Stable isotope analyses of tooth enamel carbonate of large herbivores from the Tugen Hills deposits: Palaeoenvironmental context of the earliest Kenyan hominids
Résumé
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 δ¹⁸O between hippos and terrestrial fauna. Moreover, the giraffid specimen from the Mabaget Formation exhibits one of the highest δ¹⁸O values, as expected for a drought-tolerant animal. The δ¹³C of large herbivores indicates that the earliest Kenyan hominids inhabited a mixed C₃-C₄ 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 C₃ diet which reflected browsing behaviour. The intake of C₄ plants prevailed for equids and gomphotheres and was consistent with grazing habits. Most hippos and suids were mixed C₃-C₄ feeders and consequently were generalist herbivores. Bovids and rhinos had a diet ranging from C₃ to C₄ plants. Variations in δ¹³C and δ¹⁸O 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 C₃ 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.
Domaines
Sciences de la Terre
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