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 for some of them homeothermic organisms. However, these conclusions remain contentious owing to the associated uncertainties regarding the δ18O value and oxygen isotope fractionation relative to environmental sea water. Here, we present new hydroxylapatite phosphate δ18O values (δ18Op) 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 intra-skeletal δ18Op variability failed to reveal distinct heterothermic patterns within any of the specimens, indicating either intra-body temperature homogeneity or an overriding diagenetic overprint of the original biological δ18Op bone record. Body temperature estimates have then been reassessed from new and published δ18Op values of well-preserved isolated teeth, recently revised Mesozoic latitudinal δ18O oceanic gradient and 18O-enrichment factor 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 abilities. These results improve our understanding of Mesozoic marine reptile thermoregulation and indicate that due to their limited body temperature variations, the δ18Op values from Ichthyosauria fossil remains could be used as valuable archives of Mesozoic oceans δ18Osw values that may help improve palaeoenvironmental and palaeoclimatic reconstructions.