Fossil leaves of Podocarpus subgenus Foliolatus (Podocarpaceae) from the Pliocene of southwestern China and biogeographic history of Podocarpus

Phylogenetic analyses indicate that the genus Podocarpus diverged around the Cretaceous/Paleogene boundary, and two monophyletic subgenera Podocarpus and Foliolatus had differentiated during the Paleogene. Most of described fossil remains were related to the subgenus Podocarpus, but those of the subgenus Foliolatus have not been well documented. Here, we described a new species of Podocarpus, P. yunnanensis sp. nov., and P. cf. forrestii from the lower Pliocene of western Yunnan, China based on a detailed comparisons of gross leaf morphology and cuticular structures between the studied fossils and extant Podocarpus. These two fossil taxa together with P. pliomacrophyllus H. Chen et J.Y. Wu from the same horizon, revealed a thriving of Podocarpus in western Yunnan during the Pliocene. For the subgenus Foliolatus, its early representatives might originate from Australasia and spread northward to northeastern India at least during the late Oligocene. The Neriifolius subclade within the subgenus is much older than the estimate of molecular clock studies, and the Indochinese subclade is probably derived from a rapid diversification driven by the continuous uplift of Hengduan Mountains (HDM) with the accompanying climate fluctuations since the late Miocene. Moreover, the thriving of Podocarpus in the Pliocene of western Yunnan reveals a warm and humid climate during that period. The Pleistocene glaciation and the geographic barriers posted by the HDM might contribute to the modern distribution of Podocarpus in South China and Indochina Peninsula.