T. rex was nearly as warm-blooded as a human, tooth study finds

Tyrannosaurus rex ran a body temperature of about 97 degrees Fahrenheit, or 36 degrees Celsius, nearly matching a living human’s, according to a new analysis of fossilized tooth enamel led by researchers at UCLA. The measurement is one of the first direct readings of a dinosaur’s internal body heat, rather than an estimate inferred from bone growth rings or a family tree of living relatives.

“[The team] found T. rex was 36 Celsius (97 Fahrenheit), about the same as humans,” said Robert Eagle, a UCLA geobiologist and co-author of the study. “No one’s been able to make a temperature measurement like this before.”

Ten years to build a prehistoric thermometer

The technique behind the measurement, known as clumped isotope thermometry, tracks how pairs of rare carbon and oxygen isotopes bond together inside tooth enamel; the ratio of those bonded pairs shifts in a predictable way depending on the temperature at which the enamel formed. Eagle’s team spent roughly a decade refining the method before applying it to a Tyrannosaurus rex, according to the researchers’ account of the technique, cutting the amount of fossil material required by about 90 percent along the way so that museums would allow it on irreplaceable specimens.

That patience mattered for one specific reason: enamel resists chemical change far better than bone does. “Tooth enamel has large crystalline structures that are extremely durable, making it the part of the skeleton that most resists chemical alteration,” Eagle said, according to UCLA’s newsroom release on the findings, which is why the isotope signal inside a 66-million-year-old tooth can still be trusted today.

Two teeth from Thomas the T. rex

The enamel came from small portions of two teeth belonging to a specimen nicknamed Thomas the T. rex, held at the Natural History Museum of Los Angeles County and excavated from the Hell Creek Formation in Montana. Luis Chiappe, curator of the museum’s Dinosaur Institute, signed off on sacrificing that material for the analysis. “Sacrificing small portions of two teeth to learn about T. rex’s body temperature is definitely worth the trade-off,” Chiappe said.

Researchers drilled the enamel into powder, dissolved it in phosphoric acid to release carbon dioxide gas, and ran that gas through a mass spectrometer to read the isotope ratios, arriving at 36.3 degrees Celsius with an uncertainty of roughly plus or minus 2.5 degrees, according to a technical write-up of the method.

The number lands close to, but not exactly at, human body temperature.

Modern humans run around 98.6 degrees Fahrenheit, or 37 degrees Celsius, while cold-blooded reptiles alive today typically sit between 82 and 86 degrees Fahrenheit, and an ancient crocodilian the team analyzed for comparison came in at 86 degrees Fahrenheit, or 30 degrees Celsius. Modern birds, which descended from theropod dinosaurs closely related to T. rex, run considerably hotter still, between 104 and 109 degrees Fahrenheit, according to a report on the comparisons included in the study.

What a 97-degree dinosaur could survive

A body temperature that high, sustained internally rather than absorbed from the sun, points toward an animal that generated a substantial share of its own heat rather than depending entirely on its surroundings. Aradhna Tripati, the study’s senior author and a UCLA isotope geochemist, has connected that finding to evidence that tyrannosaurs lived in places far colder than the swampy Cretaceous scenes most museum dioramas depict, including ancient environments in ways paleontologists have tied to high-latitude Alaska.

The authors are careful about how far the result travels beyond T. rex itself. The study does not establish that every dinosaur shared this metabolism, and separate coverage of the paper notes it says nothing directly about Triceratops, feathered dinosaurs, or other lineages that would need their own tooth or bone samples analyzed the same way.

Global temperatures during the Cretaceous ran an estimated 11 to 25 degrees Fahrenheit warmer than today’s, which complicates how the 97-degree figure should be read. A purely cold-blooded animal soaking up heat from that warmer atmosphere could, in principle, register a higher body temperature than a modern reptile without generating any extra heat internally at all. That is part of why the team leaned on its comparison animals rather than the T. rex figure alone: the ancient crocodilian analyzed alongside it lived through the same hotter Cretaceous climate and still measured a full six degrees Celsius cooler using the identical method.

A gap that size, measured against an animal from the same warm era, is harder to explain through climate alone and points instead toward heat production happening inside the tyrannosaur’s own body. Eagle’s team says the logical next step is running the same tooth-enamel analysis on other large theropods and on plant-eating dinosaurs like Triceratops, to find out whether a body temperature this close to a human’s was specific to T. rex or common across the animals that shared its world.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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