Crocodile teeth from the same Montana rock anchor T. rex's 36 C body temperature
A UCLA team read rare-isotope bonds in Tyrannosaurus rex tooth enamel and got about 36 degrees Celsius, a figure that took some 15 years of work to make measurable from a museum-sized sample.
The Scientist · Science desk
What happened
- The paper was published Sept. 16 in Science Advances, and it is the first time researchers have reported an actual temperature figure for the animal rather than an inferred metabolic range.
- Rare heavy isotopes of carbon and oxygen bond together more often in enamel formed at cooler temperatures, so a warm-bodied animal leaves fewer of those bonds behind in its teeth.
- A decade of refinement cut the amount of material the test consumes by roughly 90 percent, after the original bone-based version needed more sample than a museum would surrender.
Why it matters
- What's now possible capability Any fossil that kept its enamel is now a candidate for a direct temperature reading, and Eagle says avian dinosaurs and ancient mammalian ancestors are the next targets.
- What it limits constraint The signal survives in enamel because its crystals resist chemical alteration, so lineages and skeletal elements without enamel stay outside the method's reach.
- What doesn't add up contradiction Persons says growth rates, predator-prey ratios and anatomy had already made the warm-blooded case extensively, so the figure sharpens a picture the field already agrees on.
- What it sets up precedent Future physiology claims about extinct animals now have a harder standard to meet: an isotopic value paired with an ectotherm control from the same beds.
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The control sits in the same rock as the fossil. Alongside the T. rex enamel, Robert Eagle's group at UCLA measured five similarly aged fossilised crocodile teeth from Hell Creek [5]. Crocodiles are ectotherms, warming up by basking in the sun [25]. "We also found that the reconstructed temperature of T. rex was significantly elevated compared to temperatures derived from crocodile teeth from the same geological formation - Hell Creek in Montana," Eagle said [21]. That comparison rules out the dull explanation, which is that the enamel simply recorded its surroundings. The team also checked the T. rex values against ambient temperatures for prehistoric Montana reconstructed from isotopic estimates and from climate models, and the animal came out warmer than both [22]. Aradhna Tripati, a geoscientist at UCLA and a co-author, told Scientific American that "the enamel chemistry is truly recording the temperature at which it formed, and 66 million years later, we can read it using those measurements on a mass spectrometer" [17]. The bone version of the method existed a decade ago, but running it on a T. rex would have needed more bone than a museum would part with [11]. The sample had to shrink. Pressurising the released carbon dioxide into a denser jet let the mass spectrometer work from less powder, and the refinement cut the material required by roughly 90 percent [9][12]. That was small enough for the Natural History Museum of Los Angeles County to release portions of two teeth from Thomas, the most complete tyrannosaur in its dinosaur hall [4]. Enamel was the target because "tooth enamel has large crystalline structures that are extremely durable, making it the part of the skeleton that most resists chemical alteration by the environment over the eons," Eagle said [7]. He told Scientific American the result is the culmination of some 15 years of work [14]. Published accounts of the sample differ. Live Science and New Scientist describe three T. rex teeth from the Hell Creek Formation [3]; the UCLA account describes portions of two teeth from Thomas [4][28]. The figure itself appears as 36 C in most accounts and as 36.3 C in Live Science's [1][2]. Modern cold-blooded reptiles run about 28 to 30 C, and most birds run 40 to 43 C [16][18]. T. rex therefore lands 6 to 8 degrees above the reptile band and 4 to 7 below the bird one [29]. A 2022 study led by Jasmina Wiemann of Johns Hopkins had already put T. rex at the lower end of endothermy [19], and she called the new paper "absolutely mind-blowing" and "impressively precise" [26]. Scott Persons of the South Carolina State Museum, who was not involved in the work, said the existing evidence already included bone growth rates and predator-prey ratios, along with a "slew of anatomical traits consistent with high-performance athleticism" [20]. Thomas Carr at Carthage College called the study an "example of science in action, converging upon a single, robust hypothesis, which in this case reveals T. rex as an active top predator" [27]. New Scientist described the enamel result as adding weight to previous findings [23]. "Using thermal performance curves and climate modelling based on these results, we found that the entire North American continent would have been habitable for T. rex in the Cretaceous Period," Eagle said [10], a period roughly 6 to 14 C warmer than today [13]. Wiemann goes further, saying the temperature would have let T. rexes live anywhere on Earth, even the Arctic [31]. The fossil evidence at high latitude is a tyrannosaur from the Kikak-Tegoseak Quarry in Alaska, from beds with no trace of lizards, turtles or crocodiles of the same period [15].
What to watch
- Whether the enamel method reproduces on avian dinosaurs and ancient mammalian ancestors, which Eagle says he wants to try next.
- Whether an independent lab gets the same value from teeth of other T. rex individuals or from other formations.
- Whether the continent-wide habitability conclusion survives a different Cretaceous climate reconstruction, since it comes from thermal performance curves plus modelling.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence70
- Adoption
- Not scored
- Hype gap+15
- Incentives35
- Confidence65
Perspective Coverage
5 publishers- Builder
- Builder 75%
- Operator
- Operator 25%
- Investor
- Investor 0%
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
UCLA scientists measured T. rex body temperature from tooth enamel and found it was 36 degrees Celsius (97 F), almost the same as humans.
- [2]
Live Science reported the T. rex body temperature figure as 97.3 degrees Fahrenheit (36.3 degrees Celsius), comparable to that of a human or an elephant.
- [3]
Scientists estimated the dinosaur's body temperature by studying isotopes in the enamel of three T. rex teeth from specimens found in the Hell Creek Formation in Montana.
