New evidence from fossil teeth confirms warm-blooded Tyrannosaurus rex

A new analysis of fossilized Tyrannosaurus rex teeth has provided the clearest estimate yet of the dinosaur’s body temperature. Researchers found that the giant predator likely maintained an internal temperature of about 97 degrees Fahrenheit, strengthening evidence that it was a warm-blooded animal.

Ancient dental remains provide fresh insights into T. rex physiology

For decades, paleontologists have debated whether Tyrannosaurus rex should be considered a warm-blooded dinosaur capable of regulating its internal temperature or an animal whose body temperature was largely determined by its surroundings.

That question has been difficult to answer because body temperature does not survive directly in a fossil. Scientists have instead relied on indirect evidence, including growth patterns in bones, anatomy, activity levels and the environments in which dinosaurs lived.

A new study published in Science Advances offers a different approach. Researchers analyzed chemical signatures preserved in the enamel of T. rex teeth and used them to estimate the temperature at which the enamel formed.

The outcome reached roughly 97 degrees Fahrenheit, which translates to 36 degrees Celsius.

That figure places T. rex within the general range of many modern warm-blooded animals and considerably above the typical body temperatures associated with modern cold-blooded reptiles. The measurement does not by itself answer every question about dinosaur metabolism, but researchers say it provides an important physical constraint on how the animal functioned.

Robert Eagle, a geobiologist, associate professor at the University of California, Los Angeles, and coauthor of the research, characterized this measurement as one of the most direct evaluations scientists have managed to secure regarding the body temperature of a T. rex.

The discovery holds immense weight since the controversy surrounding dinosaur metabolism has persisted for decades. For close to 60 years, researchers have theorized that tyrannosaurs and various other dinosaurs might have been equipped to produce and sustain significant levels of internal warmth.

Data gathered from fossils has steadily reinforced that view. A notable turning point occurred in 2022 with the unearthing of a T. rex footprint in Alaska, which effectively demonstrated that these animals were capable of thriving in fiercely frigid habitats.

The fresh thermal estimation contributes an additional piece to that puzzle. Instead of depending exclusively on the creature’s physical structure or the surrounding conditions where its remains were discovered, scientists are currently able to analyze a molecular footprint retained directly within its dental enamel.

That evidence indicates that T. rex was not merely a cold-blooded reptile whose temperature rose alongside the ambient surroundings. Instead, it sustained a thermal baseline considerably above its external environment.

How researchers converted T. rex teeth into an ancient thermometer

The study depended on a relatively small amount of fossil material, an important consideration when researchers are working with one of the most valuable and recognizable dinosaurs ever discovered.

The team examined two tiny sections taken from teeth belonging to a specimen known as Thomas the T. rex. The skeleton is approximately 70% complete and is housed at the Natural History Museum of Los Angeles County.

Researchers were able to use only a few milligrams of enamel because the analytical technique had been refined over more than a decade. Earlier versions of the method required substantially more fossil material. Reducing the amount needed by roughly 90% made it possible to study specimens without removing large or visually significant sections from important fossils.

The approach focuses on isotopes, which are different forms of chemical elements. Carbon and oxygen occur in several isotopic forms, and certain combinations of these isotopes can form bonds in tooth enamel at rates that depend on temperature.

In simple terms, the chemical structure of the enamel retains information about the conditions that existed when it formed.

The researchers measured these isotope bonds in tiny samples from the T. rex teeth. By examining their abundance and arrangement, they were able to calculate the temperature associated with enamel formation.

That made the teeth function much like a geological thermometer.

The selection of teeth mattered as well. Enamel ranks among the toughest biological substances, capable of preserving chemical data remarkably well across geological epochs. Even though fossilization alters biological specimens, enamel remains relatively resilient against shifts that might otherwise wipe out the original temperature signature.

Aradhna Tripati, a climate scientist and UCLA geochemistry professor who served as a senior author for the research, underscored that the capacity to handle such tiny samples proved vital when analyzing a specimen as precious as T. rex.

For decades, researchers had estimates about dinosaur metabolism based on bones and biomechanics, but they lacked a direct measurement of body temperature. The chemical composition of the enamel provided an opportunity to approach that question from another direction.

The method has already been applied to other extinct animals, including dinosaurs, woolly mammoths and the enormous prehistoric shark megalodon. Each application gives scientists another way to reconstruct how ancient creatures responded to the climates in which they lived.

