Stable isotope analyses of tooth enamel carbonate of large herbivores from the Tugen Hills deposits: Palaeoenvironmental context of the earliest Kenyan hominids

摘要

Understanding shifts in past climatic and environmental conditions are crucial for throwing light on human evolution. Available reconstructions of the palaeoecology of faunal and floral assemblages indicate that the earliest Kenyan hominids, including Orrorin tugenensis, are associated with forest landscapes. In this study, we present stable isotope data of tooth enamel carbonate of large herbivores associated with these hominids in order further to evaluate their environmental context. Fossil teeth were sampled in the Lukeino Formation (6.1-5.7 Ma, Late Miocene) and the Mabaget Formation (5.3-4.5 Ma, Early Pliocene) at the foot of the Tugen Hills (Great Rift Valley). Despite chemical changes in fossil enamel, preservation of the palaeoenvironmental signals is supported by the difference in δ~(18)O between hippos and terrestrial fauna. Moreover, the giraffid specimen from the Mabaget Formation exhibits one of the highest δ~(18)O values, as expected for a drought-tolerant animal. The δ~(13)C of large herbivores indicates that the earliest Kenyan hominids inhabited a mixed C_3-C_4 environment as did Ardipithecus (Ethiopia) and Sahelanthropus (Chad), two early putative hominids. The Tugen Hills might have been a wooded grassland with patches of woodland. There is no record of closed-canopy forest at the time of deposition. We emphasize differences in food habits among herbivore groups. Deinotheres, nearly all elephantids and the giraffid individual had a C_3 diet which reflected browsing behaviour. The intake of C_4 plants prevailed for equids and gomphotheres and was consistent with grazing habits. Most hippos and suids were mixed C_3-C_4 feeders and consequently were generalist herbivores. Bovids and rhinos had a diet ranging from C_3 to C_4 plants. Variations in δ~(13)C and δ~(18)O among faunal assemblages suggest changes in local climate and vegetation. Moister conditions likely occurred in the Early Pliocene and the latest Miocene (ca. 5.7 Ma) than before. The C_3 plant biomass would have increased in the latest Miocene. There were probably denser stands of trees within the depositional environment during the Early Pliocene than in the Late Miocene, which is consistent with palaeontological studies.