Climate Reconstruction from Leaf Size and Shape: New Developments and Challenges

—Leaf physiognomy (size and shape) in fossils is commonly used to reconstruct terrestrial paleoclimate. Physiognomic leaf-climate methods are underpinned mostly by the covariation between toothed margins and mean annual temperature (MAT) and between leaf size and mean annual precipitation. Digital leaf physiognomy, a multivariate method based largely on variables that are functionally linked to climate and that can be measured by computer algorithm, minimizes many of the deficiencies present in other approaches. Nevertheless, the relationships between MAT and many physiognomic variables, especially tooth-related variables, are confounded by leaf thickness, leaf habit (deciduous vs. evergreen), and phylogenetic history. Until these factors are properly accounted for, a minimum error in MAT of ±4 °C for digital leaf physiognomy and ±5 °C for other methods should be assumed.