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 {delta}18O value and oxygen isotope fractionation relative to environmental sea water. Here, we present new hydroxylapatite phosphate {delta}18O values ({delta}18Op) 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 {delta}18Op 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 {delta}18Op bone record. Body temperature estimates have then been reassessed from new and published {delta}18Op values of well-preserved isolated teeth, recently revised Mesozoic latitudinal {delta}18O oceanic gradient and 18O-enrichment factor of fully aquatic air-breathing vertebrates. Our results confirm that Ichthyosauria were homeothermic endotherms (31{degrees}C to 41{degrees}C), while Plesiosauria were likely poikilothermic endotherms (27{degrees}C to 34{degrees}C). The new body temperature estimates of the Metriorhynchidae (25{degrees}C to 32{degrees}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 {delta}18Op values from Ichthyosauria fossil remains could be used as valuable archives of Mesozoic oceans {delta}18Osw values that may help improve palaeoenvironmental and palaeoclimatic reconstructions. Non-technical abstractSome marine reptiles from the Mesozoic such as ichthyosaurs, plesiosaurs and metriorhynchids, were capable of reaching elevated body temperatures and for some of them to maintain it few degrees above that of their marine environment, a characteristic similar to that observed in modern cetaceans. Nevertheless, the estimation of their body temperature from the chemical oxygen signature of their fossil remains (bones and teeth) is accompanied by uncertainties associated with the chemical oxygen signature of the surrounding water and the mineralisation processes of the bones and teeth. In this study, new data were collected from four ichthyosaurs, three plesiosaurs and one metriorhynchid in order to gain a deeper understanding of the mechanisms by which these marine reptiles were able to maintain a body temperature higher than that of their environment. The chemical signatures of oxygen in the bones and teeth of the specimens did not exhibit any discernible patterns indicative of specific zones of heat production or loss, as observed in modern marine vertebrates. Concurrently, we reassessed the estimated body temperature of these marine reptiles, thereby corroborating the hypothesis that ichthyosaurs were homeothermic endotherms. Conversely, our novel estimates suggest that plesiosaurs were likely poikilothermic endotherms, whereas metriorhynchids were probably also poikilothermic endotherms but with a limited capacity for heat production. Finally, the narrow range of body temperatures maintained by ichthyosaurs indicates that the oxygen chemical signature of fossilised remains could serve as a valuable marker for reconstructing variations in the oxygen isotope composition of the Mesozoic oceans, paving the way to enhance our understanding of the environment and climate of this period in Earths history.