New T. Rex discovery directly counters the long-held image of the dinosaur

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Chemical evidence embedded within Tyrannosaurus rex teeth indicates that the formidable predator was warm-blooded, maintaining an internal body temperature comparable to that of a human. Researchers noted that the discovery should fundamentally reshape public understanding of the famous dinosaur.

An examination of the chemical composition of enamel taken from three T. rex teeth unearthed in Montana revealed the creature maintained a body temperature of 97.3 degrees Fahrenheit (36.3 degrees Celsius), give or take 4.5 F (2.5 C). In comparison, average human body temperature sits at 98.6 F (37 C).

A closer biological comparison can be made to the elephant, the largest living land mammal, which averages roughly 97 F (36 C), and the ostrich, the largest living bird species, at about 102 F (39 C). Tyrannosaurus was among the largest carnivorous land animals in history.

The study’s confirmation of warm-bloodedness in Tyrannosaurus directly counters the long-held image of the species as a sluggish, slow-moving reptile.

“It stops being a reptile in the way people picture reptiles. An animal at 36 degrees Celsius is not waiting for the sun to warm up. It is thermally independent of its surroundings, and in that respect closer to an elephant or an ostrich than to a crocodile,” said UCLA geoscientist Aradhna Tripati, senior author of the research published on Wednesday in the journal Science Advances.

Tyrannosaurus inhabited western North America during the Cretaceous Period roughly 66 million years ago, near the end of the dinosaur era. Its fossil record spans from the Canadian provinces of Alberta and Saskatchewan southward through Montana, Wyoming, the Dakotas, New Mexico, and Texas.

Tripati explained that climate modeling suggests Tyrannosaurus could inhabit much of the continent, “including cold environments where it would have experienced freezing. That is a far more mobile and adaptable animal than the one people picture.”

Warm-bloodedness, or endothermy, refers to an organism’s capacity to internally generate and maintain heat. This grants evolutionary benefits such as continuous physical stamina and the ability to populate varied climates. Conversely, cold-bloodedness, or ectothermy, obligates creatures such as crocodiles and snakes to rely on external heat sources to warm themselves.

“Endothermy is common in large animals and even some fish. It comes at a cost, though,” UCLA geobiologist and study co-author Robert Eagle said.

“Higher metabolic rates mean higher food requirements and potentially more active hunting to acquire it. An endothermic T. rex would have had to acquire significantly more food to sustain its higher metabolic rate, but would also be capable of more sustained activity to find food and migrate, and live in colder environments,” Eagle said.

Mammals and birds are modern endotherms. Meat-eating dinosaurs fall within the theropod lineage, from which modern birds eventually emerged via small, feathered ancestors. Today, small flying birds maintain body temperatures around 107 F (42 C).

“Our temperature for T. rex sits at the low end of the endotherm range,” Tripati said.

Fossilized teeth provided the vital data for the team’s analysis.

“Tooth enamel forms inside the animal at body temperature, so the enamel chemistry is a thermometer,” Tripati said.

By measuring the distribution of specific carbon and oxygen isotopes preserved in the enamel—a method known as “clumped isotopes”—researchers determined internal temperatures.

Using a precise, low-speed rotary drill fitted with a tungsten carbide bit, the scientists extracted roughly 5 milligrams of enamel powder—less than a pinch of salt—from each sample.

The Natural History Museum of Los Angeles County supplied the specimens: two teeth from a well-preserved young adult individual named Thomas, and a third partial tooth from a separate Tyrannosaurus, both collected in Carter County, Montana.

For comparison, researchers applied the same test to five fossilized crocodilian teeth from the same geological formation, recording an average body temperature of 87.6 F (30.9 C), which matches modern cold-blooded reptiles and falls well below that of the T. rex.

“It’s thought that many dinosaurs, but not necessarily all, were warm-blooded,” Eagle said. “However even amongst warm-blooded organisms, there are questions of the degree. They can be warm-blooded like sloths or anteaters, with low-level endothermy; modern large mammals; or avian (bird) endotherms, which typically have higher metabolic rates than mammals.”

“On other dinosaurs, I would be cautious about generalizing,” Tripati said, regarding endothermy. “This is one species, and the evidence across dinosaurs points to a range of strategies rather than a single answer.”