Reconstructing the body temperature of extinct marine reptiles

The body temperature of vertebrates provides clues regarding their physiology and the ambient environments. The 18 O/ 16 O ratio (δ 18 O) of phosphate in bioapatite is a function of δ 18 O of body water and body temperature of vertebrate. However, because it was difficult to constrain the mixing ratios between the ingested ambient and metabolic H 2 O in body water of each vertebrate, determining the δ 18 O of body water presents a challenge. Here, we used the triple oxygen isotope compositions (Δ’ 17 O) of phosphate in bioapatite to quantify the contribution of metabolic H 2 O to body water. We discovered that the contribution of metabolic H 2 O to the body water of mosasaur and plesiosaur was negligible by comparing the Δ’ 17 O 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 δ 18 O of phosphate in bioapatite by substituting the δ 18 O of body water with that of seawater.