Evidence of early C₄ grasses, habitat heterogeneity, and faunal response during the Miocene Climatic Optimum in the Mojave Region

Major changes to landscapes, climate, and mammalian faunas occurred at the regional scale in western North America during the Miocene Climatic Optimum (MCO) between ~17 and 14 Ma, but few studies have looked at how the MCO affected basin-scale environments. Paleoenvironmental reconstructions coupled with mammalian fossil assemblages from the Crowder and Cajon Valley formations in the Mojave Desert, California, contribute insights into local-scale (10s of kilometers) responses to warming during the Hemingfordian and Barstovian North American Land Mammal Ages. By integrating lithological descriptions, phytolith analyses, carbon isotope composition of preserved soil organic matter (δ¹³C_SOM), and elemental geochemistry of paleosols, we provide reconstructions of depositional environments, vegetation, and precipitation through the MCO. Phytolith and δ¹³C_SOM evidence suggest that paleoenvironments were predominately C₃ grasslands, with the earliest potential presence of C₄ grasses in the region found within the Crowder Formation at ~17 Ma. Based on elemental geochemistry of paleosols, mean annual precipitation estimates are 807 (±182) mm yr^{− 1} at 17 Ma in the Crowder Formation and 740–800 (±182) mm yr^{− 1} from 16 to 15 Ma in the Cajon Valley Formation. Our multiproxy approach indicates that the Crowder and Cajon Valley basins were stable, large-scale braided stream and floodplain systems with intermittent paleosol development and heterogeneity in vegetation and moisture conditions across spatial scales. Overall, we find a signal of local paleoenvironmental stability during the MCO; however, a significant drying trend in the Crowder Formation does correspond with faunal turnover and reduction in the number of taxa present. High species diversity within faunal assemblages, especially among small mammals, and fewer shared species between the Crowder and Cajon Valley assemblages than expected based on bootstrap analysis indicate that peak mammal diversity was accommodated at both local and regional scales during sustained tectonic activity and the MCO in western North America.