New records of Podocarpium A. Braun ex Stizenberger (Fabaceae) from the Oligocene to Miocene of China: Reappraisal of the phylogeographical history of the genus

Abstract

Podocarpium (Fabaceae), characterized by single seeded pods and paripinnate compound leaves, is well known as an extinct legume genus having extensive fossil occurrences from the Miocene of Central Europe and eastern Asia. However, the Palaeogene unambiguous megafossils are quite scarce, especially the Oligocene record from eastern Asia, which heavily impedes our understanding of early dispersal patterns and infrageneric relationships of this enigmatic genus. Here, we described dozens of legume specimens from the Oligocene Ningming Formation of Guangxi in South China, the Early Miocene Guide Group of Qinghai in Northwest China and the Middle–Late Miocene Shengxian Formation of Zhejiang in Southeast China as Podocarpium podocarpum (A. Braun) Herendeen based on extremely similar fruit characters. Meanwhile, we reexamined variation in fruit morphology among the previously reported fruits of Eurasia and present legumes as a basis for interpreting the infrageneric relationship and subfamily affinity of Podocarpium. The high similarities presented by these fossil fruits support the previous recognition of the specimens from the Oligocene–Miocene of Central Europe and the Miocene– Pliocene of eastern Asia as P. podocarpum, but such a low species diversity of Podocarpium appears to indicate a bradytelic morphological evolution for this whole ancient lineage through geologic time. Moreover, judging from seed orientation as well as seed and fruit comparative sizes, the oblong seed is parallel to the long axis of the fruit and its placentation is near the apex of the seed chamber, which favor the previous assignment of Podocarpium to the subfamily Caesalpinioideae.Additionally, we speculated that this extinct genus may prefer a warm and humid (or mildly arid) environment by plotting the megafossil localities on the overall Eocene, Oligocene and Miocene palaeoclimate maps based on continental drift. The Oligocene occurrence from the Ningming Formation in Guangxi, South China sheds new light on the historical biogeographical pattern between Central Europe and eastern Asia, i.e. after the probable origin in low latitude tropic coastal regions of southernmost South China (i.e. Hainan) by the Middle Eocene, Podocarpium may subsequently disperse into Guangxi and then Europe via a probable low-latitude land connection located among the Tethys and Paratethys seas to the southwest of Eurasia in the Early Oligocene, independent of the gradual closure of the Turgai Strait in the middle-high latitudes during the Oligocene.