For about forty years, the argument over whether Tyrannosaurus rex ran hot or ran cold was conducted entirely without a thermometer. That is the strange part. Everyone had an opinion, and the opinions were fierce, and the whole thing rested on inference: the shape of the bones, the speed of growth rings, the latitudes where the fossils turned up, the posture, the guesswork about how fast a nine-tonne animal could move without cooking itself. Nobody had a number. Nobody thought a number was possible, because the animal has been dead for 66 million years and you cannot put a probe into something that is not there.
A team based mostly at UCLA now has the number. They took a dental drill to fossilised tooth enamel and came back with 36 degrees Celsius, about 97 Fahrenheit. That is, give or take a fraction, a human being. It is the temperature of the person reading this.
The measurement belongs to a specific animal with a name. Thomas is the most complete tyrannosaur skeleton held by the Natural History Museum of Los Angeles County, pulled out of the Hell Creek Formation in Montana a little over two decades ago. He is a museum showpiece, which is exactly why this was so difficult. You do not get to walk into an institution and grind up a piece of a specimen like that because you have a hypothesis. The permission is the hard part, and the permission is where the story actually begins.
Thomas was found in the early 2000s by a museum team working the badlands, and the excavation and preparation of him took years, which is the ordinary timescale for this work and the reason a specimen like him is treated the way a gallery treats a painting. Preparation means removing rock from bone at close to the scale of a grain of sand, and a single skull can absorb thousands of hours. Once that is done, the institution holding the specimen has an obligation that outlasts everyone currently employed there. The fossil is not a resource to be consumed. It is supposed to still be intact and still be answerable for questions nobody has thought of yet in two hundred years, which is why requests to sample one are refused far more often than they are granted, and why researchers spend as much effort reducing the size of the sample they need as they do on the analysis itself.
That refusal is not obstructive. It is the whole point of a collection. Plenty of nineteenth and twentieth century specimens were damaged or destroyed by well-meaning analysis using methods that were later superseded, and the material is gone and the question is still open. Curators remember that. So the sentence that unlocked this study was not a clever hypothesis. It was a much duller one: we now need roughly a tenth of what we used to need.
The technique is called clumped isotope palaeothermometry, and the principle is more elegant than the name. Carbon and oxygen each come in ordinary versions and rare heavy versions. When a mineral crystallises, those rare heavy isotopes occasionally end up bonded to each other rather than distributed randomly, and how often that happens depends on temperature. Cooler conditions produce more of these clumped pairings. Warmer conditions produce fewer. The ratio is not affected by what the animal ate, or what the water it drank was like, or any of the other variables that have wrecked previous attempts at ancient chemistry. It is a thermometer that reads only heat, and it has been sitting inside the tooth, locked in place, since the enamel formed in a living jaw.
Robert Eagle, the UCLA geobiologist behind much of the work, developed an earlier version of this method roughly a decade ago using bone. Bone was the problem. Bone is porous and chemically restless, and over tens of millions of years it swaps atoms with groundwater and sediment until whatever signal it once carried has been overwritten. Enamel is the opposite. Enamel is built from unusually large, tightly packed crystals, and Eagle has described it as the part of a skeleton that most stubbornly refuses to be chemically rewritten by its surroundings. If anything in a fossil was going to hold onto a 66-million-year-old temperature, enamel was the candidate.
The other breakthrough was subtraction. The earlier version of the technique needed a substantial amount of material, which is another way of saying it needed a museum to agree to the destruction of a meaningful piece of an irreplaceable specimen. The refined version needs roughly ninety percent less. That reduction is what unlocked the door. A curator can consider a request to remove a powder sample from portions of a couple of teeth in a way they could never consider a request that would visibly damage the animal. Luis Chiappe, who runs the museum’s Dinosaur Institute, is a co-author on the paper, which tells you how closely the institution was involved rather than merely permitting it from a distance.
There is a small discrepancy worth naming rather than smoothing over. The university’s own account describes portions of two teeth being sampled. Some wire coverage reported three. It does not change the result, but it is the sort of detail that gets copied wrong for years afterwards, so the honest version is that it was two or three teeth and the figure is a snapshot from a small number of samples, not a survey of the species.
What happens next is almost comically low-tech. The powder from the drill goes into phosphoric acid, which releases carbon dioxide gas. The gas goes into a mass spectrometer, which counts the isotopes and works out how often the heavy ones are bonded to each other. Out the other end comes a temperature. Published in Science Advances, the study reports an average of roughly 36.3 degrees Celsius for the tyrannosaur enamel, which the team rounds in plain language to 36, about the same as us.
The number on its own would be interesting. What makes it evidence is the control.
The team did not just measure the dinosaur. They measured a crocodilian from the same ancient ecosystem, an animal that was living in the same place, drinking the same water, breathing the same air, buried in the same sediment and subjected to the same 66 million years of geological weather. If the method were simply reading the temperature of the ground rather than the temperature of the animal, both results should have come out the same. They did not. The crocodilian came back at roughly 30 to 31 degrees Celsius, right where a cold-blooded reptile that regulates its heat by moving between sun and shade ought to sit. The tyrannosaur came back six degrees warmer.
Six degrees, in the same mud, in the same climate, is the whole argument. It means the difference is coming from inside the animals.
