THE NEOGENE TRANSITION FROM C3 TO C4 GRASSLANDS IN NORTH AMERICA: ASSEMBLAGE AND ISOTOPIC ANALYSIS OF FOSSIL PHYTOLITHS

The rapid, late Neogene (8-2 Ma) ecological expansion of C₄ grasses that paved the way for modern tropical grassland ecosystems is well documented in the fossil record of stable carbon isotopes. Potential triggers for this broadly synchronous change, long after the evolutionary origin of C₄ photosynthesis in grasses, include changes in CO₂ levels, seasonality, aridity, herbivory, and fire regime. To date, the lack of direct evidence for late Neogene floral composition and structure has hindered testing of these hypotheses. To remedy this problem, we aim to provide the first direct, relatively continuous record of vegetation change for the Great Plains of North America for the critical interval (~12-2 Ma) using plant silica (phytolith) assemblage composition and carbon isotope ratios.

Phytoliths were extracted from Late Miocene-Pliocene paleosols in Nebraska and Kansas. Phytolith analysis of the 14 best preserved assemblages indicates that habitat openness varied substantially during the Middle-Late Miocene but became more uniformly open, forming relatively open grassland or savanna, during the Late Miocene- Pliocene. In addition, grass phytoliths typical of chloridoid and other PACMAD grasses increased markedly 8-5 Ma to up to ~50-60% of grasses, resulting in mixed C₃-C₄, highly heterogeneous grassland communities by 5.5 Ma. These findings are supported by carbon isotope ratios of phytolith extracts that also record mixed C₃-C₄ grass communities by 5.5 Ma. In addition, d¹³C values indicate that most PACMAD grasses likely were C₄. Our study suggests that the rise to dominance of C₄ grasses began in the latest Miocene, consistent with interpretations of isotopic records from paleosol carbonates and ungulate tooth enamel. The rise in abundance of chloridoids, which appeared in the central Great Plains by the Early Miocene, demonstrates that the ‘globally’ observed lag between C₄ grass evolution/taxonomic diversification and ecological expansion occurred at the regional scale. These patterns of vegetation change imply that environmental alteration during the Late Neogene influenced the C₃-C₄ shift in the Great Plains. Specifically, the importance of chloridoids and the decline in the relative abundance of forest indicator taxa, including palms, point to climatic drying as a key trigger for C₄ dominance.