Cerling, T.E., J.M. Harris, B.J. MacFadden, M.G. Leakey, J. Quade, V. Eisenmann, and J.R. Ehleringer. 1997. Global vegetation change through the Miocene-Pliocene boundary. Nature 389:153-158.

A global increase in C4 biomass is indicated by changes in the del-13C of fossil tooth enamel in Asia, Africa, North America, and South America between 8 and 6 million years ago (Ma). The abrupt and widespread increase in C4 biomass may be related to a decline in atmospheric CO2 levels below a threshold important to C3 plants, which have lower quantum yields under low P(CO2) conditions, especially when accompanied by higher temperatures.