The origins and diversification of C4 grasses and savanna-adapted ungulates

C-4 grasses constitute the main component of savannas and are pervasive in other dry tropical ecosystems where they serve as the main diet for grazing animals. Among potential factors driving C-4 evolution of grasses, the interaction between grasses and grazers has not been investigated. To evaluate if increased grazing pressure may have selected for higher leaf silica production as the grasses diverged, we reconstructed the phylogeny of all 800 genera of the grass family with both molecular (combined multiplastid DNA regions) and morphological characters. Using molecular clocks, we also calculated the age and number of origins of C-4 clades and found that shifts from C-3 to C-4 photosynthesis occurred at least 12 times starting 30.9 million years ago and found evidence that the most severe drop in atmospheric carbon dioxide in the late Oligocene (between 33 and 30 million years ago) matches the first origin of C-4 photosynthesis in Chloridoideae. By combining fossil and phylogenetic data for ungulates and implementing a randomization procedure, our results showed that the appearance of C-4 grass clades and ungulate adaptations to C-4-dominated habitats match significantly in time. An increase of leaf epidermal density of silica bodies was found to correspond to postulated shifts in diversification rates in the late Miocene [24 significant shifts in diversification (P < 0.05) were detected between 23 and 3.7 million years ago]. For aristidoid and chloridoid grasses, increased grazing pressure may have selected for a higher leaf epidermal silica production in the late Miocene.