ESTIMATING EXTENT OF DIAGENETIC OVERPRINTING AND STABLE ISOTOPE VALUES OF LATE CRETACEOUS VERTEBRATE MICROFOSSIL ENAMELOID: EVIDENCE FOR PALEOECOLOGICAL RECONSTRUCTIONS

Carbonate oxygen (δ¹⁸O) and carbon (δ¹³C) stable isotope values have been obtained from vertebrate microfossil enameloid of several taxa from the Upper Cretaceous (Campanian) Mesaverde Formation, Wyoming. This analysis was conducted to deduce if expected patterns in carbon and oxygen isotope compositions could be preserved in tooth enamel from a variety of Mesozoic carnivorous and herbivorous terrestrial and aquatic vertebrates (i.e., Hybodontidae, Dasyatidae, Lepisosteidae, Amiidae, Crocodilidae, Hadrosauridae, Ceratopsidae, and Tyrannosauridae). For terrestrial vertebrates, enamel and enameloid δ¹³C values are controlled by the carbon isotope composition from diet, whereas δ¹⁸O values are controlled by environmental water and physiology. Given the known flora of this time period, we expect consumers to have δ¹³C values consistent with C3 environments and δ¹⁸O to track local surface waters. For aquatic vertebrates, enamel and enameloid δ¹³C and δ¹⁸O values are often most strongly influenced by the chemistry of environmental waters. We expect δ¹³C and δ¹⁸O for aquatic taxa to show high values for primarily marine taxa (e.g., hybodontids), intermediate values for brackish water taxa (e.g., lepisosteids), and low values for freshwater taxa (e.g., amiids). We used δ¹³C values as evidence of dietary preferences to discriminate taxa into dietary guilds. Preliminary results show minor differences in δ¹⁸O values with larger, but less than expected, variance in δ¹³C compared to modern ecosystems. These values indicate that diagenesis has altered the original isotopic composition, but to what degree has yet to be determined. Using the less dense bioapatite (e.g., dentine) as a measure of diagenesis, it is evident that carbon isotope values are higher and oxygen isotope values are lower than the original isotope values. Bivariate plots of isotope values show distinct taxonomic clusters that may correlate with interspecific diets. Results from this study, in addition to previous faunal and statistical analyses from this same locality (Barwin Quarry/Fales Rocks), are giving further insight into the paleoecology of the Campanian coastal facies of Wyoming.