The abrupt spread of grasslands using C₄ photosynthesis, sometimes referred to as the rapid increase in C₄ ecosystems (RICE), occurred in the late Miocene in North America. While fossil plant specimens from the Miocene Dove Spring Formation of California as well as data from phylogenetic studies and molecular clocks show that C₄ grasses evolved prior to the RICE, most isotopic data from paleosols and mammal tooth enamel suggest that its abundance on the landscape was minimal. However, a few recent studies from the Great Plains suggest that C₄ grasslands may have been more prominent prior to the RICE event. Here we examine stable carbon isotope values from ungulate tooth enamel from the Barstow Formation of southern California, which is geographically and temporally close to the Dove Spring Formation, and records a diverse and abundant paleofauna of medial Miocene age. Tooth enamel δ¹³C values were examined in seven ungulate genera including the hypsodont equids; Acritohippus sp. and Scaphohippus sp.; the camelids, Aepycamelus sp., Hesperocamelus sp., and Procamelus sp.; the antilocaprid, Merycodus sp.; and the proboscidean, Gomphotherium sp. More positive δ¹³C values observed within the equids, camelids, and antilocaprids are suggestive of C₄ grasses being included in the diets of these taxa. The equids exhibited the most positive mean δ¹³C values, which indicate a higher component of dietary C₄ grasses (up to 18\%) when compared to the other sampled ungulate taxa. The tooth enamel isotope values presented in this study show the presence of C₄ grasses as a significant component of ungulate diets millions of years before the RICE. The abundance of C₄ plants earlier in the Miocene may imply a more significant role in the major ungulate diversification events than previously suspected. The few pre-RICE localities showing evidence of C₄ abundance suggests that these grasslands were geographically restricted which limits, but may not exclude, the possibility that a world-wide mechanism controlled its spread.