Cenozoic paleoecological evolution of NE Tibet: A review of magnetostratigraphically-constrained mammal records

The northeastern Tibetan Plateau (NETP) is an ideal laboratory for investigating the interplay between tectonics, climate and biotic evolution. Cenozoic sedimentary basins in this region have recorded ecological shifts linked to the evolution from a wetter monsoonal to an arid climate, in response to the formation of the Tibetan Plateau and to global climate variations. Through decades of intense investigations from various international groups, the Longzhong Basin (LB) in the eastern segment of the NETP represent one of Asia's best dated continental records with a comprehensive set of climate proxies that have allowed major advances in the reconstruction of paleoenvironmental conditions and landscape evolution, as well as understanding of their driving forces. The LB also preserves a particularly rich and well-documented mammal fossil record offering key insights for reconstructing paleoecological conditions by analyzing taxonomic compositions and faunal characteristics. However, a comprehensive review synthesizing previous faunal record studies—which have primarily focused on individual sub-basins of the LB—remains to be performed in order to develop an understanding of regional patterns and drivers of ecological shifts. In this review, we compile the best-dated and richest mammalian fossil records of the LB in the Lanzhou, Xining, and Linxia sub-basins. We integrated all published mammalian fossil taxa from these sub-basins into a re-evaluated magnetostratigraphic framework dating back to the Oligocene. Subsequently, we assigned ecological scores to reflect the environments inhabited by these fossil assemblages, thereby enabling a semi-quantitative reconstruction of their habitats over time and space. Our results suggest that the LB region was characterized by open woodland during the Oligocene–Early Miocene, transitioned to dense forest during the Middle Miocene, and evolved into savanna- and steppe-dominated environments beginning in the Late Miocene. These paleoecological changes, as supported by regional mammalian fossil- and pollen-based ecology studies, strongly correlate with monsoonal precipitation patterns previously reported from terrestrial and marine records. Thus, a progressive intensification of East Asian monsoon precipitation in the LB from late Oligocene to the Early Miocene, a pronounced upsurge during the Middle Miocene, and a large periodic reduction from the late Middle Miocene to the Quaternary can be inferred. Furthermore, integrating these new paleoecological insights with published data on sedimentary facies, provenance, and sedimentation rates allows us to elucidate the regional paleoecological and geomorphological evolution extending to the late Eocene. Under the combined influence of orogenic uplift and global cooling, the LB region gradually evolved from low-elevation, low-relief landscapes characterized by drier conditions and open woodland environments following the Eocene–Oligocene Transition to relatively high-elevation, high-relief landscape featuring pronounced monsoonal seasonality, persistently drier conditions, and ecosystems dominated by C4 plants during the Late Miocene–Quaternary.