Supplementary data from: 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 bioapatite phosphate δ¹⁸O values (δ¹⁸O_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 intra-skeletal δ¹⁸O_p 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 δ¹⁸O_p bone record. Body temperature estimates have then been reassessed from new and published δ¹⁸O_p 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 δ¹⁸O_p values from Ichthyosauria fossil remains could be used as valuable archives of Mesozoic oceans δ¹⁸O_sw values that may help improve palaeoenvironmental and palaeoclimatic reconstructions.