Stable and radiogenic isotope analyses on shark teeth from the Early to the Middle Permian (Sakmarian–Roadian) of the southwestern USA

δ¹⁸O_Pvalues and⁸⁷Sr/⁸⁶Sr ratios were determined on disarticulated xenacanthiform, hybodontid and ctenacanthid shark tooth material from several Early Permian (Sakmarian–Kungurian) continental bone beds of northern Texas and southern Oklahoma as well as from the marine Middle Permian (Roadian) of northern Arizona. The δ¹⁸O_Pvalues derived from the teeth of bone beds are in the range of 17.6–23.5‰ VSMOW, and are mostly depleted in¹⁸O by 0.5–5‰ relative to proposed coeval marine δ¹⁸O_Pvalues. This indicates an adaptation to freshwater habitats on the Early Permian coastal plain by several sharks. Distinctly higher δ¹⁸O_Pvalues from two bone beds are attributed to significant evaporative enrichment in¹⁸O in flood plain ponds.⁸⁷Sr/⁸⁶Sr ratios of around 0.71077 are notably more radiogenic than⁸⁷Sr/⁸⁶Sr of contemporaneous seawater. In contrast, the isotopic composition of teeth from the marine Kaibab Formation is characterised by low δ¹⁸O_Pvalues in the range of 13.4–15.6‰ VSMOW while⁸⁷Sr/⁸⁶Sr ratios of around 0.70821 are closer to the Roadian seawater value. The distinctly depleted δ¹⁸O_Pvalues cannot be readily explained by fluvially affected freshening in a nearshore marine environment, so a diagenetic alteration of the Kaibab material seems to be more likely, excluding it from further interpretation.