QUANTIFYING GEOLOGIC CONTROLS ON THE VERTEBRATE FOSSIL RECORD: A CASE STUDY FROM THE PLIO-PLEISTOCENE OF KOOBI FORA, KENYA

Paleodiversity analyses of the vertebrate fossil record have been key to documenting the evolution of life on Earth. Though it is widely acknowledged that taphonomic effects and collection effort can potentially bias assessments of paleodiversity, we still have much to learn about the geologic controls on fossil distribution, recovery, diversity, and abundance. Here, we combine a high-resolution digital elevation model (DEM), Landsat imagery, and fossil occurrence points for a 10x10 km area in the Plio-Pleistocene Koobi Fora Formation, East Turkana, Kenya. This area spans four geologic members dating to between ~ 3.60-1.38 million years ago (Ma) and has produced a rich collection of large mammal fossils, including hominins (n = 283 specimens). For each georeferenced fossil occurrence, we collected four geologic variables: slope of the outcrop, distance to nearest stream, topographic wetness index (TWI), and reflectance (i.e., lithology). Using logistic regression analysis predicting fossil presence (1) and absence (0) across our study area, we find that three of the four variables significantly predict fossil presences: slope (p = <0.001), TWI (p= 0.029), and reflectance (p = <0.001). The probability of fossil recovery is highest in areas with higher slopes, greater water movement (low TWI), and sandstone lithologies. Additionally, the number of fossils recovered is highly correlated (r = 0.99) with both exposed outcrop area and volume. Though the positive association of fossils with more extensive outcrop exposures and sandstone lithologies was expected, our finding that slope and TWI significantly predict fossil presence feeds directly into long-standing arguments about the nature of the Turkana Basin record. Specifically, our findings reveal that, even after decades of collection, fossils continue to be actively eroded from the basin’s Plio-Pleistocene sediments. This encourages the continuation of ongoing annual fieldwork in the Turkana Basin and the East African Rift Valley more broadly.