Put the T. rex figure next to the living world and it lands in an interesting place. Modern cold-blooded reptiles typically run somewhere around 28 to 30 degrees. Modern birds, which are the direct descendants of theropod dinosaurs and the closest living relatives T. rex has, run hot, roughly 40 to 43 degrees. The tyrannosaur sits between them, above the reptiles, below the birds, and almost exactly on the mammals. Eagle has summarised it as higher than a reptile but lower than a bird, which is a plain sentence doing a lot of work, because that gap is precisely the territory palaeontologists have been arguing about.
It is worth remembering how long people have been shouting about this. For most of the first century of palaeontology, dinosaurs were understood as enormous reptiles, sluggish, tail-dragging, swamp-bound, essentially lizards scaled up until they became absurd. That picture started coming apart in the second half of the twentieth century, during a period researchers still call the dinosaur renaissance, when work on anatomy and on the birdlike theropods forced a rethink of how these animals stood, moved and probably lived. Out of that came the argument that at least some dinosaurs were active, fast, internally heated animals much more like birds and mammals than like crocodiles.
The argument was never settled, because every line of evidence brought to it was indirect. Bone microstructure showed rapid growth, and rapid growth is suggestive of a high metabolism, but not proof of one. Predator to prey ratios in fossil assemblages were used as an argument, then argued back the other way. Later work looked at eggshell chemistry and at the chemical residues of metabolism in fossil bone. Every one of those approaches produced a probability and a rebuttal. Forty years of that leaves a field with strong convictions and thin evidence, which is the exact condition in which a direct measurement lands hardest.
The old debate was usually framed as a binary. Warm-blooded, like us, generating heat internally and paying for it with a huge appetite. Or cold-blooded, like a lizard, taking heat from the environment and going still when the environment stops supplying it. The version that kept surfacing in between was gigantothermy, the idea that an animal the size of a bus does not need much of an internal furnace because sheer bulk holds heat the way a stone building holds the afternoon. A large enough body cools slowly simply because there is so little surface area relative to volume. On that reading, T. rex could have been essentially a reptile that was warm by accident of size.
That was a respectable position, and it is hard to square with a six-degree gap against a crocodile living in the same swamp. The crocodile was not small. The mechanism that made the tyrannosaur warmer was not just being big.
The result also tidies up a detail that had been sitting awkwardly in the record for years. Tyrannosaur fossils have been recovered from Cretaceous Alaska. The climate then was far milder than the Alaska of today, but it still had months of darkness and real cold, and it is not a place where an animal wholly dependent on ambient heat comfortably makes a living. Palaeontologists had used that as circumstantial evidence for internal heat production. Circumstantial evidence is what you fall back on when you cannot measure the thing directly. Now there is a measurement, and it points the same way.
None of which makes the case closed, and the team has been careful about that. A temperature taken from enamel is a temperature from the window when that enamel was forming. It is not a lifetime average, and it is not a species average. A juvenile might have run differently from an adult. An animal in a heatwave might read differently from the same animal in winter. Two or three teeth from one skeleton is a beginning, not a survey. Independent labs using other approaches will produce their own estimates, and those estimates will not all agree, and the interesting science will happen in the disagreements.
There is also a subtlety about what the number is even describing. A living body is not one temperature. The core of a large animal is warmer than its extremities, and a jaw sits somewhere in between, so a reading from enamel is a reading from a particular part of a particular body at a particular moment in its growth. Researchers using this approach elsewhere, on mammals and on ancient marine animals, have run into the same question and generally treat the enamel figure as a good proxy for core temperature rather than an exact match. It is a strong signal with a margin around it, and the honest framing is that T. rex was in the mid thirties rather than that it was precisely 36.3.
But the ceiling has moved. The assumption underneath four decades of argument was that direct measurement of a dinosaur’s body temperature was permanently out of reach, that all anyone could ever do was reason from shape and circumstance towards a probability. That assumption was simply wrong. The information was present the whole time, in the chemistry of a tooth, waiting for someone to work out how to ask.
The lead author on the paper is Randy Flores, with Eagle and the isotope geochemist Aradhna Tripati at UCLA, Alessandro Chiarenza at University College London, and Chiappe in Los Angeles. What they built is less a single finding than a tool, and the tool is the reason this matters more than one number about one famous animal. If enamel holds a readable temperature for 66 million years, then a great many extinct animals are carrying their own body temperatures around in museum drawers right now. The long-necked sauropods. The duck-billed hadrosaurs. The small feathered theropods at the base of the bird lineage. Each one is a separate question about when, and how often, and in which lineages, warm-bloodedness actually appeared.
That is the question this opens up. Endothermy is expensive. Running a body several degrees above the outside world costs an enormous amount of fuel, which is why mammals eat so much more than reptiles of the same size. Something has to make that worth paying for: sustained activity, tolerance of cold, the ability to hunt at night or in winter or at latitude. Working out where on the dinosaur family tree that bargain was first struck, and whether it was struck once or repeatedly, is now something you can approach with samples rather than arguments.
And there is the smaller, more human thing, which is probably why this story travelled so far so fast. Everything about T. rex is built for distance. The size, the teeth, the sixty-six million years, the fact that it is the standard cultural shorthand for a monster. It is not an animal most people picture as being like them in any respect.
Then a number comes out of a lab and it is 36, and the distance collapses a little. An animal you could not have stood next to, from a world with a different sky and different plants and different air, was carrying the same internal weather you are carrying right now. Warm to the touch. Roughly your temperature. It had been true the whole time. Nobody had asked the tooth.






































