The work is by Randon J. Flores, Robert A. Eagle and Aradhna Tripati, of the University of California, Los Angeles, and seven other authors. Among them is Luis M. Chiappe, of the Natural History Museum of Los Angeles County, which holds the teeth. It was published open access in Science Advances on 16 September 2026. The university press release was republished by ScienceDaily on 20 September.
The method uses clumped isotopes. Carbon and oxygen have rare, heavier forms, carbon-13 and oxygen-18. When a mineral forms, two heavy atoms sometimes end up bonded in the same carbonate group. This happens more often in the cold and less in the heat. Counting those bonds tells the temperature at which the mineral formed. Enamel forms inside the animal’s jaw, at body temperature. The press release says it resists alteration better than bone.

Delta 47 values in the enamel of three Tyrannosaurus rex tooth samples (blue) and five crocodilian teeth (orange) from the Hell Creek Formation, United States, Late Cretaceous. The diamond is each tooth’s final value, the hollow circles the single analyses. Higher values mean a lower formation temperature. Raw data deposited by the authors, not converted to degrees. Chart produced by ScienceOnline from data in Flores R.J. et al., Science Advances 2026, doi 10.1126/sciadv.aeb7653, repository doi 10.5281/zenodo.22696419 (licensed CC BY 4.0). Original newsroom chart.

According to the abstract, measurements on three T. rex teeth give 36.3 degrees, with a standard error of 2.5. The standard error shows how uncertain the mean is, not how much the animal’s temperature varied. The value is comparable to modern endotherms, animals that make their own heat, such as mammals and birds. Teeth from crocodilians that lived in the same formation give significantly lower temperatures. The crocodilian figure is missing from the abstract. The press release does, 30 degrees.
The raw data are public, in an authors’ repository on Zenodo and GitHub. They cover five crocodilian teeth and three T. rex samples, each analysed two to six times. The quantity measured is called delta 47, and it is higher when bonds between heavy isotopes are more frequent, that is, when the enamel grew at a lower temperature. In the crocodilians the mean of the final values is 0.576 per mil. In the tyrannosaur it is 0.563. The direction is the expected one, with the tyrannosaur warmer. Converting to degrees depends on a calibration described in the full text, which was not read.
The press release speaks of two teeth from Thomas, the T. rex skeleton on display in the Los Angeles museum. The abstract speaks of three teeth. In the repository the T. rex samples are three, under two different catalogue numbers.
The crocodilians act as a control. According to the press release, Eagle’s reasoning runs like this. If the rocks had altered the enamel after burial, the two animals should give similar values. They give different values, and the signal is judged to be biological.
The abstract adds a comparison with environmental temperatures taken from other proxies and from simulations of the climate of the time. T. rex kept its body warmer than its surroundings. At that temperature, most of the North American palaeocontinent, including the cold high latitudes, was potentially open to it. The press release describes a possible range from Mexico to Alaska.
The abstract does not use the phrase «warm-blooded». It speaks of a temperature comparable to endotherms. The press release instead says the result is direct evidence of warm blood. A high body temperature can have more than one cause, and an animal weighing many tonnes holds heat through its bulk alone. How the authors deal with that possibility is not in the abstract.
The sample is small. Three tyrannosaur teeth and five crocodilian teeth, all from a single formation. With 2.5 degrees of standard error the mean falls between about 33.8 and 38.8 degrees, with a probability of roughly two in three. Full methods, funding, competing interests and any statement on the use of artificial intelligence could not be checked. The press release cites partial support from the US National Science Foundation.
For a doctor, 36.3 degrees is a normal temperature.

References
Flores R.J., Eagle R.A., Trayler R.B., Larocca Conte G., Kim S.L., Chiarenza A.A., Farnsworth A., Valdes P.J., Chiappe L.M., Tripati A., «The body temperature of Tyrannosaurus rex», Science Advances, 12, 38, eaeb7653, 16 September 2026, open access (licence not verified). doi: 10.1126/sciadv.aeb7653
Flores R., Eagle R., Trayler R., Conte G., Kim S., Chiarenza A., Farnsworth A., Valdes P., Chiappe L., Tripati A., «The Body Temperature of Tyrannosaurus rex», code and data, Zenodo, 10 September 2026, licensed CC BY 4.0. doi: 10.5281/zenodo.22696419
University of California, Los Angeles, press release, republished by ScienceDaily on 20 September 2026 as «A 66-Million-Year-Old Tooth Reveals T. Rex Was Nearly as Warm as a Human».