Fossil teeth place T. rex near 36 °C

Three fossil teeth turned Tyrannosaurus rex enamel into a 66-million-year-old thermometer. The analysis estimated an average body temperature of 36.3 degrees Celsius, with an uncertainty of plus or minus 2.5 degrees. The value is close to that of living large mammals and directly supports the view that this theropod generated and retained metabolic heat. The Science Advances result does not originate the hypothesis that T. rex was endothermic — able to keep its body warm through internal processes — but gives that hypothesis a physical measurement.

The team led by Randy Flores, with Robert Eagle and Aradhna Tripati of the University of California, Los Angeles, examined how rare forms of carbon and oxygen bond within enamel. These bonds, called clumped isotopes, become more common when a mineral forms at lower temperatures and less common at higher temperatures. Because enamel grows at the animal’s body temperature and resists chemical alteration better than bone, the proportion of these bonds retains thermal information.

The procedure still removes fossil material, but the team reduced the required amount by roughly 90% over a decade. A low-speed drill removed about five milligrams of enamel from each tooth. The powder was dissolved in phosphoric acid to release carbon dioxide, and a mass spectrometer measured the isotopes in the gas. Two teeth came from the young-adult skeleton known as Thomas, while a third fragment came from another individual; all were from the Hell Creek Formation in Montana.

Five crocodilian teeth from the same deposit served as a geological comparison. They produced an average of 30.9 degrees Celsius, consistent with ectothermic crocodilians whose temperature depends more heavily on the environment. If fossilization had imposed the same chemical signal on every tooth, the two groups would tend to converge. The difference cannot exclude every form of later alteration, but it supports the conclusion that the measured signal retains a biological component.

The T. rex estimate lies at the lower end of the range observed among endotherms: close to elephants and below the roughly 40 to 43 degrees Celsius common in many living birds. Using this value, the group’s paleoclimate models indicate a potentially habitable range from Mexico to Alaska. That conclusion fits tyrannosaur fossils found at cold latitudes, but temperature does not measure running speed, hunting frequency, or the exact amount of food consumed. Those traits remain ecological inferences.

The advance lies in the method and the number, not in permission to assign one physiology to every dinosaur. The sample contains three teeth from two individuals and one deposit, while different dinosaur lineages probably used different thermal strategies. Applying the isotope thermometer to other species, ages, and environments could show whether 36.3 degrees was typical of T. rex or only part of its variation. For now, enamel supplies the most direct evidence of a large predator thermally closer to an elephant than to a crocodile.