Fossil leaves of Berhamniphyllum (Rhamnaceae) from Markam, Tibet and their biogeographic implications

A new occurrence of buckthorn fossil leaves is reported from the upper Eocene strata of Markam Basin, southeastern Tibet, China. The leaf margin is either entire or slightly sinuous. Secondary veins are regularly spaced, forming eucamptodromous venation. These secondaries exist as straight lines from midvein to near margin and then arch abruptly upward and enter into a margin vein. The tertiary veins are densely spaced and parallel, and are percurrent to secondary veins. This leaf architecture conforms with Berhamniphyllum Jones and Dilcher, an extinct fossil genus reported from America. Our fossils are characterized by their dense secondaries, with secondary veins on the upper half portion of the blade accounting for over 40% of all secondaries. A new species, Berhamniphyllum junrongii Z. K. Zhou, T. X. Wang et J. Huang sp. nov., is proposed. Further analysis shows that confident assignment among Rhamnidium, Berchemia, and Karwinskia cannot be made based on leaf characters alone. Berhamniphyllum might represent an extinct common ancestor of these genera. In this study, several fossil Berchemia from Yunnan and Shandong are emended and reassigned to Berhamniphyllum. A new complex, namely the Berchemia Complex, is proposed based on morphology, molecular evidence, and the fossil record. This complex contains the fossil leaves of Rhamnidium, Karwinskia, Berchemia, and Berhamniphyllum. The historical biogeography of the Berchemia Complex is also discussed in this paper. This complex might have originated in the late Cretaceous in Colombia, South America, and dispersed to North America via Central America during the Eocene. Subsequently, the complex moved from North America to East Asia via the Bering Land Bridge no later than the late Eocene. Besides, the complex migrated from North America to Europe via the North Atlantic Land Bridge and then migrated further to Africa. In East Asia, it first appeared in Markam on the Qinghai-Tibetan Plateau, and then dispersed to other regions of Asia.