Late Pleistocene pelagic gastropods of southern Taiwan: paleobiodiversity, first fossil records, and regional affinity

Holoplanktonic gastropods (pteropods and heteropods) are major components of modern Indo–West Pacific (IWP) plankton, yet their fossil record in this region remains sparse. Expanding the spatial and temporal coverage of fossil data is essential for reconstructing dispersal pathways of pelagic fauna within the IWP and understanding the origins of present-day diversity. Here, we describe a newly discovered Late Pleistocene assemblage of pelagic gastropods from southern Taiwan. The assemblage comprises 14 pteropod and eight heteropod taxa, most representing the first fossil records of holoplanktonic gastropods from Taiwan. We also evaluate variation in paleobiodiversity between depositional environments of the Szekou Formation. Species richness and density do not differ significantly between restricted and open lagoon settings, contrasting with patterns reported for benthic bivalves. To place these findings in a broader context, the newly reported assemblages were analyzed together with other Pleistocene assemblages across the IWP region. Only weak geographic and temporal separation was detected, suggesting a relatively cosmopolitan community composition in subtropical waters during the Pleistocene, likely reflecting low temperature variability despite glacial–interglacial cycles. Indicator species analysis further suggests a later arrival at higher latitudes for the pteropods Telodiacria quadridentata and Heliconoides inflata , which show associations with late Pleistocene sites, whereas Styliola subula displays a distribution resembling its modern range, being most closely associated with assemblages from Taiwan and southern Japan.