Early Miocene environments in East Africa are proposed to have consisted of entirely dense forest systems composed strictly of C3 vegetation, which has affected our understanding of Miocene ape habitats. However, new environmental data from the early Miocene sites of Moroto (21 Ma), Napak (20 Ma), Tinderet (20 Ma), Rusinga Island (20-18 Ma), and Bukwa (19 Ma) indicate a more open environment consisting of open canopied forests, woodlands and wooded grasslands with evidence of some C4 vegetation. This study builds on this new data by analyzing stable carbon isotopes in mammalian tooth enamel collected from Kalodirr and Moruorot, Kenya (17.5-16.8 Ma). These δ13Cenamel signatures suggest a mosaic of C3 vegetation too enriched to indicate a closed canopy forest. Rather, they are more similar to values found in woodlands, with some indications of C4 vegetation. This supports the hypothesis that the forest canopies of East Africa were opening before the middle Miocene, demonstrating that the environmental history of East Africa was more dynamic than previously described.