Tracing ancestral biogeography of Sonneratia based on fossil pollen and their probable modern analogues

Extant tropical mangrove Sonneratia is assigned in the monogeneric subfamily of Lythraceae. There are still some debates on the early fossil pollen of Florschuetzia, which is well accepted as ancestral to Sonneratia. This paper re-assesses palynological interpretations on the historical biogeography of the genus, based mainly on the updated fossil pollen records of Paleogene through Quaternary and their probable modern analogues. Florschuetzia was extensively documented from the late Eocene to middle Miocene in palaeotropics around the Tethyan region. According to the geological age of the fossil pollen and their morphological assessments, ancestral Sonneratia migrated from the center of origin in southeastern Asia probably during early Eocene, and radiated and expanded northward to China and Japan, southward to Australia, and westward to east Africa. Until the warmer period of the early middle Miocene (Langhian), Sonneratia had the largest geographical range suggested by abundant fossil pollen from southern mainland China and southwestern Japan, out of latitudinal limit of this extant genus. Quaternary glaciations, especially the Last Glacial Maximum (LGM), played a significant role in shaping the current biogeography of Sonneratia. However unequivocal assignments of early Florschuetzia and the associated variants to the evolved Sonneratia remain an issue due to the lack of intensive morphological comparison from different fossil sites. Thus, we examined the evolutionary trends of the extinct genus Florschuetzia towards Sonneratia on the basis of our synthesis on the updated published data and our recent pollen morphological investigation.