栎属的历史植物地理学研究

THE FOSSIL HISTORY OF QUERCUS

The original time and geological distribution of Quercus are disscussed based on fossil records. The fossil record of Quercus was thought to extend far back into the Cretaceous. Most Cretaceous fossils assigned to Quercus are however poorly preserved and none have been proven to be Quercus. The first believable record of Quercus is from the middle Eocene Clarno Formation of Oregon, United States. This fossil showed that Quercus was already diverse in the Eocene. Furthermore study on fossil pollen in the Sanshui basin, southwest Guangzhou, China has shown that the percentages of Quercus in the spore-pollen assemblage of deposits are increased grom 1- 3% to 2- 38% from Paleocene to O-ligocene. These indicate an origin of Quercus no earlier than the late Cretaceous and Certainly no later than the Eocene, probaly originating in the early Paleocene. The first reliable megafossils in China are from the Fushun, Liaoning northeast China. The geological distributions have shown that the modern distribution pattern of subg. Cyclobalanopsis had formed in the Miocene. The fossil records of Quercus sect. Brachylepides, evergreen sclerophyllous, are first found in the Miocene and this section become dominant element in the vegetation since the Miocene in Hengduan Mt., southwest China. The occurrence of deciduous oaks is much later than those of evergreen ones.