Abstract
The Late Miocene site of Shuitangba in southwestern China has yielded a diverse vertebrate fauna and abundant plant fossils from swamp-edge and shallow lake sediments, indicating a warm, humid climate with strong temperature and precipitation seasonality, and higher regional rainfall amounts than in modern times at the site. Subtropical, evergreen broad-leaved forest dominated the ecosystem, accompanied by aquatic vegetation, lianas, widespread grasses at woodland margins, and broad-leaved deciduous forest and evergreen trees and shrubs. This heterogeneous ecosystem formed a forest-wetland refugium in a time of global cooling and aridification, supporting water birds, crocs, fish, otters, apes, and other mammals typical of forests. Among the mammalian assemblage are the remains of the archaic stegodontid proboscidean Stegodon zhaotongensis , comprising the most complete skeletal record of basal stegodonts yet recovered. Craniodentally, St. zhaotongensis exhibits numerous plesiomorphies, some reminiscent of its stegolophodont predecessors, such as a low, broad cranium narrow across the temporal lines and extremely brachyodont molars with modest numbers of plates. Absence of lower tusks and enamel bands on upper tusks, lack of independent molar accessory conules, and possession of pentalophodont intermediate molars confirm the species to belong in Stegodon within Stegodontidae. Size and robusticity of appendicular elements show that St. zhaotongensis individuals were powerful and massive, considerably larger and heavier than extant elephants, implying an important role in the maintenance of its habitats. The morphology of this species supports the hypothesis that stegodonts are of Asian derivation from stegolophodonts, and that traits shared in common with elephants are convergences. It also corroborates the validity of the species. Despite the brachyodont condition of its molars, dental isotopic analysis shows that individuals of St. zhaotongensis had broad dietary preferences, including mixed C 3 /C 4 browsing/grazing, suggesting a shift in feeding behavior prior to evolution of morphological adaptations as a way of coping with changing ecological circumstances.