Isotope bonds in two fossil teeth measure T. rex's body temperature at 36 C

Rare isotopes of carbon and oxygen bond to each other inside tooth enamel, and how often they do depends partly on temperature. Cooler mineral holds more of those bonds, warmer mineral fewer ^[10]. So a warmer animal leaves fewer of them behind ^[10]. "That's the basis of using the isotopes as a thermometer," Eagle said ^[13].

Eagle said the measurement could in theory be made on any part of the skeleton, but that bones in the body are constantly remodeled and dissolved ^[14].

The obstacle was mass. The first version of the method, built about a decade ago on skeletal material to sort warm-blooded extinct animals from cold-blooded ones ^[6], needed more fossil than a museum would reasonably give up from an important and irreplaceable specimen ^[7]. Bringing the requirement down to roughly a tenth of what it had been ^[22] persuaded the Natural History Museum of Los Angeles County to release enamel about 66 million years old ^[9][25].

Thirty-six degrees sits 6 to 8 degrees above the range Eagle gives for modern cold-blooded reptiles and 4 to 7 degrees below the range he gives for most birds ^[20][21]. It is a middling number, and it is close to where he expected to land. "We found T. rex was 36 Celsius (97 Fahrenheit), about the same as humans. The temperature is about what I would have guessed -- higher than a reptile or a slow mammal like a sloth, but lower than an avian," Eagle said ^[12].

The enamel gives a temperature. Metabolic rate is a separate quantity, and UCLA argues from the first toward the second: the university says the result supports a picture of an active predator or scavenger with a relatively fast metabolism, not an animal that relied on basking in the sun ^[16]. Other evidence already pointed that way, including tyrannosaur fossils found as far north as Alaska ^[17]. The animal was still a scaly, egg-laying reptile ^[18]. And two teeth from one skeleton fix one animal's temperature ^[23]; this sample says nothing about variation between individuals, between juveniles and adults, or across seasons.

For more than a century, T. rex biology has been reconstructed from bones, teeth, footprints and other traces ^[19]. "No one's been able to make a temperature measurement like this before," Eagle said ^[11].