Fossil involucres and a nutlet of Ostrya (Betulaceae) from the upper Eocene of Shaanxi and their biogeographic implications

Abstract

Ostrya Scopoli, family Betulaceae, is mainly distributed in the Northern Hemisphere, with a rich fossil record in the Miocene. This study presents a new record of Ostrya involucres and nutlet fossils from the upper Eocene Bailuyuan Formation in Weinan City, Shaanxi, China. The fossil involucres are saccate, ellipsoid or ovate, with reticulate veins; the nutlet is ellipsoid-ovate, one side surface has five ribs, and is wingless. Based on morphological comparisons, a new species, Ostrya parajaponica Huang and Jia, n. sp. is proposed, which represents the earliest fossil record of Ostrya involucres globally. In this study, the historical distribution of Ostrya is investigated, and it is speculated that Ostrya originated in Asia before the late Eocene. During the late Eocene, Ostrya may have spread from Asia to North America via the Bering Land Bridge and then spread from North America to Europe via the North Atlantic Land Bridge. During the Oligocene, the closure of the Turgai Straits may have facilitated widespread exchange of Ostrya across the Eurasian continent. During the Miocene, during warm climates, Ostrya was widely distributed across the American and Eurasian continents. Subsequently, the cooling and drying of climates beginning in the Pliocene shaped the current geographical distribution pattern of Ostrya.