Unravelling the palaeo-biogeographical history of the living fossil genus Rehderodendron (Styracaceae) – an Asian relic element of the Eurasian Cenozoic flora

Abstract

Background The relic genus Rehderodendron (Styracaceae), which species are restricted to mostly warm to tropical climate (C and A climate of Koeppen-Geiger) in East Asia today, was present as fossil fruits and pollen in Europe during warmer periods from lower Eocene to Pliocene times. To understand which extant species are closest related to the fossils, pollen and fruit morphology of six extant species and fossils of Rehderodendron are compared. Results Both, fossil pollen and fruits resemble a morphological mixture of four extant species R. kwantungense, R. macrocarpum, R. microcarpum, and R. indochinense, thus implying that these four extant and the fossil European taxa represent an old Eurasian lineage, whilst the pollen and fruit morphology of the extant R. kweichowense and R. truongsoense differ considerably from the fossils and other extant species investigated, and are considered to have evolved independently. Conclusions The palaeo-biogeographical history of Rehderodendron reveals that its fossil members of the European lineage were most prominent during climate optimums such as the Palaeocene-Eocene Thermal Maximum, Early Eocene Climate Optimum and Middle Miocene Thermal Maximum. However, when during the Pliocene the climate changed to colder and less humid conditions, the genus went extinct in Europe, but migrated eastwards, most likely in two dispersal events, probably along the Tethys, prior to extinction. One of the former most westerly stepping stones is presented by the refugial occurrence of R. microcarpum in the southeastern Himalayas whereas the more to the east distributed taxa are R. macrocarpum and R. kwantungense, might have migrated eastwards already before the Miocene