FOSSIL LEAF VENATION AS AN INDICATOR OF PRIMARY PRODUCTIVITY THROUGH TIME

As long as underlying physiological mechanisms are understood, the failure of a proxy to perform as expected can be as informative as its success. Leaf gas exchange rates, including transpiration and assimilation, have been linked to leaf vein density in living plants and vein density has been used as a readily measurable proxy of physiology for fossil plants. However, problems arise when physiological expectations are compared to details of the fossil record. Declining CO₂ over the Cretaceous has been suggested as an evolutionary driver of the high leaf vein densities (7 to 28 mm mm^-2) that are unique to the angiosperms throughout all of Earth history. Photosynthetic modeling indicated the link between high vein density and productivity documented in the modern low-CO₂ regime would be lost as CO₂ concentrations increased, but also implied that plants with very low vein densities (less than 3 mm mm^-2) should experience substantial disadvantages with high CO₂. Thus, the hypothesized relationship between CO₂ and plant evolution can be tested through analysis of the concurrent histories of alternative lineages since an extrinsic driver like atmospheric CO₂ should affect all plants, not just the flowering plants. No such relationship is seen. Regardless of CO₂ concentrations, low vein densities are equally common among non-angiosperms throughout history and common enough to include forest canopies and not just obligate shade species that will always be of limited productivity. Modeling results can be reconciled with the fossil record if maximum assimilation rates of non-flowering plants are capped well below those of flowering plants, capturing biochemical and physiological differences that would be consistent with extant plants but previously unrecognized in the fossil record. Although previous photosynthetic modeling suggested that productivity would double or triple with each Phanerozoic transition from low to high CO₂, productivity changes are likely to have been limited before a substantial increase accompanying the evolution of flowering plants.