STABLE ISOTOPE VALUES FROM UNGULATE TOOTH ENAMEL SHED LIGHT ON THE MIOCENE BIOGEOGRAPHY OF C4 PLANTS IN THE UNITED STATES

The spread of grasslands in the USA during the early to middle Miocene is associated with the diversification of mammalian clades and evolution of morphological characteristics, such as hypsodonty. Although many modern ecosystems include an abundance of C₄ plants, the early to middle Miocene spread of grasslands is widely thought to have only included C₃ grasses. Prior studies have shown that C₄ plants proliferate in ecosystems 8-5 Ma, during the Rapid Increase in C₄ Ecosystems (RICE). However, a few recent studies have shown some abundance of C₄ plants in particular localities from the western USA prior to 8 Ma. Although C₄ plants represent only 5% of modern plant diversity, they account for over 25% of global net primary productivity. Consequently, they play important roles in many modern ecosystems. Thus, understanding the chronology and geography of the spread of C₄ plants has implications for floral and faunal evolution. Here we use tooth enamel carbon isotope (δ¹³C) values from >500 Miocene ungulate specimens from across the USA as a proxy for landscape C₄ abundance. We investigate three questions about the role of C₄ plants in Miocene grassland expansion: (1) Were there centers of C₄ abundance in the Miocene that may have served as sources for the RICE? (2) During the early Miocene, did C₄ grasses play any role in the evolution of hypsodonty? (3) Do Miocene ungulate δ¹³C values correlate with the global climate isotope record? Our results show a low abundance of C₄ plants at some localities before the RICE, primarily from the mid‑Miocene of southern California. These localities may have served as centers for C₄ spread during the RICE. Additionally, the abundance of C₄ plants at localities that also contain hypsodont ungulates suggests a connection between C₄ plants and the evolution of this feature. Further, the occurrence of C₄ plants in only a few localities prior to the RICE suggests its presence was controlled by local mechanisms (e.g., local temperature). Although the ungulate δ¹³C values measured here do not correlate with the global isotope climate record, the sudden increase in C₄ abundance 8-5 Ma as shown in many previous studies suggests that a global mechanism (e.g., low atmospheric CO₂) triggered the RICE, resulting in the predominance of C₄ plants in some ecosystems.