A technique called clumped-isotope thermometry, usually used for reconstructing ancient climates, is shedding light on the age old debate over whether dinosaurs were warm- or cold-blooded. Computer models of heat loss and energy use, oxygen isotope measures, and other techniques have all yielded conflicting results. While evidence of energetic behaviors and high growth rates have suggested that the extinct reptiles’ veins may have carried warm blood throughout the body, some paleontologists have argued that an endothermic animal of that size would quickly overheat.

Comparing levels of various chemical isotopes in the enamel of fossilized dinosaur teeth, which form at different rates depending on the temperatures at the time of the chemical decomposition, evolutionary biologist Robert Eagle of the California Institute of Technology in Pasadena estimated that two large species of dinosuars—the 30-ton Brachiosaurus and the 50-ton Camarasaurus—had body temperatures between 36° and 38°C, nearly identical...

But that doesn’t mean that dinosaurs maintained their body heat the same way today’s mammals do. Their warm blood may simply be the result of the warm environment they inhabited, for example. In fact, if the ancient giants were endothermic like modern mammals, the insulating properties of their enormous size would have likely caused their temperatures to rise above 40°C—which would have killed them. If dinos were indeed endothermic, they must have also had a cooling system in place, Eagle and his colleagues suggest, perhaps losing heat through their necks and tails.

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