An assessment of the use of epidermal micro-morphological features to estimate leaf economics of Late Triassic–Early Jurassic fossil Ginkgoales

• Leaf economics are widely used in ecological/physiological studies of modern plants.
• Leaf mass per area (LMA) can predict investment in leaves and leaf lifespan.
• It is not possible to directly quantify the leaf economics of fossil plants.
• Extant Ginkgo biloba leaves with high LMA have smaller more densely packed cells.
• The LMA of fossil Ginkoales from Triassic–Jurassic sediments was reconstructed.

The leaves of extant plants occur within a well-defined economic spectrum reflecting the nutrient and carbon investment in the construction of the foliage and the photosynthetic return over the life of the leaf. The dry leaf mass per area (LMA) is commonly used to predict leaf lifespan, photosynthetic capacity, stomatal conductance, nutrient investment and ecosystem productivity. However, direct quantification of LMA in fossil plants (fLMA) is not possible due to the loss of internal leaf structures and leaching of solutes during the fossilisation process. Estimation of fLMA based upon leaf fossil leaf macro-morphology has also been restricted due to the frequency of fragmentation incurred during transport prior to deposition. Therefore, application of this important ecological parameter to key episodes of evolutionary and environmental change in deep-time has been restricted. Here we utilise a correlation between adaxial epidermal cell density and LMA of extant Ginkgo biloba to reconstruct fLMA of fossil Ginkgoales during a major global warming event at the Triassic–Jurassic boundary. fLMA values suggest reduced ecosystem productivity, possibly indicative of increasing ecological stress, towards the Late Triassic; coincident with increased fire frequency and palaeo-atmospheric [CO2] as the Central Atlantic Magmatic Province developed. Early Jurassic fLMAs indicate the recovery of ecosystem function to pre-boundary levels. Analyses of leaf micro-morphological characters may have widespread application to the reconstruction of fossil leaf economics from fragmentary plant fossils through Earth History.