Ancient Teeth Reveal T. Rex Ran as Warm as a Human Being

A 66-Million-Year-Old Thermometer

For decades, the question of whether Tyrannosaurus rex was warm-blooded or cold-blooded has loomed over paleontology like the dinosaur itself. Now, scientists have finally cracked it — and the answer came from teeth. A new study published on September 16 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 result is striking: researchers have put a number on just how warm tyrannosaurs likely ran — about 97 degrees Fahrenheit, or 36 degrees Celsius, similar to humans.

The research team, led by UCLA scientists Robert Eagle and Aradhna Tripati, used clumped-isotope analysis on two teeth from "Thomas," a nearly complete T. rex skeleton housed at the Natural History Museum of Los Angeles County. "Tooth enamel forms inside the animal at body temperature, so the enamel chemistry is a thermometer," Tripati said. In other words, locked inside those fossilized teeth was a biological record that survived 66 million years — and scientists finally had the tools to read it.

A Decade of Painstaking Science

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. That breakthrough mattered enormously. While paleontologists had long suspected that carbon-oxygen chemical bonds could pinpoint a T. rex's blood temperature, earlier iterations of the process required a large fossil sample. Drilling so much material from a bone also damages it — and while there were plenty of megalodon teeth in museum archives, institutions weren't willing to donate a much rarer T. rex chomper.

The researchers ultimately used a minimally destructive technique — a low-speed rotary drill with a tungsten carbide bit — to take about 5 milligrams of enamel powder, less than a pinch of salt, from each tooth. 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 measuring those bonds, the team could reconstruct the creature's internal temperature with remarkable precision.

What It Means for the King of the Dinosaurs

T. rex's internal temperature sits ahead of today's cold-blooded reptiles and even sloths, but short of their avian descendants, which range between 104 and 109 degrees Fahrenheit. For comparison, the researchers also applied the technique to five similarly old crocodilian teeth from the same rock formation, finding average body temperatures of 87.6°F (30.9°C), in line with modern crocs and lower than in Tyrannosaurus. That gap is telling. "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."

A warm-blooded T. rex makes sense for the apex predator's active hunting lifestyle and fast metabolism. It also helps explain how the dinosaur roamed such a large territory encompassing wide temperature shifts — a big reason why paleontologists have found T. rex fossils in places like present-day Alaska that are otherwise devoid of cold-blooded ancient reptiles. 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. Their models suggested that the dinosaur could have inhabited a broad stretch of North America, from what is now Mexico to Alaska.

A New Tool for Unlocking Prehistoric Life

Researchers have estimated dinosaur metabolism and growth rates before, but Tripati says the new study's method is especially precise at estimating internal temperature because tooth enamel and thermodynamic estimates aren't as sensitive to confounding factors such as environmental temperatures. That precision opens a door that paleontologists are eager to walk through. In the future, researchers say it would be interesting to explore the question of 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 that question.

For decades, paleontologists relied on skeletal structures and biomechanics to infer metabolism. This interdisciplinary approach merging geochemistry with paleontology offers a direct, chemical measurement of ancient biology. The T. rex has always captured the human imagination, but science is only now catching up to the full reality of what this animal was. A warm-blooded, continent-spanning apex predator that ran at human body temperature — the king of the