Why did the museum finally hand over T. rex teeth?

UCLA researchers first built their prehistoric thermometer roughly a decade ago, but the early version required more fossil material than a museum would reasonably sacrifice from an irreplaceable T. rex specimen. Over ten years of refinement, the team cut the amount of material needed by about 90%.

That improvement persuaded the Natural History Museum of Los Angeles County to provide tiny portions of two teeth from Thomas the T. rex, the most complete tyrannosaur skeleton displayed in the museum's dinosaur hall. The sampled teeth were not on public display, according to Luis Chiappe, curator of the museum's Dinosaur Institute.

"We're asked for fossils for use in destructive analysis all the time," Chiappe said. "The museum contains tens of millions of specimens of minerals, animals and fossils that are irreplaceable. We have to make decisions that balance the damage to the specimen against gaining knowledge about the natural world. Sacrificing small portions of two teeth to learn about T. rex's body temperature is definitely worth the trade-off."

How warm is 97 degrees for a reptile?

T. rex was still a scaly, egg-laying reptile, but its body temperature sat far closer to a mammal's than a modern reptile's. Cold-blooded reptiles today typically maintain temperatures around 28–30 °C (82–86 °F). Birds — the evolutionary descendants of dinosaurs — run considerably hotter, often 40–43 °C (104–109 °F), according to Eagle.

For context, the study's numbers place T. rex between those groups: warmer than a crocodile, cooler than a chicken.

Why does warm blood matter for an Alaskan T. rex?

Internal heat regulation may have opened cold environments to tyrannosaurs that shut out other reptiles. Co-author Alessandro Chiarenza, a paleontologist at University College London, said an internally regulated body temperature could have allowed tyrannosaurs to live where cold-blooded reptiles struggled.

T. rex lived during the Cretaceous Period, when Earth was roughly 11–25 °F warmer than today. Even so, Cretaceous Alaska would have had brutal winters for any animal that could not generate its own heat. The fossil record reflects that: paleontologists have found no fossils of lizards, turtles, or crocodiles in Cretaceous Alaska, Chiarenza said, while Tyrannosaurus fossils do turn up there.

"Now we have empirical evidence using this geologic thermometer," Chiarenza said. "Using paleoclimate models of the past, we were able to reconstruct a range in North America 66 million years ago that stretched from Mexico to Alaska, based on where T. rex could have survived with a 97 °F body temperature."

How did the researchers check their work?

Thomas the T. rex was excavated from Hell Creek in Montana, a site that also produced ancient crocodilian fossils. The team measured that crocodilian at 30 °C (86 °F) — sharply lower than T. rex's 36 °C. That gap matters. If geological processes at the site had altered both animals' chemistry in the same way, Eagle explained, the two species would have shown similar readings. Their different temperatures support the conclusion that the signals reflect genuine biology, not contamination.

The study covers a single iconic specimen, so the 97-degree figure represents one T. rex rather than a survey of the species. Still, the measurement anchors decades of indirect evidence about dinosaur metabolism in hard chemical data, and the refined technique — which now works on roughly 90% less material — could be applied to other fossils in museum collections. The research was supported in part by grants from the National Science Foundation.