Biogeochemistry at the crossroads between environment and ecology: use of oxygen isotopes to track environment preference variation in fossil crocodilians

International audienceThe Shungura Formation of the Lower Omo Valley (southwestern Ethiopia, dated between 3.6 Ma and ⁓1.0 Ma) recorded major environmental change occurred during the Plio-Pleistocene (aridification, landscape opening, intensification of seasonality, change in hydrography). In these conditions, the continuous persistence of several crocodilian species, throughout the time interval and different environments, is explained either by their adaptation to habitats that remain available despite global change, or by their adjustment to new habitats over time. As for extant crocodilians, we suspect that past species were able to occupy a wide range of habitats, so that the diversity of habitats they occupied documents at least a significant part of available aquatic environments.We used stable isotope geochemistry (oxygen) recorded in crocodilian teeth to describe the environment(s) occupied by the fossilized specimens from different levels across the Shungura series. Indeed, the isotopic oxygen composition of each tooth is known to be mainly dependent on that of the water ingested during dental formation (nearly during a year). The ingested water corresponds to the water of the habitat, which is itself closely related to the isotopic composition of local and upstream rainwaters, as well as evaporation conditions. The distribution and range of part of the signal indicate the diversity, nature, and conditions of the occupied aquatic environments.The use of this approach on the fossil crocodilians from various horizons of the Shungura Formation shows they occupied different environments, and alternately wide or narrow range of habitats over time. Comparison with independent information on past environmental contexts of different levels strengthen these conclusions