The early Pleistocene aquatic palaeoecology in Central Java, Indonesia as recorded in mollusc assemblages from the Puren and Sangiran layers of the Sangiran Dome

Abstract

This study explores quantitative patterns recorded in mollusc assemblages with the goal of elucidating the palaeoenvironmental reconstruction, and the relationship between the established palaeodatasets of analysis and the levels of variability in palaeontological data. Mollusc fossil assemblages obtained from the Upper Puren to the Lower Sangiran layers of the early Man Site, Sangiran Dome in Central Java, Indonesia were analyzed by using bulk samples that were collected from the different facies of the dome which were deposited during the early Pleistocene. A total of 58,216 specimens from 18 samples were collected from the Blue Clay Puren, Corbicula Bed, Lower Lahar, and Black Clay Sangiran. All specimens were identified up to the genus level with a total of 61 mollusc taxa, which were further classified in terms of their ecology. These samples were then subsequently analyzed using ecological statistics methods. Specimens were compared based on the overall composition and taxa abundances, as well as the autoecological investigation of their life habit, substrate preference, depth range, and the ecological implication of the assemblages The Puren mollusc assemblage exhibits generally similar overall composition and ecological marine structure with those that inhabited the soft bottom sublittoral zone. This assemblage contained predominant species of the family Arcidae (Anadara sp., Arca sp. and Arcopsis sp.) and the genus Turritella sp. (Turritellidae) suggesting the development of a shallow body of water in Central Java of the Sunda arc around 2 million years ago (Ma). Species from the Corbicula Bed assemblages, characterized by the presence of numerous Corbicula, contained shell species that originated from both freshwater and marine environment. Mollusc assemblages from the Lower Lahar and Black Clay Sangiran were pure freshwater shells. The examined mollusc assemblages in the Lower Sangiran were associated with marine and swampy coasts and lotic ecosystems with landward flowing waters indicating landmass deposits. Their presence characterizes the start of marine episodes linked to shift in regression and continental lacustrine environments, and counterparts of Pleistocene glacials/interglacials and volcanic activities within the last 2 Ma. These lines of evidence suggest that the molluscan diversity in Sangiran Dome in Central Java is associated with sub-environmental diversity of the localities with patterns likely resulting from a combination of ecological, environmental, and taphonomic processes. Overall, this study illustrates the usefulness of fossil shells obtained from Sangiran Dome. Species richness of mollusc assemblages is useful in tracing the environment and its sequence of early Pleistocene succession of the Sangiran Dome in Central Java. The changing environment has resulted in significant shifts in the density and distribution of Sangiran molluscs as revealed by the assemblages that were deposited on the different facies of Puren and Sangiran layers. The approach provides a narrative of the relevant transformation of the aquatic ecology in Sangiran Dome. It also elucidates the sources of variability on paleontological patterns.