Fossil herbivores and crocodiles as paleoclimatic indicators of environmental shifts from Bed I and Bed II times of the Olduvai Gorge, Tanzania

Abstract

The radiation and extinction of East African vertebrate fauna have been linked to key climatic changes that occurred around 2.8, 1.7, and 0.5 Ma. This study tests whether vegetation and diet changes were linked to these climatic fluctuations, focusing on the climatic event at 1.7 Ma, by analyzing the stable isotope composition of fossil herbivore teeth from the Olduvai Gorge's stratigraphic units Upper Bed I, Lower Bed II, and Upper Bed II, which span ~1.83–1.33 Ma, in order to detect shifts in herbivore diets from C3 to C4 plants in conjunction with their drinking water isotope geochemistry. C4 grass expansion is known to be favored by drier conditions, and this study tests whether herbivore diets permanently changed to being C4 dominant at this key time period. Crocodile teeth are also tested to determine if they may be used as paleoclimatic indicators, since large crocodiles prey on terrestrial herbivores in wetland settings.Vertebrate bioapatite carbon and oxygen isotope compositions are found to show similar trends as those of pedogenic carbonates (carbonate nodules, or concretions), which reflect a drying environment that was changing from C3 woodland dominated habitats to mixed woodlands and grasslands from Bed I to Lower Bed II times. While a return to more humid conditions in Upper Bed II might be expected to lead to a reversal to C3 plants, herbivores in this study retained mostly the drought-resistant C4 plant diet. These results indicate that vegetation and ecosystems passed through a threshold to C4 domination that did not revert to C3 despite partial reversals in climate that occurred later. Crocodile teeth were not useful in tracking C4 plant expansion, but the oxygen isotopes from modern crocodiles are able to show seasonal cycles from wet to dry seasons, and Bed I crocodiles indicate a degree of annual seasonal changes during that time as well.