Herbivore Isotopic Dietary Ecology of the Middle Miocene Maboko Formation, Kenya

Middle Miocene deposits at Maboko Island in the Nyanza Rift of Western Kenya (-15-14 Ma) have yielded a rich fossil mammalian record that documents a mid-Miocene faunal shift. Palaeoecological proxies for Maboko have previously been interpreted to indicate heterogeneous habitats, ranging from grassland to closed canopy forest and implicated in this turnover. Stable carbon and oxygen isotope data of fossil herbivore enamel from catarrhine-bearing deposits at Maboko were analyzed to reconstruct the nature of C3 vegetation (i.e., waterstressed or subcanopy), as well as determining if any C4 biomass, representative of more open woodland or grassland habitats, were consumed. Taxa sampled include representatives of ruminants, suoids, rhinocerotids, and proboscideans. 813Cenamel and 818Oenamel values of Maboko fossil herbivores indicate foraging strategies consistent with a C3 dominated ecosystem, exhibiting a range of 813Cenamel signatures similar to those of extant browsing herbivores foraging in mosaics of open forest/woodland habitats. Within the Maboko sequence, isotopic evidence indicates varying environments based on variable dietary spectra associated with discrete fossiliferous units within the succession. Relative to other stratigraphic beds, isotopic signals of herbivore enamel from Bed 5b, for example, reflect more closed woodland/forest foraging. The overall -4%o range of 813Cenamel values from Maboko (-14.1%o to - 10.2%o) is statistically similar to 813Cenamel values from the slightly younger middle Miocene site of Fort Ternan and is consistent with faunal and paleosol evidence from Maboko suggesting ecological variability. However, the isotopic evidence from Maboko indicates that environmental variability is more constrained than previously reconstructed, instead ranging from more open canopy forest to open woodland habitats, albeit with some spatial and temporal heterogeneity. Closed canopy forest plants and C4 biomass were not detectable as dietary components for any herbivores sampled thus far; nor was there evidence of significantly water-stressed C3 vegetation (possibly C3 grasses) being consumed.