Fossil tragulids of the Siwalik Formations of southern Asia

Tragulids are common in the Early Miocene through Pliocene Siwalik faunas of the Indian Subcontinent where they are represented by as many as 17 species. Large collections of Siwalik fossils have recently been made by collaborative projects from Harvard University, the University of Arizona, the Geological Survey of Pakistan, and the Pakistan Museum of Natural History. The collections together comprise over 3700 specimens, including dental, cranial, and postcranial elements. Most of this fossil material is from northern and southwestern Pakistan from well-dated stratigraphic sections. The oldest definite tragulids are from the Early Miocene Vihowa Formation and are around 18.7 Ma, while the youngest are in the Pliocene Tatrot Formation and are 3.3 Ma. The fossil tragulids of the Siwaliks differ from the extant species in a number of ways. Importantly, they have a much wider range of body sizes, ranging from 1 to nearly 76 kg. Consequently the small species overlap with the smallest species of extant Tragulus, while the large species approach medium size bovids and cervids. Compared to other ruminants, Siwalik tragulids are also relatively abundant and species rich. Although the status of some described species is uncertain, preliminary analysis indicates there are many as yet undescribed species. Three genera are known and typically at least four species co-exist at any one time during the Miocene. The history of the south Asian tragulids can be correlated to documented environmental changes. The Siwalik deposits formed in a large fluvial system, with mostly forested or wooded low relief floodplains having abundant cover and fruit. Isotopic analyses of tooth enamel and soil carbonates indicate the vegetation was dominated by C3 plants until 9 Ma, after which there was a shift to a more seasonally dry monsoon climate, undoubtedly accounting for a Late Miocene change in the relative abundance of tragulids.