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 junrongiae 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.

Late Eocene sclerophyllous oak from Markam Basin, Tibet, and its biogeographic implications

Sclerophyllous evergreen broad-leaved forests,mainly made up of sclerophyllous oak,Quercus section Heterobalanus(Øerst.)Menitsky,Fagaceae,represent the most typical forest type in the Hengduan Mountains.Their distribution pattern is closely related to the growth and formation of the Qinghai-Tibetan Plateau(QTP).The oldest fossil record of Quercus sect.Heterobalanus so far discovered is from the middle Miocene of the Gazhacun Formation in Namling County,southern Tibet.However,our recent discovery of leaf fossils from the upper Eocene of Lawula Formation in Markam Basin,southeastern Tibet,illustrates that their origin is nearly 20 Myr older than previously assumed.By integrating the results from geometric morphometrics,geographical range expansion,and ecological niche shifts of this section in what is now the QTP and the Hengduan Mountains,we infer that the leaves of Quercus sect.Heterobalanus were already adapted to cool and dry conditions in some local regions no later than in the late Eocene.Then,with the growth of the QTP and late Cenozoic global cooling,the expansion of cooler and drier habitats benefited the spread and development of this section and their leaves exhibited morphological stasis through stabilizing selection.Based on published fossil records and recent discoveries,we argue that Quercus sect.Heterobalanus appeared in the subtropical evergreen and deciduous broad-leaved mixed forests of the southeastern margin of what is now the QTP no later than in the late Eocene.Some taxa spread westwards along the Gangdese Mountains and later the Himalaya,and others spread eastwards and southeastwards,gradually becoming a dominant group of species in the Hengduan Mountains.This dispersal route is contrary to the previous“northwards hypothesis”of this section,and further supports the hypothesis of an East Asian origin for Quercus section Ilex Loudon.