PETROGRAPHIC AND ISOTOPIC ANALYSIS OF PHYTOSAUR TOOTH DENTIN REVEALS ARIDIFICATION-RELATED TRENDS IN THE DIAGENETIC ENVIRONMENT OF THE CHINLE FORMATION

The Upper Triassic (Norian; 228-208 Ma) Chinle Formation of Petrified Forest (PeFo) National Park spans a critical period in Earth’s history, during which major tectonic breakup of Pangea coincided with profound changes in atmospheric circulation and faunal compositions. Declining mean annual precipitation estimates and increasing carbonate content from Chinle paleosols suggests gradual regional aridification resulting from collapse of the Late Triassic Megamonsoon. While teeth can be a valuable paleoclimate proxy, studies frequently focus on tooth enamel reliability. Tooth dentin is often ignored in favor of enamel owing to the former’s greater susceptibility to chemical alteration, under the presumption that dentin records a mostly diagenetic signal. However, dentin’s sensitivity makes it excellent for studying early-stage diagenesis, which may reflect near-surface conditions. Phytosaur dentin provides a means to investigate changes in diagenetic conditions through the Norian in western equatorial Pangea. Teeth from the lowermost Blue Mesa preserve enamel and exhibit primary cements of calcite and secondary cements of goethite and barite, indicating a range of oxidative and alkaline diagenetic conditions during fossilization. Teeth from the topmost Petrified Forest Member are most altered, exhibiting primary and secondary cements of calcite and barite, with large portions of dentin replaced by calcite, suggesting a temporal trend toward more oxic and alkaline conditions by end Norian. Carbonate d¹³C_PDB values of dentin apatite (n=11) and apatite-calcite mixtures (n=18) range from -10.3 to -5.1‰ and -9.7 to -5.3‰ respectively, whereas d¹⁸O_PDB values range from -14.9 to -8.0‰ and -14.2 to -8.3‰, respectively. Long-term positive δ¹⁸O_PDB and δ¹³C_PDB trends similar to contemporaneous PeFo paleosol carbonates suggest a regional climate trend of Norian aridification. Aridification onset coincides with a faunal turnover documented stratigraphically near a persistent red silcrete in the lower Jim Camp Wash Beds of the Chinle, that defines the boundary between the Adamanian and Revueltian biozones in PeFo. A decline in herbivore diversity and body size, and reorganization in phytosaur taxa, occurs across this boundary, suggesting onset of water-stressed conditions in association with aridification.