Late Miocene occurrence of monogeneric family Oleandraceae from southwest China and its implications on evolution of eupolypods I

Yunnan in SW China is a world renowned hotspot for diverse species of vascular plants such as ferns. However, fossil records of the Cenozoic ferns there are insufficient to clarify their phylogeny and historical biogeography from a geological perspective. Among these derived ferns, the monogeneric family Oleandraceae, with a pantropic distribution, is natural and distinctive; however, its origin and evolution remains unknown because of the absence of fossil records. In this study, we identified a new fossil species belonging to the genus Oleandra cav. (Oleandraceae s. s.) from the upper Miocene of Yunnan, China based on a detailed comparison of morphologically similar genera and within this genus. Oleandra bangmaii sp. n. is characterised by a simple fertile frond with an entire cartilaginous margin. Moreover, the venation system in this genus is unique and comprises a prominent midrib along with parallel, closely-spaced secondary free veins. The sori are dorsally borne, with round kidney-shaped indusia. It represents the first fossil occurrence of the genus Oleandra (Oleandraceae) within its extant distribution, suggesting that its fossil history has persisted for at least 10 Ma in Asia. Our finding, together with other existing evidence, suggests that the crown of eupolypods I (the node including families Oleandraceae, Davalliaceae and Polypodiaceae) has diversified and that the climate and vegetation of Yunnan have remained relatively stable since the late Miocene.