CROCODILIAN ENAMEL STABLE ISOTOPES REVEAL 16 MILLION YEARS OF HYDROCLIMATE AND ECOLOGICAL HISTORY IN THE TURKANA BASIN

Crocodilian enamel has historically been overlooked in stable isotope analyses and paleoecological and paleoenvironmental reconstructions, even though the semiaquatic behavior and pantropical presence of crocodilians could provide novel insights into ecological structure and long-term climatic trends. Due to concerns with homeostatic temperature fluctuations causing isotope fractionation, reptilian enamel has been largely removed from previous analyses, eliminating a key view into terrestrial and aquatic ecosystems. Nevertheless, preliminary evidence suggests reptilian enamel acts as an effective and consistent method for stable isotope analyses. We created a modern spatial reconstruction of crocodilian enamel stable isotope values—“isoscape”—of Africa for stable carbonate oxygen values (δ¹⁸O_C, n = 20) in conjunction with known precipitation oxygen isotope values (δ¹⁸O_p) to demonstrate the validity of crocodilian enamel as a stable isotope proxy. This modern spatial representation of crocodilian enamel was then placed in conjunction with δ¹⁸O values of fossilized crocodilian enamel from fossil sites in the Turkana Basin—including Buluk (16 Ma) and Lomekwi (3 Ma)—to demonstrate the potential of crocodilian enamel in long-term regional climatic and ecological modeling. From paleoenvironment and paleoecological reconstructions of the Turkana Basin, stable isotope values from crocodilians demonstrate the lowest δ¹⁸O values at each fossil site while still falling within anticipated ranges from known mammalian enamel δ¹⁸O values. δ¹⁸O values from crocodilians closely correlate to semiaquatic mammalian δ¹⁸O values present at the fossil sites, indicating crocodilian δ¹⁸O trends likely relate to dietary and behavioral habits, such as the consumption of aquatic animals and overall semiaquatic activity. δ¹³C values from crocodilians predictably vary at fossil sites consistent with documented vegetation patterns, reinforcing current understandings of the greater local ecological context. These anticipated stable isotope trends from crocodilians strongly correlate to environmental conditions, demonstrating the potential crocodilian enamel holds as a proxy for paleoenvironmental and paleoecological reconstructions and for providing novel insights into the hydroclimate and ecological history of Turkana over the past 16 million years.