Middle Eocene aridification drove changes in Asian mammal faunas prior to the Eocene–Oligocene Transition

The Eocene–Oligocene Transition (EOT; 34 Ma) is characterised by global cooling, growth of the Antarctic ice sheet, sea-level drop, aridification, and faunal turnover. In Asia, previous studies have shown a transition from mammal communities dominated by large-bodied perissodactyls to those dominated by smaller-bodied Glires (rodents and lagomorphs) better adapted to a dry environment with limited resources. However, more recent paleoenvironmental records show that drying was already occurring in Central Asia during the middle Eocene due to global cooling, the growing Tibetan Plateau, and the retreating inland Paratethys Sea. Using fossil mammal occurrence records from the Paleobiology Database, the NOW database and from the primary literature, we applied a common sampling standardisation technique (shareholder quorum subsampling; SQS) to reconstruct sampling-corrected fossil mammal species and genus richness across Central and East Asia. Our results show a significant increase in biodiversity across the Paleocene/Eocene boundary, followed by high total mammal richness during the hothouse climate of the early Eocene. We recover substantially lower diversity in the middle Eocene, concurrent with the onset of aridification in this region according to proxy data and climate model results. Mammal richness increased throughout the remainder of the Eocene, but declined again across the EOT and throughout the Oligocene. In addition, we recover increased mammal turnover and decreases in mean and maximum body size across the Paleocene/Eocene boundary, in the middle Eocene and across the EOT, concurrent with shifting environmental conditions. This suggests that regional mid-Eocene aridification in Central Asia may have driven faunal shifts similar in magnitude to those linked to the global cooling event at the EOT, necessitating a reappraisal of the timing and patterns of early Cenozoic mammal evolution in Asia. Furthermore, although previous studies found evidence for a transition from perissodactyl-dominated to rodent-dominated faunas across the EOT, our analyses indicate a diversity drop in both perissodactyls and rodents across the EOT, suggesting that this signal could be at least partially influenced by sampling biases in the fossil record.