New Material of Karakoromys (Ctenodactylidae, Rodentia) from Late Eocene-Early Oligocene of Ulantatal (Nei Mongol): Taxonomy, Diversity, and Response to Climatic Change

The Eocene-Oligocene Transition (EOT) was one of the most profound climate changes in the Cenozoic era, characterized by global cooling around 34 million years ago. This time period also witnessed major faunal turnovers, such as the "Mongolian Remodeling" of Asia, characterized by the dominance of rodents and lagomorphs after the EOT. Previous studies have primarily focused on overall faunal change across the EOT. Here, we examined one genus, the earliest ctenodactylid Karakoromys, based on rich fossils from continuous sections at Ulantatal, Nei Mongol, magnetostragraphically dated to latest Eocene-Early Oligocene. Based on a systematic paleontological study of these fossils, we recognized four species of Karakoromys (Karakoromys decussus, K. arcanus, K. chelkaris, and K. conjunctus sp. nov.), indicating a relatively high diversity of the most primitive ctenodactylids during the latest Eocene-Early Oligocene (similar to 34.9-30.8 Ma). The turnover of ctenodactylids primarily occurred during a regional aridification event around 31 Ma rather than during the EOT cooling event, suggesting that regional precipitation variation in the semi-arid area may have played a more important role than global temperature change in the evolution of early ctenodactylids.