INTRA-BASINAL HETEROGENEITY OF TERRESTRIAL ECOSYSTEMS IN THE TURKANA BASIN IN THE EARLY PLEISTOCENE

Wide-spread outcrop exposure in the Turkana Basin in the East African Rift Valley offers the opportunity to investigate questions that link paleontology and paleogeography. Between ~2-1.4 Ma, well-documented lake level fluctuations and changes in hydrological systems may have resulted in the restructuring of terrestrial ecosystems in the Basin. This study incorporates the three major formations of the Turkana Basin (Koobi Fora, Nachukui, and Shungura) to investigate the relationship between changes in basin hydrology and ecosystem dynamics. Specifically, the continuous presence of a river throughout the Shungura Formation has been hypothesized to maintain paleoenvironments that served as an ecological buffer for mesic-adapted taxa during cycles of increased aridity.

This study incorporates multiple proxies of ancient habitats (e.g. faunal abundance, functional trait community analysis, and enamel stable carbon isotopes) to investigate ecological dynamics between ~2-1.4 Ma. Faunal abundance analysis was performed using count data of cranial and post-cranial elements attributed to the families Bovidae (n > 4,000), Suidae (n > 1,500), and Equidae (n > 500). Carbon isotopic analysis derive from 887 specimens. Functional traits of 75 fossil species from the Turkana Basin and 172 modern sites were analyzed in a correspondence analysis. The results indicate that the ancient ecosystems of the three formations were more arid than many known modern assemblages, yet each responded differently to changing basin hydrology. While the Koobi Fora Fm. increases in aridity through time, the Nachukui and Shungura fms. become relatively mesic. During a period of maximum lake recession (~1.8-1.5 Ma), an increase in the number of mesic-adapted taxa (Reduncini and Kolpochoerus) were recovered from the Shungura Fm. In contrast the Koobi Fora and Nachukui fms. display an increase of arid-adapted taxa (Alcelaphini and Metridiochoerus). Additionally, Eurygnathohippus persisted in the Shungura Fm. despite periods of decline and even extirpation in the other two formations, suggesting this taxon may be more mesically-adapted than previously recognized. Overall, our results indicate that the Shungura Fm. was likely a buffer zone for mesic-adapted taxa during periods of fluctuating lake size and aridity.