While the oxygen isotopic composition of pedogenic carbonate has successfully been used to address the effects of global climate change on the hydrologic cycle, detailed regional paleohydrologic studies are lacking. Since the hydrologic cycle can vary extensively on local or regional scales due to events such as such as mountain building, and since pedogenic carbonates (calcite) form in a narrow moisture regime, other proxies, such as vertebrate remains, must be used to decipher local versus regional variations in paleohydrology. In this study, the oxygen isotopic composition (δ¹⁸O_p) of phosphatic remains from a diverse set of vertebrate fossils (fish, turtles, crocodiles, dinosaurs, and micro-mammals) from the Mussentuchit Member (MM) of the Cedar Mountain Formation, Utah, USA (Aptian to Cenomanian) are analyzed in order to determine differences among the available water reservoirs and water utilization of each taxon. Calculated changes in water reservoir δ¹⁸O_w over time are then used to determine the effects of the incursion of the Western Interior Seaway (WIS) and the Sevier Mountains on paleohydrology during the MM time.

Calculation of δ¹⁸O_w from the results of isotopic analysis of phosphate oxygen suggests that turtles and crocodiles serve as another proxy for meteoric water δ¹⁸O that can be used as a measure of average local precipitation δ¹⁸O_w similar to pedogenic calcite. Pedogenic calcites can be slightly biased toward higher values, however, due to their formation during evaporative conditions. Turtles and crocodiles can be used in place of pedogenic calcite in environments that are not conducive to pedogenic carbonate formation. Remains of fish with rounded tooth morphology have δ¹⁸O_p values that predict temperatures consistent with other estimates of mean annual temperature for this latitude and time. The δ¹⁸O_p of ganoid scales and teeth with pointed morphology, however, indicates that these skeletal materials were precipitated from water that is ¹⁸O-enriched due to migration to either evaporatively enriched water, or ¹⁸O-enriched estuarine waters of the Western Interior Seaway (WIS). Another possibility that cannot be discounted and assuming all morphological remains are from the same taxon, is that the pointed teeth and ganoid scales precipitated at different temperatures than rounded teeth. Mammal and herbivorous dinosaur δ¹⁸O_p suggests they primarily drank isotopically depleted river water. Co-existence of crocodiles, turtles, and mammals allows for calculation of relative humidity from site to site and these calculations suggest humidity averaged ~ 58% and ranged between ~ 42% and ~ 76%.

The δ¹⁸O_w values estimated from semi-aquatic taxa and pedogenic calcite suggest dominance of WIS-derived moisture during their growth. Herbivorous dinosaurs particularly indicate that altitude and catchment effects from the Sevier Mountains are seemingly important for river water δ¹⁸O_w in the fall through early spring. These data suggest that temporal changes in the isotopic composition of the MM fauna are produced by the small-scale regressive–transgressive cycles of the WIS.