Unravelling the palaeobiogeographical history of the living fossil genus Rehderodendron (Styracaceae) with fossil and extant pollen and fruit data

BACKGROUND: The relict genus Rehderodendron (Styracaceae), the species of which are restricted to mostly warm temperate to tropical climate in East Asia today, is known from fossil fruits and pollen in Europe during warmer periods from the lower Eocene to Pliocene. To infer which extant species are most closely related to the fossils, new data of pollen and fruit morphologiesy of six extant species, and additional new data of fossil pollen and previously described fossil fruits of Rehderodendron, are compared. RESULTS: Both fossil pollen and fruits resemble a morphological mixture of the extant species R. indochinense, R. kwantungense, R. macrocarpum, and R. microcarpum, thus implying that these extant taxa and the fossil European taxa represent an old Eurasian lineage, whereas the pollen and fruit morphology of the extant R. kweichowense and R. truongsonense differ considerably from the fossils and other extant species investigated, and are considered to have evolved independently. CONCLUSIONS: The palaeobiogeographical history of Rehderodendron reveals that its fossil members of the European lineage were most prominent during climatic optima such as the Palaeocene–Eocene Thermal Maximum (PETM), Early Eocene Climate Optimum (EECO) and Middle Miocene Thermal Maximum (MMTM). 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 along the Tethys Sea prior to extinction. One of the former most westerly stepping stones is suggested by the refugial occurrence of R. microcarpum in the southeastern Himalaya, whereas R. macrocarpum and R. kwangtungense, the taxa distributed more to the east, might have migrated eastwards already before the Miocene.