STABLE ISOTOPE ANALYSIS OF A HELICOPRION TOOTH WHORL: TAPHONOMY AND UNDERLYING PALEOTEMPERATURE TRENDS

Helicoprion was one of the most distinctive large predatory fish of the Permian and remains one of the most enigmatic, with basic questions remaining about its life history, feeding habits, and diet. However, Helicoprion is unusual among chondrichthyans in that mineralized teeth are retained by the animal in a spiral whorl on the lower jaw rather than shed. This unique growth pattern may provide a way to address these questions. In some organisms that accrete and keep skeletal materials more or less continuously through their lives, (e.g., molluscan shells or coral skeletons), serial sampling along the growth axis can reveal cyclical geochemical patterns useful for reconstructing past temperatures, seasonality, habitat, and lifespan. Phosphate skeletal minerals, such as those found particularly in teeth, are commonly considered more resistant to taphonomic change than similar carbonate materials, and here we present initial oxygen isotopic analyses of samples from a partial Helicoprion tooth whorl consisting of six teeth (IMNH 49603). The sampled δ¹⁸O_phosphate values range from 14.7‰ (± 0.4) to 15.7‰ (± 0.1), corresponding to projected temperatures of 53 to 58 °C, dependent upon which paleotemperature equation is used. These results suggest a level of diagenetic alteration consistent with reports from chondrichthyan teeth of similar geologic setting. Despite this apparent alteration, the samples exhibit an underlying negative trend in δ¹⁸O (increase in temperature) along the whorl, which is difficult to explain by taphonomic overprinting alone. These initial results suggest that δ¹⁸O_phosphate may be useful in understanding the life and environments of these late Paleozoic predators. Further work will focus on evaluating the extent of alteration by utilizing other geochemical techniques, such as Raman spectroscopy, isotopic analysis of carbonate components to the same fossils, and analyzing larger, more-complete tooth whorls that may reveal the cyclical patterns in greater detail.