Paleocarpological studies of the early Eocene Jianglang site (Bangor County, central Tibetan Plateau): biogeographic and paleoenvironmental implications
Résumé
The Jianglang site from the central Tibetan Plateau, now at 4800 m of altitude, is dated to the early Eocene and is brand new for paleobotanists (Tang et al., 2019). Indeed, other sites with plant remains from the Tibet are not in this time span. Many fossil specimens from that new site were collected, including fruits and seeds. Among others, this site could greatly increase the understanding of not only local but also the global distribution pattern ofbiodiversity during the Paleogene, filling the gap between floristic relationship between the Tibetan Plateau and other parts of the world during the early Eocene. A first view of the fruits and seeds from Jianglang was compiled and show a great diversity of plant families, e.g. Simaroubaceae, Fabaceae, Menispermaceae, and Apocynaceae. Some fruits belonging to Ailanthus (Liu et al., 2019) and Lagokarpos (Tang et al., 2019) were already published. However, the site is still in need to more precise taxonomic studies for other taxa. Here we reported Menispermaceae and Illigera (Hernandiaceae) fruits that are rich (with Lagokarpos) in term of number of specimens. In addition, we studied a species attributed to the former Asclepiadaceae group (now Apocynaceae). We described 5 new species, which probably have close affinity to the Eocene North American, European and other Asian species. Collectively, the new paleocarpological data from Jianglang site shows that there is a tropical flora in the Tibet during the early Eocene, close to that we found in Europe and North America in the same epoch. This pattern could be explained by the expansion of an homogeneous boreotropical flora in the Northern Hemisphere during the early Eocene following the climate warming. The modern affinity of these flora is clearly from Asia but we may also argument early African link as shown by the presence of the Asclepiadaceae species. This tropical flora is also congruent with the hypothesis of no high uplift of the central Tibetan Plateau during the early Eocene.
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