Lizard ecology through time is largely unknown. Understanding ecology is important because of today’s drastic climate change, but this is not a unique event. Early Cenozoic hyperthermals were comparable to the perturbations currently experienced by living species. Understanding ecology through time must acknowledge the dynamic relationship between an organism and its environment on multiple scales. Ecological inferences can be based on form equaling function, correlating certain features (e.g. leaf-shaped dentition) with certain behaviors (e.g. herbivorous diet). Though this applies to specific taxa, there are confounding examples. Ecology can also be inferred through indirect means, but these are disconnected from the taxon of interest. Stable isotope geochemistry, however, provides an independent test. I analyzed stable isotope ratios (δ¹⁸O, δ¹³C) from enamel, providing new data on the connection between morphology, diet, and environment. I find a trophic separation in δ¹³C, and indications of aridity through δ¹⁸O. I applied this framework to extinct lizards from an Early Eocene (Wa4) assemblage, a key time between two major global warming events (Paleocene-Eocene Thermal Maximum and Early Eocene Climatic Optimum). I identify xenosaurid and glyptosaurine squamates and alethinophidian snakes. The xenosaurid is one of the youngest representatives of Restes rugosus, and I provide the first testable hypothesis of its ecology. These δ¹⁸O values corroborate hypotheses of a wet, tropical environment, and the δ¹³C values indicate an insectivorous or carnivorous diet for both taxa. My study provides an independent test of ecology of both extant and fossil lizards, with implications for differing survivorship throughout the early Cenozoic. / MS

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