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 (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.