A Tyrannosaurus rex that lived tens of millions of years ago maintained a body temperature of roughly 97 degrees Fahrenheit (36.3 degrees Celsius), according to a UCLA-led study that used the chemistry of fossil tooth enamel to settle a long-running question about dinosaur metabolism. The finding, published in the peer-reviewed journal Science Advances, places the giant predator in the same thermal range as modern humans and confirms that T. rex was an endotherm — an animal that generates and regulates its own body heat.
A thermometer built from atomic bonds
The measurement relied on a geologic thermometer developed by the UCLA team. The instrument does not look at the shape or size of a fossil. Instead, it counts rare chemical bonds between carbon and oxygen atoms that become locked into tooth enamel during an animal’s lifetime. These bonds form at a frequency that depends on temperature: fewer bonds indicate a warmer body, while more bonds indicate a cooler one. Because tooth enamel is highly durable, it can preserve that chemical signal across vast stretches of time.
To extract the signal, researchers drilled a tiny amount of enamel powder from a T. rex tooth. The powder was dissolved in acid, and the resulting gases were measured. From those measurements, the team calculated the temperature at which the enamel formed — effectively reading the dinosaur’s internal thermostat from a fossil.
Specimen Thomas and the crocodilian control
The T. rex tooth came from a specimen known as Thomas, held at the Natural History Museum of Los Angeles County. But the researchers did not rely on the dinosaur alone. They also tested crocodilian fossils excavated from the same geological site in Montana, using the same method. Those crocodilians — animals that depend on external heat sources to regulate their temperature — registered about 86°F (30°C).
That gap of roughly 11 degrees Fahrenheit is central to the study’s conclusion. If both the T. rex and the crocodilians had simply been reflecting the ambient temperature of their environment, their body temperatures should have been similar. The fact that the crocodilians were cooler suggests that the T. rex’s warmth came from within, not from its surroundings.
Why warm-bloodedness mattered
Endothermy is not just a physiological curiosity. It supports a fast metabolism, which in turn fuels an active lifestyle. For T. rex, that likely meant the energy to hunt and scavenge across its range. It also opened doors to environments that would have been off-limits to cold-blooded reptiles. The study notes that warm-bloodedness would have allowed T. rex to thrive in cold regions such as prehistoric Alaska — places where crocodiles and turtles could not have survived.
The result helps resolve a debate that has persisted for decades over whether T. rex was cold-blooded or warm-blooded. Previous uncertainty about its metabolism and activity level left open questions about how such a large predator lived. The new temperature measurement, based on direct chemical evidence from the fossil itself, points firmly toward endothermy.
A durable record in enamel
The method’s reliance on tooth enamel is part of what makes the finding notable. Enamel is the hardest tissue in a vertebrate body, and it can survive conditions that destroy bone and soft tissue. Within that durable matrix, rare carbon and oxygen isotopes pair up in ways that record the temperature of the animal when the enamel formed. The UCLA team‘s geologic thermometer reads that record.
The study was conducted at the University of California, Los Angeles, and the findings appear in Science Advances. The article detailing the work was published on September 24, 2026. For paleontologists, the result adds T. rex to the growing list of dinosaurs for which direct evidence of warm-bloodedness is available — and it does so using a specimen that has been studied for years.
What emerges is a of a predator that ran hot, much like a modern mammal or bird. Its body temperature sat close to our own, and its metabolism allowed it to range into chilly northern landscapes. The crocodilians from the same Montana site, by contrast, stayed cooler, a reminder that not all reptiles of the Mesozoic operated the same way. The debate over T. rex’s blood temperature may not be entirely closed, but the chemical evidence from a single tooth has made the warm-blooded case considerably stronger.
