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Covariation of relative atmospheric CO2 concentration (RCO2) and fossil guard cell size (as guard cell width cubed) over the last 400 Myr. The shaded envelope represents uncertainties owing to the weathering rates of basalt rocks. Each point is an individual fossil species, listed in the electronic supplementary material, table A1. RCO2 data are from Berner [4], where RCO2 is ca relative to the pre-industrial low of approximately 280 ppm.
