STABLE ISOTOPE ANALYSES OF TOOTH ENAMEL IN EOCENE CROCODYLIANS

Carbon and oxygen isotope analyses from tooth enamel carbonates were conducted on a variety of Paleogene crocodylian taxa in order to help test hypotheses regarding their diets as well as regional and local paleoclimates. Additionally, the results will increase our understanding of isotopic variation across complex ancient landscapes and ecosystems. This study utilizes a vast collection systematically sampled over the past 25 years in the Green River Basin of Wyoming where a diverse assemblage of crocodylians is preserved in the latest Wasatchian (Wa7) through middle Bridgerian (Br2) Wasatch, Green River, and Bridger Formations. These deposits represent Basin-Margin (upland) alluvial fan and braided stream to Basin-Center (lowland) meandering stream, lake-margin, and lake environments. The crocodylians occupying the basin at the time were distributed across this landscape with some taxa restricted to lake and lake-margin, or upland regions, while most occupied the river systems in between. The niches of these forms were further partitioned by body size and dietary preferences presumably as reflected in their varying snout width and dentitions. Sampled taxa included Borealosuchus wilsoni, “Crocodylus” affinis, Cf. “Crocodylus” acer, Boverisuchus (Pristichampsus) vorax, Allognathosuchus, and Procaimanoidea. The samples were chosen based on condition and their representation of different taxa, depositional environments, individual body size (age), and horizon. Once selected, these teeth were prepared at Albion College and sent to The University of Michigan Stable Isotope Laboratory for isotope analysis. Preliminary results from ¹³C analyses reveal inter- and intraspecific variation. Although the flow of nutrients from primary aquatic and terrestrial producers up through the food web to high-level consumer crocodylians is highly complicated; at least some of this variation may be due to diet preference for different species and evolving diets from juveniles to adults. Preliminary results of ¹⁸O analyses show across-landscape, through-time, and interspecific temperature variation, interpretation of which awaits additional results and analyses.