UTILIZING STRATIGRAPHY AND STABLE ISOTOPE GEOCHEMISTRY TO RECONSTRUCT THE ANCIENT ENVIRONMENTS OF OUR PRIMATE ANCESTORS: LOPEROT, GREAT RIFT VALLEY, KENYA

Apes and monkeys split from a common ancestor during the late Oligocene and despite their abundance in East Africa, are only rarely found together at the same site. However, a site located on the western side of Lake Turkana called Loperot, in northwestern Kenya, preserves an Early Miocene primate community that contains both ape and monkey taxa and provides a unique opportunity to study the paleoenvironmental conditions that suited both primates. The monkey and apes that we were identified form a new and unexpected primate community that raises the question, “What geologic and/or environmental factors caused these otherwise segregated primate species to co-mingle at Loperot?”

During the 2016 field season, we excavated a ~28m step-trench and measured 30 individual sedimentological units. Fossils found within the Loperot region were correlated to units within the step trench; specifically, a crocodile skull (CS), a monkey mandible (MM), and an ape molar (AM). In each fossil-bearing unit, we collected rhizoliths (n=123). We drilled individual rhizoliths every half-millimeter across the short axis in an offset pattern to obtain powdered microsamples for isotopic analysis in order to assess local meteoric water conditions (𝛿¹⁸O) and the abundance of C3 or C4 plants (𝛿¹³C).

Our data indicates that rhizoliths from unit MM have a higher average 𝛿¹³C value (-8.23‰) compared to the other two units (CS 𝛿¹³C = -10.51‰, AM 𝛿¹³C = -10.58‰). We interpret this to mean that when monkeys inhabited the region, the landscape was an open, mixed C₃-C₄ habitat representing a grassy woodland or open shrubland. However, during deposition of the CS and AM units (when crocodiles and apes were more abundant) the region was more dominated by C₃ vegetation or a forest or woodland habitat. Based on the stratigraphic position of the CS, MM, and AM units, we propose that the early Miocene environment at Loperot shifted from woodland to grassy woodland/open shrubland and back to woodland within ~10m of stratigraphy, most likely due to the migration of a large meandering river and its adjacent riparian forest. As the river (and the associated riparian forest) moved across the region, the primates followed their preferred habitats.