Mammalian Paleoecology of the Tugen Hills Succession, Baringo Basin, Kenya: Implications for Hominoid Evolution

Abstract

Reconstructing the evolutionary environments of early apes and humans is critical for developing and testing hypotheses relating to their paleobiology and life histories, as evolutionary innovations are intricately linked to environmental contexts. However, establishing the biotic and abiotic conditions of these paleoenvironments is often difficult due to poor preservation of fossil material, as well as temporal averaging of fossil material. To better evaluate palaeobiological hypotheses concerning hominoid and hominin evolution, it is important to understand how the scaled effects of temporal averaging on long-term (hundreds of thousands to millions of years) and short-term (tens of thousands of years) scales influence paleoecological analyses used to extrapolate hominoid paleoenvironments. To address to this issue, this project analyzed mammalian stable carbon and oxygen isotopic values from the Tugen Hills Succession (THS) of the Baringo Basin (Kenya), a nearly continuous geologic succession spanning the Middle Miocene (~15.5 Ma) to the Late Pleistocene (~240 ka), at both long and short-term temporal scales to contextualize paleoecological change concurrent with hominoid and hominin fossil sites in the succession. The THS has yielded an extensive faunal record, including one of the largest concentrations of hominoids from 14 to 4 Ma, including several hominin specimens from ~6.0 to 0.2 Ma, and he continuity of the THS thus provides a unique opportunity to characterize mammalian paleodietary change to evaluate how preservation biases such as temporal averaging influence the analysis of paleoecological datasets. In the older units within the THS (>10 Ma), stable carbon isotopic values from fossil mammalian tooth enamel suggest the presence of open forest to closed woodlands vegetation, with evidence for dietary C4 grasses first arising after 10 Ma. Equidae (horses) were the earliest group of mammals to demonstrate variable mixed feeding behaviors from browsing to grazing, while species in other mammalian families gradually tracked increasing C4 grazing signals after ~8 Ma. Both stable carbon isotopic and mesowear values show that Bovidae (antelopes) differentially shifted their dietary ecologies at the tribe level in conjunction with Late Miocene environmental change. Some bovids (Alcelaphini) maintain strong grazing behaviors throughout the sequence, while most other tribes adopted either browsing or grazing dominated mixed-feeding strategies in the early Pliocene. Many taxa adopted mixed-feeding strategies after the Middle/Late Miocene, concurrent with the purported decline of closed forest contexts in the Plio-Pleistocene. Compared to preexisting paleobotanical proxy data from the THS, carbon values suggest that most of the mammalian families maintained diverse and opportunistic foraging strategies in response to C4 grassland expansion. This interpretation may reflect temporal averaging in the datasets, condensing patterns of environmental heterogeneity. Alternatively, dietary patterns may indicate foraging patterns in which ‘opening’ environments, indicated by increasing C4 consumption, continue to maintain elements of dietary C3 vegetation. Temporally long and short-term isotopic dataset for Plio-Pleistocene mammalian groups suggest that while there is no ‘ideal’ scale of interpretation of paleoecological analysis, both long- and short-term scales might potentially overrepresent environmental heterogeneity in reconstructions of human evolutionary environments. However, by comparing paleoecological datasets at differing levels of temporal resolution, temporally long-term events (i.e. C4 grassland expansion) can be more carefully characterized by smaller, ‘site-specific’ datasets to better approximate the timing and mode of vegetation turnover within and between hominin evolutionary environments.