Rising Himalaya and climate change drive endemism in the Western Ghats: Fossil evidence insights

Abstract

Endemism, driven by geological and climatic transformations, is a hallmark of biodiversity hotspots. Fossil evidence offers unique insights into the historical biogeography and evolutionary trajectories of endemic taxa, particularly in biodiversity-rich regions like northeast India and the Western Ghats. This study reports the discovery of two Nothopegia species (Anacardiaceae) from late Oligocene sediments in the Makum Coalfield, Assam, northeast India. These fossils represent the earliest global record of the genus and display striking morphological similarities to the extant Nothopegia travancorica and N. castaneifolia, currently restricted to the Western Ghats, indicating a significant biogeographic shift over millions of years. Paleoclimatic and paleolatitudinal reconstructions suggest that the late Oligocene climate of northeast India mirrored the present-day equable climate of the Western Ghats, enabling the survival of Nothopegia in the region. However, the uplift of the Himalayan-Tibetan Plateau and a pronounced decline in the cold month mean temperature (CMMT) across northern and northeastern India likely drove the extinction of Nothopegia from these areas, confining it to the warmer environments of the Western Ghats. Present fossil evidence, along with previous fossil records of other plant taxa, underscores deep-time floristic connectivity between northeast India and the Western Ghats, likely facilitated by evergreen Paleogene forest corridors. These findings illuminate the biogeographic history of megadiverse ecosystems of South Asia and highlight the interplay between climatic shifts and geological events in shaping endemic biodiversity. Furthermore, the study demonstrates the critical role of paleobotanical data in guiding modern conservation strategies, particularly for relict taxa confined to isolated biodiversity hotspots.