Stable isotope composition of crocodilian teeth provides new information on climatic change in the East-African Rift along the Plio-Pleistocene period (Shungura Formation, Lower Omo Valley Ethiopia)

Isotopic fluctuation of certain stable isotopes, notably Oxygen, provide important information on paleoenvironmental change along geological times. However, interpreting isotopic change along continental series depends on our ability to understand its recording, for instance in soils or in mammal teeth. In the case of continental series yielding most of available information on hominid diversification and expansion within and beyond Africa, isotopic information even seems to show discrepancies depending on the archive. In our study, we use isotopic composition in crocodilian tooth enamel. We assume that, for these ectotherms that regulate their temperature, isotopic composition recorded in their teeth mainly depends on drinking water, itself depending on precipitation. Moreover, crocodilian fossil teeth are abundant and widely distributed within continental series, thus constituting an interesting archive. We sampled crocodilian teeth from the Shungura Formation (Lower Omo Valley, Ethiopia), which spans major steps of human evolution between 3.6 Ma and ~1.0 Ma, tentatively correlated with major environmental changes in eastern Africa (intensification of seasonal contrasts, increasing aridity and landscape opening). The analyses of δ¹⁸O of hundreds of crocodilian teeth have identified environmental changes. Whereas the isotopic composition of paedogenic carbonates displays a different trend over time, that of crocodilian teeth relates changes already observed in mammal teeth, notably a major shift between 2.6 Ma and 2.3 Ma toward more arid conditions. Our study indicates that crocodilian teeth are a relevant archive of environmental change in continental contexts, and calls for further study to strengthen interpretations of isotopic composition in fossil archives.