Ventilago (Rhamnaceae) Fruit from the Middle Eocene of Central Tibet, China

Abstract

Premise of research. The fossil record of Rhamnaceae is geographically widespread and ranges from the Cretaceous to the Pleistocene, but only a few fossil fruits, namely, Ventilago engoto from the Oligocene of Mexico and V. lincangensis from the late Miocene of southwestern China, have been attributed to the tribe Ventilagineae based on the recognition of their typical fruits with an apical wing. Here we document new fossils from the middle Eocene of Jianglang, central Tibetan Plateau. We discuss their systematic affinity, the biogeographic history of the tribe, and the paleoenvironment in central Tibet on the basis of this discovery. Methodology. Fossil fruits were studied using a smart digital microscope and were extensively compared with the relevant literature as well as with specimens of extant species from herbaria. Pivotal results. These fossil fruits conform to the typical fruit type of Ventilago in having a prominent apically extended elongate-elliptical wing appendage and a hypanthium enclosing two-thirds of a seed chamber. We describe a new species, V. tibetensis sp. nov., based on the conical shape of the hypanthium, the pointed apex of the seed chamber, and the ratio of the length of the fruit and the seed chamber, which differ from other fossil records of Ventilago. Conclusions. Ventilago tibetensis is the earliest fossil record of Ventilagineae, and the age of the diversification of Ventilago should be reevaluated. The Indo-Tibetan area may represent the region of origin of this genus. It is difficult to directly assess the paleoenvironment of Jianglang solely on the basis of the occurrence of V. tibetensis, even if the group mainly lived in a subtropical to tropical area in Asia, but together with previous paleobotanic studies, the floristic assemblage indicates a subtropical flora in central Tibet during the middle Eocene.