A temperature between reptiles and birds

At around 36 degrees Celsius, the estimated temperature of T. rex is considerably warmer than that of many modern reptiles but does not reach the upper range observed in some birds.

Modern reptiles are typically characterized as ectothermic, implying that external heat sources are crucial for them to manage their body temperature. For instance, a crocodile raises its warmth by basking in sunlight and lowers it by retreating into the shade or submerging in water.

Birds and warm-blooded animals, on the other hand, typically preserve fairly constant internal temperatures via metabolic reactions. Such a capacity demands substantial energy while simultaneously enabling these creatures to stay active throughout a broader spectrum of external conditions.

The new estimate places T. rex closer to the warm-bodied end of that spectrum.

That does not mean the dinosaur’s physiology was identical to that of a modern mammal or bird. Dinosaurs occupied a different evolutionary position, and their metabolism cannot simply be equated with that of living species.

Nevertheless, the temperature provides useful information about how much energy T. rex may have been able to produce and sustain.

Robert Eagle noted that some modern mammals, including sloths and anteaters, can have body temperatures in the low 90s Fahrenheit, while some birds can exceed 104 degrees Fahrenheit, or 40 degrees Celsius.

Modern cold-blooded reptiles generally maintain internal temperatures hovering around the low-to-mid 80s Fahrenheit, though the precise reading fluctuates depending on the species and ambient surroundings.

The difference matters because body temperature is closely connected to activity and energy use.

An animal capable of maintaining a high internal temperature can potentially sustain physiological activity for longer periods than an ectothermic animal whose performance is strongly dependent on its surroundings.

That does not necessarily mean T. rex was a fast sprinter. Researchers emphasize that the temperature estimate should not be interpreted as proof that the dinosaur could run continuously at high speed.

Instead, a warm-bodied metabolism could have supported prolonged activity and helped the animal remain physiologically active under conditions that would have been more challenging for an ectothermic predator.

The distinction is important. Crocodiles, for example, can move rapidly for short bursts but cannot maintain intense activity indefinitely. A warm-bodied T. rex may have had greater capacity for sustained physical performance.

The Arctic may have been within T. rex’s range

One of the most interesting implications of the temperature estimate concerns where T. rex could have lived.

The unearthing of tyrannosaur tracks and bones in far northern regions has previously proven that these creatures could thrive in habitats vastly distinct from the tropical settings commonly linked to prehistoric reptiles.

Alaska during the late Cretaceous was not identical to the Arctic environment of today, but it still experienced long periods of darkness and cold conditions. A large predator living there would have faced physiological challenges that would be difficult for a strongly ectothermic animal to overcome.

A cozy indoor atmosphere would have altered those limitations.

Using paleoclimate models, the researchers reconstructed temperatures across North America approximately 66 million years ago, near the end of the Cretaceous Period. They then compared those environmental conditions with the estimated body temperature of T. rex.

Their findings indicated that this dinosaur might have inhabited a vast regional expanse reaching from present-day Mexico all the way to Alaska.

That possibility changes the way scientists can think about the animal’s ecology.

A predator that relied heavily on sunlight to warm its body would have been more restricted by climate and season. A warm-bodied T. rex, however, could have remained active even when environmental temperatures dropped significantly.

Tripati pointed out that this distinction matters significantly. Should T. rex have kept its internal warmth notably above ambient levels, it could have inhabited regions largely unreachable for a creature relying mostly on external thermal sources.

The evidence from Alaska thus aligns with the chemical findings instead of standing in isolation.

Together, the findings support the idea that tyrannosaurs were physiologically capable of functioning in a wide range of environments across the continent.

A warmer body also meant higher energy demands

Maintaining an elevated body temperature comes with a cost.

Warm-blooded creatures typically require a continuous energy supply to keep their metabolism running. Consequently, T. rex must have secured ample nourishment, not merely to power its locomotion, development, and breeding, but also to maintain its core body temperature.

Thomas Holtz Jr., a vertebrate paleontologist based at the University of Maryland who remained unconnected to the research, noted that an endothermic T. rex probably would have demanded a greater supply of food than a similarly proportioned cold-blooded creature.

That has implications for the dinosaur’s role within its ecosystem.

