The disappearance of Metasequoia (Cupressaceae) after the middle Miocene in Yunnan, Southwest China: Evidences for evolutionary stasis and intensification of the Asian monsoon

Metasequoia glyptostroboides Hu et W.C.Cheng (Cupressaceae) is a relic plant with a narrow natural distribution in central China. Historically, the genus Metasequoia Hu et W.C.Cheng has rich fossil records from the Cretaceous to the Pleistocene in the Northern Hemisphere, but fossil records of Metasequoia in China are still rare, which limits our knowledge of its biogeographic history under paleoenvironmental changes. Here, we describe leaves including cuticles and ovuliferous cones from the middle Miocene of Zhenyuan, Yunnan, Southwest China, comprising the southernmost fossil record of the genus worldwide. Judging by the great morphological similarity between these fossils and extant M. glyptostroboides, we identify them as Metasequoia sp. (cf. Metasequoia glyptostroboides). The material documents obviously a long lasting morphological stasis within the genus. Metasequoia sp. (cf. M. glyptostroboides) considerably expands the spatial distribution range of Metasequoia in the geological past. Furthermore, possible reasons for its disappearance in Southwest China are discussed. Together with other evidence, it is concluded that the disappearance of Metasequoia from Southwest China might be related to the evolutionary stasis of Metasequoia, most likely preventing necessary adaptations of the plants to increasing winter and spring aridity induced by the intensification of the Asian monsoon in this region during the Neogene.