Plio-Pleistocene environmental shifts in the Bumiayu sub-basin revealed by mollusca and facies records: The environmental setting of a potential coastal dispersal route for large mammals into Sundaland

The Bumiayu sub-basin of western Central Java preserves an important Plio–Pleistocene transition from shallow-marine to increasingly marginal-marine and terrestrial environments along the southern margin of Sundaland. This study reconstructs the environmental evolution of the Kalibiuk–Kaliglagah succession using integrated sedimentary facies analysis, molluscan paleoecology, mangrove-associated indicators, and vertebrate taphonomic observations. The results document a systematic upward shift from storm-influenced shallow-marine deposits to estuarine, lagoonal, mangrove-influenced, and ultimately fluvio-deltaic settings. Quantitative analyses of molluscan assemblages, including diversity indices, ecological ratios, hierarchical clustering, and Principal Component Analysis (PCA), show a coherent turnover from marine and brackish assemblages to increasingly freshwater-dominated communities. These trends are consistent with progressive reduction of marine influence under repeated transgressive–regressive oscillations, likely driven by the interaction of relative sea-level change, sediment supply, and local basin controls. Root traces, crustacean remains, and carbonate nodules indicate the development of vegetated intertidal flats and mangrove-related environments during sheltered phases of deposition. Two Bovidae specimens a humerus (BMY.25.GLG.32) and a femur (BMY.25.GLG.33) preserved above the lagoonal black clay provide additional evidence for the presence of terrestrial vertebrates within this coastal transition zone. Although the stratigraphic context of hominin remains from Bumiayu remains uncertain, the reconstructed succession provides an important paleoenvironmental framework for evaluating vertebrate habitats and regional environmental connectivity in western Central Java during the Plio–Pleistocene.