Fossil tooth analysis confirms Tyrannosaurus rex was warm-blooded

By Taylor Nicioli, CNN

(CNN) — Scientists have suspected for nearly 60 years that Tyrannosaurus rex was warm-blooded.

When a fossilized footprint of T. rex turned up in Alaska in 2022, it added to the evidence that these predators could thrive in frigid environments, where most cold-blooded reptiles could not. Now, researchers have put a number on just how warm tyrannosaurs likely ran: about 97 degrees Fahrenheit, or 36 degrees Celsius, similar to humans.

A new study published Wednesday in the journal Science Advances provides the first direct estimate of T. rex’s body temperature, using a technique that analyzes chemical signatures preserved in fossil teeth. The finding adds a crucial piece of evidence to the decades of research suggesting the giant predator was warm-blooded and capable of maintaining an active metabolism even in cold climates.

“It’s probably the most direct and least complicated constraint on the actual body temperature of a T. rex that’s been possible before,” said study coauthor Robert Eagle, a geobiologist and associate professor at the University of California, Los Angeles.

Having an estimated body temperature for T. rex provides a better glimpse into what life was like for the dinosaur 69 million to 66 million years ago. “This was a warm-bodied organism, which really places new constraints on its behavior, its physiology, how it interacted with other species and how much food it needed, for example,” Eagle added.

So far, the technique has been used on a handful of ancient species, including other dinosaurs, woolly mammoths and megalodons, giving scientists a clearer picture of how extinct animals regulated their body temperatures in ancient climates.

The temperature is unusually high for a reptile, but Eagle said the number itself was not surprising. Dinosaurs, while classified as reptiles, share an evolutionary link with birds. The new number reveals that T. rex sits between the two — hotter than reptiles but colder than modern avians — and allows scientists to form new hypotheses about how exactly the prehistoric predator might have moved and lived.

Analyzing T. rex teeth

Eagle and a team of scientists used two small portions of teeth from Thomas the T. rex — an approximately 70% complete Tyrannosaurus rex skeleton held at the Natural History Museum of Los Angeles County — to estimate the dinosaur’s body temperature from chemical bonds preserved in its tooth enamel.

The researchers spent more than a decade perfecting the method to make the measurement possible. Over the years, their work reduced the amount of fossil material needed by roughly 90%, enabling the museum to provide tiny samples of two teeth that weren’t on display.

“Nobody can measure a T. rex tooth unless you can show them you only need a few milligrams,” said senior study author Aradhna Tripati, a climate scientist and professor of geochemistry at UCLA.

“For an animal this famous, it is remarkable how little we actually knew. We had guesses about T. rex metabolism, mainly from bone and biomechanics. We did not have a temperature,” Tripati added in an email. “The enamel chemistry recorded the temperature at which it formed, and 66 million years later we can read it.”

The researchers essentially used the T. rex teeth as a prehistoric thermometer. Inside tooth enamel are different forms of the elements carbon and oxygen, called isotopes. These isotopes form bonds with one another at rates that depend on temperature — cooler temperatures produce more bonds, while warmer ones produce fewer. By drilling out a few milligrams of enamel and measuring those bonds, scientists could determine the temperature at which the enamel formed, revealing the T. rex’s body temperature.

This technique can be used on any part of the skeleton, but teeth work best because their hard enamel is less likely to change over millions of years, Eagle explained.

“We have wondered about whether dinosaurs were warm-blooded or not literally since the first time dinosaurs got their name,” said vertebrate paleontologist Thomas Holtz Jr., referring to Richard Owen’s 1842 paper, which introduced the name Dinosauria and suggested that dinosaurs might have had characteristics of warm-blooded animals.

Holtz added that while there has been various evidence supporting the idea that dinosaurs were endothermic, or warm-blooded, this new method offers another way to examine the question. Comparing T. rex’s body temperature with those of cold-blooded crocodiles and clams from the same time and place “gives high confidence that the values they find for Tyrannosaurus really show a high body temperature,” said Holtz, a principal lecturer in the department of geology at the University of Maryland. He was not involved in the study.

In the future, Holtz said it would be interesting to explore the question on whether all dinosaurs were warm-blooded. Using this method next on groups such as Triceratops, Stegosaurus and Brachiosaurus, which have sometimes been considered “cooler-blooded,” could help answer the question, he added.

T. rex was hotter than some modern mammals

Although T. rex was a reptile, its estimated body temperature of 97 F falls within the range seen in modern warm-blooded animals. Warm-blooded species vary considerably in body temperature. Some mammals, including sloths and anteaters, can have temperatures in the low 90s Fahrenheit (around 32 C), while small, high-metabolism birds can exceed 104 F (40 C), Eagle said.

Modern cold-blooded reptiles typically have body temperatures around 82 F to 86 F (28 C to 30 C). Eagle added that these temperatures align with animals’ energy levels — reptiles are often more idle, whereas birds have to expend a lot of energy while in flight.

A higher, regulated body temperature also would have helped T. rex stay active for longer periods.

“I don’t think T. rex is thought to be a particularly fast sprinter, but the implications of this are maybe that it could maintain an energetic performance over longer periods, probably, than a cold-blooded organism would be able to,” Eagle said. For example, “crocodiles can run over short distances, but they’re not able to keep it up for a long time.”

A warm-bodied T. rex supports the idea that tyrannosaurs were active predators with fast metabolisms. The dinosaur’s ability to maintain a high body temperature also would have allowed it to survive in cooler climates where cold-blooded animals could not.

“What the teeth tell us is that T. rex was warmer than the environment around it,” Tripati said. “A warm-blooded T. rex is a T. rex that can go almost anywhere on the continent, including the Arctic. That is a very different animal from a sun-basking reptile.”

To explore where the dinosaur could have lived, the researchers used paleoclimate models to reconstruct what temperatures were like across North America about 66 million years ago during the late Cretaceous. The researchers then considered those conditions alongside T. rex’s estimated body temperature. Their models suggested that the dinosaur could have inhabited a broad stretch of North America, from what is now Mexico to Alaska.

The high body temperature also implies that the T. rex would have had to pursue prey and eat more than other reptiles, such as the cold-blooded giant crocodiles of the Mesozoic, to keep up with their high metabolism, Holtz added.

Eagle said that researchers hope eventually to apply the temperature-measuring technique to other ancient species, potentially reaching even further back in evolutionary history.

“I think the ancestors of mammals are particularly exciting to look at. There are not great constraints about when warm-blooded mass occurred in the evolution of mammals,” Eagle said. “That’s in itself an extra challenge because then we’re getting even further back in time. But that would be really exciting if that works.”

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