Reassessment of body temperature and thermoregulation strategies in Mesozoic marine reptiles

Ichthyosauria, Plesiosauria, and Metriorhynchidae were apex predators in Mesozoic oceanic trophic networks. Previous stable oxygen isotope studies suggested that several taxa belonging to these groups were endothermic and that some of them were homeothermic organisms. However, these conclusions remain contentious owing to the associated uncertainties regarding the δ ¹⁸ O value and oxygen isotope fractionation relative to environmental seawater. Here, we present new bioapatite phosphate δ ¹⁸ O values (δ ¹⁸ O _p ) of Ichthyosauria, Plesiosauria, and Metriorhynchidae (Middle Jurassic to Early Cretaceous) recovered from mid- to high paleolatitudes to better constrain their thermophysiology and investigate the presence of regional heterothermies. The intraskeletal δ ¹⁸ O _p variability failed to reveal distinct heterothermic patterns within any of the specimens, indicating either intrabody temperature homogeneity or an overriding diagenetic overprint of the original biological δ ¹⁸ O _p bone record. Body temperature estimates have been reassessed from new and published δ ¹⁸ O _p values of well-preserved isolated teeth, recently revised Mesozoic latitudinal δ ¹⁸ O oceanic gradients, and ¹⁸ O-enrichment factors of fully aquatic air-breathing vertebrates. Our results confirm that Ichthyosauria were homeothermic endotherms (31°C to 41°C), while Plesiosauria were likely poikilothermic endotherms (27°C to 34°C). The new body temperature estimates of the Metriorhynchidae (25°C to 32°C) closely follow ambient temperatures and point to poikilothermic strategy with no or little endothermic ability. These results improve our understanding of Mesozoic marine reptile thermoregulation and indicate that due to their limited body temperature variations, the δ ¹⁸ O _p values from Ichthyosauria fossil remains could be used as valuable archives of Mesozoic oceans δ ¹⁸ O _sw values that may help improve paleoenvironmental and paleoclimatic reconstructions.