Fossil fruits of Koelreuteria (Sapindaceae) from the Miocene of northeastern Tibetan Plateau and their palaeoenvironmental, phytogeographic and phylogenetic implications

Abstract

Fruits of Koelreuteria are well known in the Tertiary deposits of western United States, eastern Asia and Central-East Europe, yet unequivocal reproductive organs of this genus from other areas remain rare in the fossil record. Here, a new species Koelreuteria quasipaniculata Xiangchuan Li, Liang Xiao et Wenlong He sp. nov. is recognized from the Early to Middle Miocene Garang Formation of Zeku in northeastern Tibetan Plateau, Central Asia. This new species is characterized by dehiscent, membranous capsules with an asymmetrical, narrowly ovate shape, and reticulate venation including subparallel, straight lateral veins and one to two ovoid or ellipsoidal seeds each valve. Its capsular valves closely resemble those members of the K. paniculata-type group in gross form, i.e. the extant K. paniculata Laxmann distributed in eastern Asia and the Oligocene-Pliocene K. macroptera (Kováts) Wilfred Norman Edwards from Central-East Europe. The newly discovered fossil materials corroborate the occurrence of Koelreuteria in the Tibetan Plateau during the Early to Middle Miocene, and add to the diversity of Koelreuteria in Central-East Asia during the Miocene, which has been well represented from the Cenozoic deposits of the northern hemisphere. This fossil record together with other co-occurring arborescent plants (e.g. Acer, Ulmus and Fraxinus) contributes to our understanding of past vegetation change in the northeastern Tibetan Plateau. Furthermore, the historical distribution range of Koelreuteria is significantly expanded by the present record into Central Asia formerly known as a geographic region lacking extant and fossil records.