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Grasslands dominated by taxa using the C₄ photosynthetic pathway first developed on several continents during the Neogene and Quaternary, long after C₄ photosynthesis first evolved among grasses. The histories of these ecosystems are relatively well-documented in the geological record from stable carbon isotope measurements (of fossil vertebrate herbivores and paleosols) and the plant microfossil record (pollen and/or phytolith assemblages). The distinct biogeography and ecophysiology of modern C₃ and C₄ grasses have led to hypotheses explaining the origins of C₄ grasslands in terms of long-term changes in the Earth system, such as increased aridity and decreasing atmospheric pCO₂. However, quantitative proxies for key abiotic drivers of these hypotheses (e.g., temperature, precipitation, pCO₂) are still in development, not yet widely applied at the continental or global scale or throughout the late Cenozoic, and/or remain contentious. Testing these hypotheses globally therefore remains difficult. To understand better the potential links between changes in the Earth system and the origin of C₄ grasslands, we undertook a global-scale comparison between observational records of C₄ plant abundances in Miocene and Pliocene localities compiled from the literature and three increasingly complex models of C₄ physiology, dominance, and abundance. The literature compilation comprises >2,600 δ¹³C-values each of fossil terrestrial vertebrates and of paleosol carbonates, which we interpret as primarily proxies for the abundance of C₄ grasses, based on the modern contribution of C₄ grasses to terrestrial net primary productivity. We …