NICHE PARTITIONING AND PALEOHYDROLOGY: MULTI-TAXA OXYGEN ISOTOPIC ANALYSIS OF VERTEBRATE PHOSPHATES, CRETACEOUS CLOVERLY FORMATION, MONTANA, USA

The Cretaceous Cloverly Formation of Wyoming and Montana has produced one of the most complete continental vertebrate faunas known from the Early Cretaceous of North America, yet regional paleoclimatic and paleoecological conditions under which the Cloverly fauna evolved are poorly understood. Here we present some preliminary oxygen isotope data derived from phosphate of some fossil reptiles and fish from a single microfossil bonebed (OMNH V1075) in the Cloverly of Bighorn County, Montana. We calculated a δ¹⁸O_{meteoric_water} value of -7.2‰ (VSMOW) from pleurosternid turtle plastron and carapace fragments (δ¹⁸O_phosphate = 14.9‰, σ = 1.5‰), and a similar δ¹⁸O_{meteoric_water} value of -6.2‰ from enamel of button-like crocodylomorph teeth (δ¹⁸O_phosphate = 16.7‰, σ = 1.9‰). These meteoric water values both generally agree with a previously established δ¹⁸O-latitude gradient calculated for the Early Cretaceous based on δ¹⁸O_{pedogenic_carbonate} and plant proxies. Our preliminary estimate of habitat water temperature (16.1°C) from pleurosternid turtle δ¹⁸O_phosphate is consistent with previous independent analyses of δ¹⁸O_phosphate in Cloverly aquatic turtles. Our data shows insignificant (p > 0.05) variation between most taxa and between ecophysiological groups, but this is likely due to under sampling of most groups analyzed. However, due to the exceptional abundance of crocodylomorph teeth at the V1075 site, crocodylomorph tooth morphotypes were well sampled. Analysis of variance reveals that δ¹⁸O_phosphate of atoposaurid-like teeth (cf. Wannchampsus sp., δ¹⁸O_phosphate = 17.4‰, σ = 2.5‰, p = 0.018) and button-like teeth (cf. Bernissartiidae, δ¹⁸O_phosphate = 16.7‰, σ = 1.9‰, p = 0.053) differ from conical teeth (cf. Goniopholidae, δ¹⁸O_phosphate = 14.2‰, σ = 2.8‰), which we interpret as evidence of niche partitioning. Further sampling within some taxonomic and ecophysiological groups across time and space is required to draw more conclusive paleobiological and paleohydrological interpretations. Still, these preliminary results further suggest that oxygen isotopes from fossil vertebrate phosphates can offer insight into regional climates and evolution of terrestrial ecosystems of Early Cretaceous North America.