Oxygen isotopes from biogenic apatites suggest widespread endothermy in Cretaceous dinosaurs

The much debated question of dinosaur thermophysiology has not yet been conclusively solved despite numerous attempts. We used the temperature-dependent oxygen isotope fractionation between vertebrate body water (δ18Obody water) and phosphatic tissues (δ18Op) to compare the thermophysiology of dinosaurs with that of non-dinosaurian ectothermic reptiles. Present-day δ18Op values of vertebrate apatites show that ectotherms have higher δ18Op values than endotherms at high latitudes due to their lower body temperature, and conversely lower δ18Op values than endotherms at low latitudes. Using a data set of 80 new and 49 published δ18Op values, we observed similar and systematic differences in δ18Op values (Δ18O) between four groups of Cretaceous dinosaurs (theropods, sauropods, ornithopods and ceratopsians) and associated fresh water crocodiles and turtles. Expressed in terms of body temperatures (Tb), these Δ18O values indicate that dinosaurs maintained rather constant Tb in the range of endotherms whatever ambient temperatures were. This implies that high metabolic rates were widespread among Cretaceous dinosaurs belonging to widely different taxonomic groups and suggest that endothermy may be a synapomorphy of dinosaurs, or may have been acquired convergently in the studied taxa.