A new fossil record of Palaeosinomenium (Menispermaceae) from the Upper Eocene in the southeastern margin of the Tibetan Plateau and its biogeographic and paleoenvironmental implications

Palaeosinomenium Chandler (Menispermaceae) was established to accommodate the fossil endocarps that share similar morphological characteristics with the tribe Menispermeae in Menispermaceae. Previous fossil records suggested that Palaeosinomenium was distributed in North America, Europe, and East Asia before the Middle Eocene. A new fossil species, Palaeosinomenium hengduanensis Meng-Xiao Wu et Zhe-Kun Zhou sp. nov., was es-tablished based on an endocarp impression fossil from the Upper Eocene (35 +/- 1 Ma) Shuanghe Formation, Jianchuan Basin, southwestern China. The new species is characterized by a horseshoe-shaped endocarp, an excavated central area, surrounded by a slightly asymmetrical C-shaped lateral ridge, and an elliptic aperture lo-cated near the longer endocarp limb. The fossil site is located in the modern distribution area of the living species Sinomenium acutum (Thunb.) Rehd. et Wils, a potential nearest living relative of the new species. The finding of P. hengduanensis supports that the divergence within the tribe Menispermeae might have occurred by the Late Eocene and the species similar to modern S. acutum appeared in the southeastern margin of the Tibetan Plateau as early as in the Late Eocene.(c) 2022 Elsevier B.V. All rights reserved.