T. rex teeth indicate it ran as warm as an elephant - Ars Technica

Overview

T. rex teeth indicate it ran as warm as an elephant

Isotope ratios provide a hint that the giants were actively managing temperatures.

Details

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.

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 spots on the riverbank. The gap between the T.rex and contemporary crocodiles also mirrors the one measured today between large mammals and crocodilians.

The next thing to check was whether the T. rex could maintain stable body temperature or was simply as warm as its surroundings. To answer that, the scientists looked at clumped isotopes in fossil freshwater mussels from the same area in the Hell Creek Formation. Mussels mostly record summer water temperatures, and those turned out to average about 26°.

The team also ran a new high-resolution climate model of the late Cretaceous, with a grid of roughly 60 kilometers, under two scenarios: colder and hotter. Even in the hotter scenario, the warmest summer months at Hell Creek peaked at around 33°, with mean annual temperatures near 21°.

So T. rex was consistently warmer than the world it lived in. But body temperature, the authors note, is not an unambiguous indicator of metabolism.

A very large animal can stay warm simply because its bulk loses heat slowly, a phenomenon called inertial homeothermy, or gigantothermy. The juvenile T. rex’s temperature was higher than what body size scaling models predict for a cold-blooded animal of its weight. Although those models are debated, the team behind the new work argues that its results add to a growing body of evidence that T. rex was a homeothermic endotherm, an animal that kept a steady body temperature using its own metabolic heat, while acknowledging that the question is not yet settled.

To explore how far into colder climate zones T. rex could spread, the team built a computer model called a virtual species. They took thermal tolerance data for 465 living mammals and birds that maintain a stable body temperature and can cope with anything from -13° to 43.6°. Then they focused on the 34.7° to 37.3° range measured in the T. rex teeth and combined that with seasonal rainfall from the climate simulations. The resulting suitability curve was projected onto the map of late Cretaceous North America, accounting for different shorelines of the Western Interior Seaway, the shallow sea that split the continent at the time. The conclusion was that T. rex could live almost anywhere on the continent.

Known T. rex fossil sites fell mostly in areas of high predicted suitability but covered only a fraction of the estimated habitable territory, which the team thinks reflects where fossils get preserved and collected rather than where T. rex lived. The model also showed good suitability at the latitudinal extremes, consistent with tyrannosaurid finds in Alaska and a possible T. rex from the Trans-Pecos region of Texas. Cold weather aside, heat was probably not a limiting factor either.

The team calculated a combined measure of heat and humidity that sets a lethal limit for modern endotherms and found that threshold was never crossed anywhere in their Cretaceous simulations. A cold-tolerant T. rex also fits recent evidence that its lineage reached North America from Asia across the Bering Land Bridge.

To avoid damaging the fossils, the team sampled only small portions of each tooth, which makes it possible that they captured just a single season of growth. Two different areas of two different teeth from the juvenile T. rex yielded statistically consistent results, and none of the three teeth stood out as an outlier, but the researchers acknowledge that with three teeth, they cannot fully rule out a seasonal bias.

But if the study’s results hold, T. rex apparently was a warm-bodied predator with the physiological flexibility to roam from the humid south to the polar north of its continent. Using the same clumped isotope thermometry, the team now hopes to test how widespread this kind of physiology was among other dinosaurs and when it first evolved.

Science Advances, 2026.  DOI: https://doi.org/10.1126/sciadv.aeb 7653

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Key Takeaways

• Isotope ratios provide a hint that the giants were actively managing temperatures
• For most of the last century, T
• A team of researchers led by Randon J
• Paleontologists have long argued about dinosaur physiology based on indirect evidence like bone microstructure, growth rates, and where fossils turn up on the map