Paper No. 225-14
Presentation Time: 9:00 AM-6:30 PM
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.