RECONSTRUCTION OF PALEOECOLOGY AND WHOLE-PLANT HABIT USING STABLE CARBON ISOTOPES FROM DISPERSED LEAVES OF LAUROZAMITES POWELLII, AN UPPER TRIASSIC BENNETTITALEAN

Stable carbon isotopes are a powerful tool for examining ecological processes across landscapes and time. Isotopic fractionation of atmospheric carbon into plant tissue occurs during photosynthesis, and the magnitude of the δ¹³C difference is related to variation of photosynthetic rates. As available light in forest canopies decreases, so too does photosynthetic rate and δ¹³C_leaf; the so-called canopy effect. We apply the canopy effect to interpret the paleoecology and habit of the bennettitalean Laurozamites powellii, based on abundant and well-preserved leaves from the Upper Triassic Chinle Formation near Fort Wingate, NM, USA. Fossils were collected from the Monitor Butte Member, which records fluvial and lacustrine deposition in a seasonally dry tropical regime on the western margin of Pangea. The high quality of preservation of the fossil leaves, including cuticles, reflects burial in standing-water deposits in basal parts of a paleovalley, where a persistently high water table and rapid sediment aggradation precluded decay. Leveraging the mean and variance of δ¹³C_leaf measurements at different canopy heights from modern forests, evidence from fossil δ¹³C_leaf suggests an open canopy with uniformly high sunlight conditions. Our data support the findings of previous studies indicating the presence of open woodland, or savanna-like plant communities containing bennettitaleans. Measurements of leaf physiognomy and epidermal cell shape, indicative of positional changes within a crown, covary with δ¹³C. The magnitude of variation indicates little within canopy shading of leaves and is consistent with a divaricate habit, a result in contrast to the traditional reconstruction of bennettitaleans. The application of empirical observations of changing light conditions and its effect on δ¹³C and morphology from modern systems to the fossil record demonstrates the potential of this approach for paleoecological reconstructions in deep time, and may provide significant insight into the evolution of terrestrial ecosystems.