Fig. 1: Atmospheric pCO₂ levels are derived from leaf carbon isotope fractionation (Δ_leaf), Stomata Density (SD), and stomata pore length (p).

a Δ_leaf is determined from the carbon isotope composition (δ¹³C) of the fossil flora (green), marine carbonates (blue) and calculated atmospheric CO₂ (light blue) (Supplementary Table 4). b Plants grown under higher ambient CO₂ levels yields a higher Δ_leaf and/or lower stomata density. The curves represent gas-exchange model calibrated for modern lycophytes with a reference CO₂ assimilation rate at modern atmospheric levels (A₀) of 3.5 µmol m⁻² s⁻¹, an operational stomatal conductance efficiency (ratio of operational to maximal stomatal conductance) ζ = 0.2, stomata pore shape β = 0.6 and p = 18 ± 2 µm comparable to ancient and modern lycophytes (see supplement for details). Here, a fossil lycophyte from the Devonian is used as an example. Devonian lycophytes typically display Δ_leaf values close to 20‰ and stomata densities between 15 and 25 mm⁻¹ (Supplementary Data 1).