Pieces of the puzzle: Lack of significant C4 in the late Miocene of southern California

The stable carbon isotope ratios of tooth enamel from 89 herbivores from the Dove Spring Formation (DSF) of the El Paso Basin, western Mojave Desert, California, were analyzed to determine if C4 plants may have been present in the late Miocene of southern California. Taxa analyzed include antilocaprids, castorids, camelids, equids, gomphotheriids, merycoidodontids, and rhinocerotids. The atmospheric CO2-δ13C value has varied significantly in the geologic past, leading to changes in the δ13C values of plants and tooth enamel. The estimated δ13C value of atmospheric CO2 in the middle to late Miocene is − 5.8 ± 0.2‰, making the pure C3 diet cutoff value higher than the modern by ~ 2‰. Given this, much of the evidence of an early spread of C4, which inappropriately uses the modern C3 diet cutoff value, may be a false signal from water-stressed C3 plants and the amount of C4 biomass in the geologic past may have been over-estimated. The enamel-δ13C values of all taxa sampled are − 10.1 ± 1.1‰ (n = 89). Accounting for the changes in the δ13C values of atmospheric CO2, enamel-δ13C values from the DSF indicate that herbivores from that area had C3 diets and lived in an environment dominated by C3 plants. Serial enamel isotope data from the DSF revealed negligible intra-tooth variations in the δ13C values, confirming that these herbivores consumed C3 plants year-round. If the C4 photosynthetic pathway did spread to southern California prior to the rapid, global rise in C4 in the late Miocene, the taxa sampled did not incorporate C4 biomass into their diets. The results of the stable carbon isotope analysis, along with the evidence from the rock record and the life habits of the fossil taxa present, indicate that the El Paso Basin consisted of diverse habitats including C3 grasslands, wooded C3 grasslands, and riparian forests.