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The Cloverly Formation of Montana and Wyoming preserves abundant nonmarine vertebrate fossils from the mid-Cretaceous, yet its paleoenvironment and faunal niche structure remain poorly understood. We analyzed δ¹⁸Ο_phosphatein over 100 fossil individuals from multiple vertebrate taxa collected from a single microfossil bonebed in Carbon County, Montana.To infer habitat preferences and water-use strategies, we compared δ¹⁸Ο_phosphatevalues within and across taxa. We reconstructed δ¹⁸O_{surface_water}from semi-aquatic reptile values using regressions calibrated with data from modern environments and extant taxa. Using a multi-taxon framework, we estimated warm-season water temperatures from δ¹⁸O_{surface_water}and δ¹⁸Ο_phosphateof lepisosteid (gar) scales, then converted these to air temperatures using a modern climate transfer function. δ¹⁸Ο_phosphatevalues ranged from 9.5‰ to 23.2‰ (VSMOW) and varied across taxa. Aquatic and semi-aquatic groups exhibited lower values than dinosaurian taxa. Our reconstructed mean δ¹⁸O_{surface_water}was −7.9‰ (95% CI: −10.1 to 5.5‰), yielding a warm-season water temperature of 26°C and an air temperature of 24°C. Intertaxon differences reflect niche partitioning and suggest primary isotopic signals are preserved. Unexpectedly high values in Bernissartiid-like neosuchian teeth may indicate greater ecohydrological diversity than previously recognized. Our δ¹⁸O_{surface_water}estimate aligns with other Aptian-Albian proxies but exceeds model-based predictions, likely due to outdated assumptions underlying the model. The MAWSAT estimate falls within the upper range of model-data assimilation outputs. These results provide new context for ecological structure in the Cloverly fauna and offer the first quantitative temperature estimate for the Formation, helping to define baseline conditions between the Aptian-Albian Cold Snap and the Cretaceous Thermal Maximum. 

Introduction:The Yucca Formation is a Lower Cretaceous sedimentary unit present in West Texas. Based on its relative stratigraphic position in the Cretaceous succession of West Texas, it is expected that the Yucca Formation is of Albian and/or Aptian age. It is also expected that the carbon isotope excursions associated with OAE 1a and OAE 1b should be identified in the Yucca Formation. The goals of this project are to 1. construct a carbon isotope chemostratigraphic record of the Yucca Formation, and 2. correlate the Yucca Formation with strata of similar age using chemostratigraphy. Methods:163 samples were collected from Big Bend Ranch State Park (BBRSP) to determine the δ¹³C value of bulk sedimentary organic matter. Results:C-isotope values range from −27.02‰ to −18.42‰. Discussion:Carbon isotope excursions (CIEs) that are associated with the Aptian-Albian Boundary are identified as well as CIEs associated with Oceanic Anoxic Events (1a and 1b). This allows us to conclude that the Aptian-Albian boundary is recorded within the Yucca Formation strata at about 71 m above the base of the section exposed in the Lower Shutup of the Solitario in Big Bend Ranch State Park. Regional correlation of the Yucca Formation to other chemostratigraphic records from other Cretaceous strata suggests that the Yucca Formation in BBRSP is time equivalent to the Sligo, Pine Island, James, Bexar, and a portion of the lower Glen Rose Formation on the Comanche Platform and to a portion of the lower Glen Rose Formation in Big Bend National Park.