Isotope ratios provide a hint that the giants were actively managing temperatures. For most of the last century, T. rex was pictured as a sluggish, tail-dragging reptile that had to warm up in the sun before it could go anywhere. Then, further research changed that view, replacing it with the active, bird-like animal shown in the movie Jurassic Park.

But whether T. rex used warm blood to power that activity remained a question. Now we might have gotten closer to an answer. A team of researchers led by Randon J.

Flores and Robert A. Eagle, geochemists at the University of California, Los Angeles, has measured the T. rex body temperature by analyzing its teeth. This dental thermometer read about 36° Celsius—roughly the body temperature of a modern elephant.

Paleontologists have long argued about dinosaur physiology based on indirect evidence like bone microstructure, growth rates, and where fossils turn up on the map. Some studies suggested many dinosaurs were endotherms, generating their own body heat like birds and mammals. Others argued that each lineage may have had its own thermal strategy.

Earlier attempts to measure T. rex’s temperature relied on oxygen isotopes in bones and teeth. The problem was that those ratios depend not only on temperature but also on the composition of the water present in the animal’s body, which was something nobody could pin down for an extinct creature. Flores, Eagle, and their colleagues got around that with a technique called clumped isotope thermometry, which Eagle introduced to dinosaur research over a decade ago in work on Jurassic sauropods.

Carbonate minerals in tooth enamel contain rare, heavy isotopes of carbon and oxygen (carbon-13 and oxygen-18). How often these heavy atoms bond with each other, or clump together, depends on the temperature at which the mineral formed. Since tooth enamel forms inside a living body, the number of those clumped bonds records the animal’s body temperature, regardless of the water content in its body.

The team analyzed three T. rex teeth provided by the Natural History Museum of Los Angeles County, all from Montana’s Hell Creek Formation, which preserves the final period of time before the asteroid impact. Two of the teeth belonged to a young adult T. rex that likely weighed over 3 tons. The third one was an isolated partial tooth from another individual.

For comparison, the researchers also analyzed five teeth of crocodilians that shared the same rivers and floodplains. First, though, the team had to make sure that 66 million years underground hadn’t changed the chemistry of the teeth. They focused on enamel, which is far more resistant to alteration than bone or dentin, and ran multiple checks.

It turned out the enamel and dentin of the same teeth had different isotopic signatures, which would be unlikely if their chemical composition had been changed after burial. Infrared spectroscopy showed that the fossil enamel looked a bit like that of the modern alligator, and its carbonate content matched that of modern reptilian enamel. Carbon isotopes even showed the dietary signal expected from predators.

Once the team was sure that the millions of years underground did not change the teeth, they used their method to infer the body temperature. A warm reptile The two teeth from the juvenile T. rex yielded temperatures of 37.3° and 35.9° Celsius, while the tooth from the second individual came in at 34.7°. Averaged together, T. rex had a body temperature of 36.3°, give or take 2.5°.

That lines up well with Indian and African elephants, which run at around 36°, and falls within the margin of error of large flightless birds like ostriches and emus. It’s a bit cooler than smaller flying birds, which average above 41°. The crocodilians the team used as a comparison came in at an average of 30.9°—in line with the preferred range of modern crocodilians, which keep themselves at around 30° to 35° by shuttling between the water and basking...