T. rex was already an enormous predator, with a powerful skull and teeth capable of processing large prey. A high metabolic demand would have added another factor to its ecological requirements.

Researchers can leverage this data to formulate more accurate models regarding the food consumption of tyrannosaurs, as well as the frequency of their hunting and feeding habits.

It could also assist researchers in evaluating how they interact with other massive wildlife inhabiting identical ecosystems.

The question extends beyond individual behavior. Metabolism affects growth rates, reproduction, movement, activity patterns and the amount of energy an animal needs to survive.

Consequently, determining the approximate body temperature of T. rex provides a foundation for investigating many other aspects of its biology.

The measurement does not establish precisely how fast the dinosaur grew, how frequently it hunted or how much food it consumed. Those questions require additional evidence. But having an estimated body temperature gives researchers a parameter that can be incorporated into future models.

The discovery might help settle an even older dinosaur controversy

The question of dinosaur metabolism is nearly as ancient as the scientific study of dinosaurs themselves.

In 1842, British anatomist Richard Owen coined the term Dinosauria while outlining the traits that set dinosaurs apart from alternative reptiles. Ever since, scholars have continually argued over whether dinosaurs ought to be understood mainly through the physiological lens of present-day reptiles or treated as creatures possessing significantly higher metabolic rates.

Over the following decades, accumulated evidence suggested that at least a portion of dinosaurs were endothermic or possessed metabolic systems capable of producing significant internal heat.

Bone microstructure, growth patterns, posture, activity levels and discoveries from high-latitude environments have all contributed to that discussion.

The new chemical technique does not replace those lines of evidence. Instead, it provides another independent method for examining the question.

Holtz said the comparison between T. rex and animals such as crocodiles and mollusks from similar periods and locations gives researchers additional confidence that the high temperature measured in the tyrannosaur represents a genuine biological signal rather than simply reflecting the surrounding environment.

The subsequent phase will involve ascertaining whether comparable temperatures were typical of different dinosaurs.

Not every dinosaur occupied the exact same ecological niche, and considerable debate persists regarding whether distinct dinosaur lineages relied on varying metabolic strategies.

Applying the method to creatures like Triceratops, Stegosaurus and Brachiosaurus might yield insightful comparisons. Should these animals similarly exhibit comparatively elevated core temperatures, researchers could infer that endothermic traits were prevalent across the dinosaur lineage.

If their temperatures were substantially different, the results could point to greater metabolic diversity than previously assumed.

The method could also be used beyond dinosaurs.

Researchers are interested in applying it to ancient relatives of mammals, particularly species living during periods when the evolutionary transition toward modern warm-blooded physiology was taking place.

Tracing those changes farther back in time could help scientists understand when and how the ability to regulate internal temperature became established.

A better understanding of the lifestyle of T. rex

The estimated 97-degree-Fahrenheit body temperature does not answer every question about Tyrannosaurus rex, but it provides a significant new piece of information about the animal’s physiology.

The chemical evidence from its teeth supports decades of research suggesting that tyrannosaurs were more metabolically active than modern cold-blooded reptiles. It also helps explain how such a large predator could inhabit environments that included relatively cold regions of ancient North America.

More broadly, the study demonstrates how even tiny fragments of fossil material can preserve information about animals that disappeared tens of millions of years ago.

The enamel of a T. rex tooth may look like an ordinary piece of fossilized tissue, but its microscopic chemistry contains clues about the conditions under which it formed. By developing techniques sensitive enough to read those signals without requiring large portions of a specimen, researchers can investigate questions that were once considered nearly impossible to answer.

For T. rex, the outcome points toward a creature that was capable of maintaining a high internal temperature and sustaining significant physiological activity.

That discovery introduces a fresh layer to the portrait of the renowned carnivore. Far from being merely a massive reptile suited for balmy climates, T. rex seems to have been equipped with a metabolic rate that granted it enhanced resilience against ambient thermal conditions.

Its capacity for maintaining warmth may have allowed it to inhabit a massive expanse of North America, stretching from comparatively mild southern territories to significantly chillier northern environments.

Future measurements from other dinosaurs will determine how widespread that physiology was. For now, however, the chemistry locked inside two small pieces of T. rex tooth enamel has provided scientists with one of the most direct estimates yet of the animal’s internal temperature, offering a new window into how the predator lived roughly 66 to 69 million years ago.