative of the crown node for Bauhinia. 

The oldest fossil record of Bauhinia was published in Journal of Systematic Palaeontology. 

Combined with paleoecological niche simulations and ancestral state reconstructions, the researchers suggested a Paleocene origin of Bauhinia in the Afrotropical realm, with subsequent dispersal to the Neotropical and Indomalayan realms. 

"The discovery of the oldest Bauhinia from the southern Tibetan Plateau updates our understanding of the biogeographical history of this genus and shows that the Kohistan-Ladakh Island Arc is an ancient corridor for floristic exchange between Africa and India," said SU Tao of XTBG. 

The study was supported by the Second Tibetan Plateau Scientific Expedition Program and the National Natural Science Foundation of China. 

Fossil leaves of Bauhinia tibetensis. (Image by GAO Yi)