Sri Lankan snail diversity: faunal origins and future prospects

WA Museum Records and Supplements | Updated 7 years ago

ABSTRACT – Sri Lankan snails have a complex history that might be linked to events ranging from Mesozoic plate tectonics and climate change on a global scale to more recent regional climate change and human impact. The fauna could include ancient groups that may have been present in Pangaea, diversified in Gondwana and survived in Deccan Plate refugia at the KIT boundary and Pangaean taxa that arrived via Eurasia. Taxa could have been assimilated from continental fragments encountered during the northward rafting of the Deccan Plate. Fauna colonising immediately following Eurasian contact could have been a regional fauna associated with Tibetan plateau plate fragments. There may have been a brief period when an Arabian corridor was open to African faunas and there was likely to have been a succession of immigrants through the Assam corridor or through long distance dispersal. The presence of Acavus and Oligospira as endemic acavoids in Sri Lanka and the complete absence of acavoids from India as well as the presence of at least 10 species of CoriIla and only one in India, give a greater potential Gondwanan identity to Sri Lanka's snail fauna. A similar pattern with Tortulosa may also have ancient origins. It is possible, but perhaps doubtful, that this picture has been sustained since the KIT boundary. A more likely explanation might be that during successive Deccan Trap lava flows Gondwanan snails survived as a small relict fauna in a southern Deccan Plate refuge and possibly a north eastern refuge. Snail faunas could have later dispersed across the intertrappean and supratrappean landscape from a refuge or refugia to be largely replaced in India by a subsequent combination of climate change and competition from a succession of colonizing species. A notable feature of the Sri Lankan snail fauna is the level of endemism and lack of shared components with India at the species level. A high level of endemism may have been present in the highland areas of Sri Lanka through the Cenozoic and earlier, and current endemism may be a mixture that includes Mesozoic elements. However, species turnover could be high and many of the current species could have become established in the Quaternary. Most of the Sri Lankan land snail species diversity and endemism is confined to forests in the wet zone of the Southwest. The maintenance of high levels of endemism and generation of high species diversity in a confined area through long periods is thought to be linked to the varied topography in Sri Lanka and the capacity for altitudinal faunal shifts in response to climate change. The remaining fragmented forests in a mosaic of transformed habitats no longer offer such capacity for altitudinal adjustment giving a poor prognosis for the long-term survival of the fauna.