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

delta O-18(P) values and Sr-87/Sr-86 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 delta O-18(P) values derived from the teeth of bone beds are in the range of 17.6-23.5 parts per thousand VSMOW, and are mostly depleted in O-18 by 0.5-5 parts per thousand relative to proposed coeval marine delta O-18(P) values. This indicates an adaptation to freshwater habitats on the Early Permian coastal plain by several sharks. Distinctly higher delta O-18(P) values from two bone beds are attributed to significant evaporative enrichment in O-18 in flood plain ponds. Sr-87/Sr-86 ratios of around 0.71077 are notably more radiogenic than Sr-87/Sr-86 of contemporaneous seawater. In contrast, the isotopic composition of teeth from the marine Kaibab Formation is characterised by low delta O-18(P) values in the range of 13.4-15.6 parts per thousand VSMOW while Sr-87/Sr-86 ratios of around 0.70821 are closer to the Roadian seawater value. The distinctly depleted delta O-18(P) values 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.