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. 2017 May 11;174(2):650–664. doi: 10.1104/pp.17.00204

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Figure 3. — A comparison of two different empirical pCO₂ proxy models applied to stomatal index data (from Steinthorsdottir et al., 2011b) of Triassic and Jurassic fossil Ginkgoales. The Barclay and Wing (2016) model is based on an SI-pCO₂ response curve from historical herbarium specimens over a CO₂ range from 290 to 429 ppm (CO₂ = 9920.9 × (SI_fossil)^−1.484). The McElwain (1998) calibration is based on a comparison of fossil SI with that of an NLE: Carboniferous standardization: pCO₂ (paleo) = SD_NLE/SD_Fossil × 600 and the recent standardization, pCO₂ (paleo) = SD_NLE/SD_Fossil × pCO_2NLE.