- [4]
The Natural History Museum of Los Angeles County provided portions of two teeth from Thomas the T. rex, the most complete tyrannosaur skeleton in its dinosaur hall.
- [5]
Robert Eagle and colleagues analysed the chemistry in the enamel of three T. rex teeth and compared them with five similarly aged fossilised crocodile teeth.
- [6]
Rare heavy isotopes of carbon and oxygen bond together more abundantly in enamel that forms at cooler temperatures, so warm-blooded animals have fewer of these bonds in their teeth than cold-blooded animals.
- [7]
Eagle said: "Tooth enamel has large crystalline structures that are extremely durable, making it the part of the skeleton that most resists chemical alteration by the environment over the eons."
- [8]
The study was published Sept. 16 in Science Advances and marks the first time scientists have had an actual number for T. rex body temperature.
- [9]
Pressurizing the gas allowed the team to create a denser jet of CO2, providing a large enough sample for the mass spectrometer from a smaller amount of material.
- [10]
Eagle said: "Using thermal performance curves and climate modelling based on these results, we found that the entire North American continent would have been habitable for T. rex in the Cretaceous Period."
- [11]
UCLA researchers developed the method a decade ago using bones, but testing it on a T. rex required more bone than any museum would part with.
- [12]
Over the decade the UCLA team refined the process, cutting the amount of material needed for ancient dental testing by roughly 90 percent.
- [13]
The Cretaceous was one of the warmest periods in Earth's history, about 11 to 25 F (6 to 14 C) warmer than today.
- [14]
Eagle, who pioneered the approach to measuring prehistoric body temperatures, says the findings are a culmination of some 15 years of work.
- [15]
A tyrannosaur fossil has been found in the Kikak-Tegoseak Quarry in Alaska, although there is no trace of other reptiles such as lizards, turtles or crocodiles from that period in the region.
- [16]
Modern cold-blooded reptiles run about 28 to 30 degrees Celsius (82 to 86 F).
- [17]
Aradhna Tripati, a geoscientist at UCLA and a co-author, says: "The enamel chemistry is truly recording the temperature at which it formed, and 66 million years later, we can read it using those measurements on a mass spectrometer."
- [18]
Most birds, T. rex's evolutionary descendants, often run much warmer at 40 to 43 degrees Celsius (104 to 109 F).
- [19]
A 2022 study showed dinosaurs likely had a range of metabolisms and estimated T. rexes that were on the lower end of endothermy; its lead author was Jasmina Wiemann, a paleobiologist at Johns Hopkins University.
- [20]
Scott Persons of the South Carolina State Museum, who was not involved in the new study, says there was already extensive evidence that T. rex was warm-blooded, including bone growth rates and predator-prey ratios and a "slew of anatomical traits consistent with high-performance athleticism".
- [21]
Eagle said: "We also found that the reconstructed temperature of T. rex was significantly elevated compared to temperatures derived from crocodile teeth from the same geological formation - Hell Creek in Montana."
- [22]
The researchers say T. rex body temperatures were higher than the ambient temperatures in prehistoric Montana, reconstructed both from isotopic estimates and from climate models.
- [23]
New Scientist reported that the 66-million-year-old tooth enamel evidence suggests T. rex was probably warm-blooded like birds and mammals, adding weight to previous findings.
- [24]
Eagle says of endothermy: "It comes at a cost though - higher metabolic rates mean higher food requirements and potentially more active hunting to acquire it."
- [25]
Ectothermic, or so-called cold-blooded animals like crocodiles, often warm up by basking in the sun instead of regulating body heat through metabolism.
- [26]
Wiemann says the new study is "absolutely mind-blowing" and "impressively precise".
- [27]
Thomas Carr at Carthage College in Wisconsin says the new work is an "example of science in action, converging upon a single, robust hypothesis, which in this case reveals T. rex as an active top predator".
- [28]
The published accounts differ on the sample: two describe three T. rex teeth from the Hell Creek Formation, while the UCLA account describes portions of two teeth from Thomas at the Natural History Museum of Los Angeles County.
- [29]
The 36 C reading sits 6 to 8 degrees above the modern cold-blooded reptile range and 4 to 7 degrees below the typical modern bird range.
- [30]
Eagle hopes to use the same technique to determine the body temperatures of avian dinosaurs and ancient mammalian ancestors next.
- [31]
Wiemann says the findings suggest that, based on their temperature, T. rexes likely could have lived anywhere on Earth, even the Arctic.
Sources
5 independent publishers whose own reporting we read for this story.
- discovermagazine.comDrilling Into 66-Million-Year-Old <em>T. Rex</em> Teeth Reveals a Human-Like Body Temperature
1 article · September 17, 2026
- livescience.comScientists finally figured out the temperature of T. rex's blood — and it was as hot as ours
1 article · September 16, 2026
- newscientist.comT. rex teeth show the enormous predatory dinosaur was warm-blooded
1 article · September 16, 2026
- phys.orgTyrannosaurus rex teeth reveal a body temperature close to the human average
1 article · September 16, 2026
- scientificamerican.comT. rex was as warm-blooded as humans, new study finds
1 article · September 16, 2026
Topics and entities
Topics
- Dinosaur thermoregulation
- Paleontology
- Clumped isotope thermometry
Entities
- Natural History Museum of Los Angeles County
- Hell Creek Formation
- Thomas the T. rex
- University of California, Los Angeles
- Science Advances
- Tyrannosaurus rex
- Alessandro Chiarenza