Fossil Samaras of Acer in the Lower Miocene of Central Inner Mongolia, China, and Their Phytogeographical Implications

Samara fossils of Acer were unearthed from the Early Miocene Hannuoba Formation in central Inner Mongolia, China. Based on macro- and micro-characteristics, they were identified as Acer pretataricum sp. nov. of section Ginnala, and cf. Acer mono of section Platanoidea. We reconstructed the dispersal routes of these two sections according to their fossil records. During the Early Eocene, section Ginnala was confined to North America. In the Late Eocene, this section expanded westward to East Asia. It was distributed widely in East Asia. In the Late Tertiary, it potentially recolonized the European and American continents. Meanwhile, section Platanoidea was distributed disjunctively in East Asia and North America during the Eocene. Members of this section likely expanded westward from East Asia into Europe in the Oligocene. By the Miocene, it had achieved extensive distribution in the Northern Hemisphere. However, by the Pliocene, it was confined to East Asia and Western Europe. It may have spread eastward from East Asia to North America during the Holocene, finally forming its current existence in North America and the Eurasian continent. This investigation reveals distinct differences in the dispersal pathways of two sections, suggesting that the reconstruction of dispersal routes for Acer taxa should be conducted separately at the section level.