Reconstruction of Upper Siwalik palaeoecology and palaeoclimatology using microfossil palaeocommunities

Abstract

Upper Siwalik sediments, exposed in the vicinity of Chandigarh, have yielded microvertebrates in addition to large vertebrates, invertebrates (molluscs, ostracods and decapods), charophytes, pollen and spores. These assemblages derive from at least four horizons of the Tatrot and Pinjor Formations (spanning the period between ∼4 and 1.8 Ma). Among the microvertebrates, a diverse assemblage of micromammals, including rodents, insectivores and lagomorphs, has been recovered from these deposits. These accumulations have been found to be primarily of scatological origin. The most common microfossil communities belonged to pond, pond bank, wooded grassland, sandy plain, bushland and temperate montane habitats. Presence of gerbil rodents such as Tatera associated with lizards (e.g. Uromastyx, Calotes and scincid) in some deposits might indicate semi-arid conditions or intermittent dry periods. The replacement of survival-oriented cricetids by reproduction-oriented murid rodents in the Late Miocene is indicative of inter-annual seasonal variations (intensification of monsoons). Across the Mio–Pliocene boundary a change in the dietary habits among the murid rodents has also been observed. The Mid-Miocene non-grazers such as Antemus and Progonomys gave way to Late Miocene (Parapelomys and Karnimata) and Pliocene forms (Dilatomys, Bandicota, Nesokia, Hadromys, and Golunda), which were predominantly grazers. An increase in body size among the Pliocene murid rodents has been inferred from the size of their first upper molars. Within the Pliocene, across the Tatrot–Pinjor boundary (around 2.5 Ma), among the rodents a significant change at the generic level, leading to the appearance of modern forms such as Hadromys, Bandicota, Nesokia, Rhizomys, and Tatera, has been observed. This period (∼2.5 Ma) is marked also by a great diversification of murid species, probably indicating an intensification of monsoons. This interpretation is further strengthened by the occurrence of a diverse palynological assemblage reflecting alternately wet and dry conditions.