Isotopic reconstruction of Proboscidean habitats and diets on Java since the Early Pleistocene: Implications for adaptation and extinction

Since its sub-aerial emergence, Java has experienced multiple tectonic, geographic and climatic changes, which affected the megafaunal occupation, adaptation and succession. Six Proboscidean taxa have been found from various localities throughout Java extending back to the Early to Late Pleistocene. The six taxa are: Stegoloxodon indonesicus, Sinomastodon bumiajuensis, pygmy Stegodon sp., Stegodon trigonocephalus, Elephas hysudrindicus and Elephas maximus, in which respective taxa are included in successive faunal stages. The aim of this research was to reconstruct the succession of Proboscidea in Java and the adaptation of each taxon to environmental changes by incorporating stable isotope analysis with the fossil faunal record, geology and stratigraphy. We conducted stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotope analysis of the carbonate phase in the tooth enamel of six proboscidean taxa from numerous localities and ages. Our results suggest that for the earliest terrestrial fauna from Java, represented by Stegoloxodon indonesicus, the feeding ecology was restricted to a closed canopy rainforest during the earliest Pleistocene, while towards the late Early Pleistocene, the successive taxon, Sinomastodon bumiajuensis, was adapted to different or increasingly drier grassy habitats. C₄ expansion on the island took place since the later part of the Early Pleistocene (before 1.5 Ma) and continued until the Middle Pleistocene, as suggested by the carbon and oxygen isotope composition of assemblages of proboscidean taxa from Sangiran, Kedung Brubus, Trinil, and western Java, which also predominantly display the expected range of C₄-dominant feeders. However, the occurrence of mixed C₃/C₄ feeder Proboscidea in the Ngandong Fauna suggests that fragmented dense evergreen forests, shrubby or woodland vegetation reappeared towards the end of the Middle Pleistocene. This environmental shift is detected in the later stage of the Middle Pleistocene, as the dietary preference of all analysed samples from individuals from this age shifted back from a C₄-dominated into a C₃-dominated diet, which suggests a change from a dry to more humid climate conditions.