Now, scientists have achieved what was once considered impossible: they have measured the body temperature of a Tyrannosaurus rex.

UCLA researchers developed a method to estimate body temperature from fossilized teeth and applied it to specimens housed at the Natural History Museum of Los Angeles County. Their results show that Tyrannosaurus maintained a body temperature of approximately 97 degrees Fahrenheit, or 36 degrees Celsius—surprisingly close to human body temperature.

The finding supports the idea that Tyrannosaurus was an active predator or scavenger with a relatively fast metabolism, rather than a sluggish animal that relied heavily on sunbathing. The ability to generate and regulate internal heat may also help explain how tyrannosaurs survived in cold regions that were inhospitable to many other reptiles.

A fossil thermometer improved over 10 years

UCLA researchers first developed the prehistoric thermometer method about a decade ago. The technique uses skeletal material to investigate whether extinct animals were warm-blooded or cold-blooded.

Initially, applying the method to Tyrannosaurus presented a major problem. Early versions required more fossil material than museums could reasonably sacrifice from important and irreplaceable specimens.

Over the next decade, however, the UCLA team dramatically improved the technique. Researchers reduced the amount of material needed to analyze ancient teeth by about 90 percent.

This improvement allowed the Natural History Museum of Los Angeles County to provide a small portion of two teeth from Thomas II, the most complete Tyrannosaurus rex skeleton on display in the museum’s Dinosaur Hall.

Although other lines of evidence have increasingly suggested that Tyrannosaurus was warm-blooded, the study—published on September 16—provided the first direct temperature measurement for the animal.

“No one has been able to measure temperatures like this before,” said UCLA geobiologist and study co-author Robert Eagle. “We found the temperature was 36 degrees Celsius, or 97 degrees Fahrenheit, about the same as in humans. The temperature is higher than in reptiles and slow-footed mammals like sloths, but lower than in birds.”

Although Tyrannosaurus was still a scaled, egg-laying reptile, its body temperature of 36 degrees Celsius was much closer to that of warm-blooded mammals than to modern reptiles.

Eagle said modern cold-blooded reptiles typically maintain body temperatures of about 28 to 30 degrees Celsius, or 82 to 86 degrees Fahrenheit. Most birds, the evolutionary descendants of dinosaurs, often have much higher body temperatures of 40 to 43 degrees Celsius, or 104 to 109 degrees Fahrenheit.

How scientists measured Tyrannosaurus rex body temperature

The secret lies in microscopic chemical bonds preserved in tooth enamel.

Rare isotopes of carbon and oxygen can combine within enamel. How often these bonds form depends partly on temperature. More bonds form under cooler conditions, while fewer form at higher temperatures.

This relationship allows researchers to use chemical signatures preserved in fossil teeth as a kind of ancient thermometer. Warm-blooded animals should leave fewer of these isotopic bonds than cold-blooded animals.

“This is the basis of using isotopes as thermometers,” Eagle said. “Theoretically, measurements could be made using any part of the skeleton, but the bones in our bodies are constantly being rebuilt, dissolved and replaced. Tooth enamel has a large crystalline structure that is extremely durable, making it the part of the skeleton most resistant to chemical changes from the environment over many years.”

To make the measurements, Eagle and first author Randy Flores used a dental drill to collect enamel from fossil teeth. They then dissolved the powdered enamel in phosphoric acid, releasing carbon dioxide gas containing the isotopic bonds they wanted to study.

A mass spectrometer measured the isotope ratios in the gas. The researchers also pressurized the gas to create a denser stream of carbon dioxide, allowing the instrument to obtain usable measurements from much smaller amounts of fossil material.

A warm-blooded Tyrannosaurus was built for an active life

The 97-degree measurement adds physical evidence to the idea that Tyrannosaurus was an energetic animal with a relatively high metabolism.

Maintaining its own body temperature would have helped Tyrannosaurus remain active while hunting or scavenging, although it would also have needed enough food to support such a metabolism, Eagle said. The finding also helps scientists place the dinosaur along the evolutionary path that eventually produced modern birds, which have even higher body temperatures.

Another important benefit of warm-bloodedness may have been access to colder environments. Study co-author Alessandro Chiarenza, a paleontologist at University College London, said tyrannosaurs could have lived in areas where cold-blooded reptiles had difficulty surviving because their body temperature was regulated internally.

Tyrannosaurus lived during the Cretaceous period, when Earth’s climate was much warmer than it is today. Global temperatures were about 11 to 25 degrees warmer than modern temperatures, and dinosaurs took millions of years to adapt to their hot world.

Even under these generally warm conditions, however, ancient Alaskan winters would have been extremely harsh for cold-blooded dinosaurs.

That pattern is reflected in the fossil record. Paleontologists have not found fossils of lizards, turtles, crocodiles or other reptiles from Cretaceous Alaska, Chiarenza said. Tyrannosaur fossils, however, have been found there.

“We now have empirical evidence using this geothermometer,” Chiarenza said. “Using historical paleoclimate models, we were able to reconstruct the extent of North America as it was 66 million years ago, stretching from Mexico to Alaska. Tyrannosaurus could have survived with a body temperature of 97 degrees Fahrenheit.”

Why scientists were allowed to analyze Tyrannosaurus teeth

Obtaining material from famous fossils is not an easy decision. Analyses that require drilling or removing portions of a specimen permanently change objects that cannot be replaced.

After years of collaboration between Eagle and UCLA, the Natural History Museum concluded that the improved technology could produce valuable results while requiring only a very small sample.

The museum therefore allowed researchers to collect portions of two teeth from Thomas II. The teeth were not on public display, said Luis Chiappe, curator of the museum’s Dinosaur Research Institute and head of research and collections at the Natural History Museum.

“We are constantly asked for fossils to use in destructive analysis,” Chiappe said. “Museums house tens of millions of irreplaceable mineral, animal and fossil specimens. We must make decisions that balance damaging specimens with gaining knowledge about the natural world. Tyrannosaurus body temperature is definitely worth the trade-off.”

An ancient crocodile provided an important check

The researchers also analyzed ancient alligator fossils excavated from Hell Creek in Montana.

These crocodile remains provided important comparison material. The researchers measured the crocodile’s body temperature at 30 degrees Celsius, or 86 degrees Fahrenheit—significantly lower than the 36 degrees Celsius, or 97 degrees Fahrenheit, measured in Tyrannosaurus.

This difference helped rule out the possibility that geological processes at the fossil site altered the chemical characteristics of both animals in the same way. If the surrounding geology had changed their molecular composition, the two species would have been expected to produce similar temperature measurements, Eagle said.

Instead, their vastly different temperatures support the conclusion that the chemical signals preserved in the fossils reflect genuine differences in the animals’ biology.

This research was supported in part by a grant from the National Science Foundation.