Fifty Years in the Foothills: Ecosystem Evolution in the Neogene Siwalik Record of Pakistan

Abstract

The continental fossil record has exceptional, long sequences of fossiliferous strata that are the basis for evaluating ecosystem dynamics and their formative influences. The Siwalik sequence of South Asia is one example. It occurs in the Potwar Plateau (Punjab Province, Pakistan) and spans 18–1 Ma. The sequence consists of alluvial sediments deposited in a foreland basin created by the collision of the Indian and Eurasian tectonic plates. Sediments representing large and small river channels and their associated floodplain deposits correspond to mountain-sourced large rivers and foothill-sourced smaller rivers. Vegetation attributes are recorded in stable carbon isotopes and biomarkers in paleosols. Molluscs, fishes, crocodilians, turtles, lizards, snakes, birds, and mammals are preserved throughout the sequence. Mammalian faunas had exceptionally high species richness (116 species) at their peak and included up to 18 species of co-occurring megaherbivores (>800 kg). Significant changes over time in species richness, taxonomic composition, and ecological structure of mammalian faunas coincided with major changes in climate and vegetation. ▪ Siwalik strata and fossils document a long continuous sequence of South Asian continental sediments and ecosystems south of the Himalaya Mountains. ▪ A multidisciplinary analysis of tectonics, fluvial systems, climate history, and vertebrate diversity documents ecosystem dynamics from 18 to 6 million years ago. ▪ A sparse portion of the Siwalik record coinciding with the Miocene Climatic Optimum raises the possibility that humid heat stress limited occupancy of the floodplain by most mammals for much of this time. ▪ The timing and magnitude of change in mammalian species richness and ecological structure are consistent with environmental forcing as a significant influence on these features.