Abstract

The body temperature of vertebrates provides clues regarding their physiology and the ambient environments. The 18O/16O ratio (δ18O) of phosphate in bioapatite is a function of δ18O of body water and body temperature of vertebrate. However, because it was difficult to constrain the mixing ratios between the ingested ambient and metabolic H2O in body water of each vertebrate, determining the δ18O of body water presents a challenge. Here, we used the triple oxygen isotope compositions (Δ’17O) of phosphate in bioapatite to quantify the contribution of metabolic H2O to body water. We discovered that the contribution of metabolic H2O to the body water of mosasaur and plesiosaur was negligible by comparing the Δ’17O values of phosphate in their teeth with those of phosphate at isotope equilibrium with seawater, as predicted from the in vitro experiments using pyrophosphatase. Therefore, we accurately estimated their body temperature to range from 23 to 26°C, with an uncertainty of ± 5°C, from the δ18O of phosphate in bioapatite by substituting the δ18O of body water with that of seawater.