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Five Discoveries That Changed Our Understanding of T. Rex Forever, From Warm Blood to Lips
Paleontologists are still unraveling the secrets of the world’s most famous dinosaur, such as its outward appearance or the way that it hunted. With each new finding, our view of the “tyrant lizard king” has gotten a little clearer—and sometimes has radically shifted
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No dinosaur fires our collective imagination quite like Tyrannosaurus rex. Ever since it was named in 1905, the Cretaceous carnivore has been the subject of public awe, silver screen celebrity and intense scientific study. Decades of research and debate have investigated everything from what the infamous predator ate to its walking speed, body temperature, social life and body coverings.
“In the early 20th century, when Tyrannosaurus rex was first discovered, there was this concept of a great and savage world dominated by slow dinosaurs, engaging what felt like continuous battles,” says Cassius Morrison, a paleontologist at University College London. But the initial image of a monstrous, ever-hungry T. rex has since transformed into a predator very different from most other giant, carnivorous dinosaurs.
T. rex was originally envisioned as a slow-moving, tail-dragging terror of ancient swamps, facing off against Triceratops in epic, blood-spattered battles. Now, the T. rex we know and love is understood as a hot-blooded, possibly feathery carnivore that played a critical role in its ancient ecosystem, feeding on naive and injured herbivores while also cleaning up the carcasses of adult Triceratops and Edmontosaurus that were too large to hunt.
The shift didn’t happen all at once, but it was gradually pieced together, discovery by discovery. Here are five findings that contributed to the ever-changing image of the tyrant lizard.
A big head requires small arms
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The tiny arms of T. rex were a mystery as old as the dinosaur’s name. The 1905 paper that coined the name Tyrannosaurus rex also pointed out how surprisingly puny the carnivore’s forelimbs were. The appendages wouldn’t have been any help in capturing prey, so perhaps they were only useful in helping one T. rex cling onto another while mating, one scientist suggested. But no one really knew for sure. The puny arms of T. rex and related tyrannosaurs simultaneously became a puzzle and a joke, as if small, two-fingered arms somehow contradicted an animal that was otherwise big and stout from nose to tail.
Fun fact: T. rex’s fingers
Paleontologists originally thought T. rex had three fingers on each stubby forearm, aligning with earlier predators like Allosaurus. They didn’t have fossil evidence of the carnivore’s hand or claws. But after more closely related dinosaurs, such as Gorgosaurus, were found with two claws in Canada, experts realized T. rex must have had two fingers, too. The T. rex forearm fossil that confirmed this was found in 1988.
But small arms make sense for a predator with a massive head, heavily threaded with muscle. Especially at the giant sizes reached by dinosaurs like T. rex, a powerful bite required some heavy cranial anatomy. Arms became a biomechanical tradeoff, as concluded by a study of theropod arms published in May. Large carnivorous dinosaurs like the tyrannosaurs captured their prey and dismantled carcasses with their mouths. To use their clawed arms, the dinosaurs would have had to run right into their prey and wouldn’t have been able to see what they were doing. Smaller forelimbs meant the dinosaurs had more space for neck muscles that could support the head.
Maintaining a massive head as well as densely muscled arms would have made dinosaurs like T. rex too front-heavy. We can make fun of the dinosaur all we want, but those little arms are a sign of just how devastating a bite T. rex possessed.
Lips covered T. rex’s fearsome teeth
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The banana-size teeth of T. rex are part of the carnivorous dinosaur’s charm. In museums, paleoart and, of course, Jurassic Park, T. rex is often shown grinning with serrated teeth peeking out from the edges of its maw. But if we had seen the living animal, those famous teeth would have been hidden under lips, some research suggests.
Despite being otherwise out of date, one of the first paintings of T. rex seems to have gotten that detail right. Charles R. Knight’s 1919 depiction of T. rex showed the dinosaur with lips. The design was soon discarded, with even Knight opting for toothier renditions in his later work, but a 2023 paper revived the idea. Crocodiles and alligators, with their many-toothed grins, have often been used as inspiration for envisioning what dinosaurs were like. But these modern reptiles can have exposed teeth because they spend so much time in the water, minimizing the risk of their chompers drying out from exposure.
For the study, the team analyzed the fossil teeth of a close T. rex relative, Daspletosaurus. They found that the tooth enamel had worn evenly on both sides, suggesting protection by lips. Based on the size of T. rex’s skulls, their teeth could have fit neatly within their mouths, the team suggested, and they found that T. rex jaw bones contain holes for nerves and blood vessels that resemble those of modern, lipped reptiles. The most likely scenario, the researchers proposed, is that T. rex had protective lips over its teeth, not unlike those of Komodo dragons. The teeth of T. rex would have been hidden until the dinosaur opened its mouth.
T. rex hunted by ambush
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The fact that T. rex ate meat has been obvious since the dinosaur’s discovery. Its long jaws full of serrated, ever-replaceable teeth don’t suggest a life munching leaves. But how much did T. rex hunt, and how much did the dinosaur scavenge? Paleontologists have argued over the point since the dinosaur was discovered, with T. rex described as everything from a slow-walking scavenger to a supercharged predator of horned dinosaurs.
T. rex, paleontologists have found, was not a fast runner. The dinosaur was likely agile and better able to turn while moving than other large, carnivorous dinosaurs were, but the tyrant lizard’s top speed was likely about 12 miles per hour. And yet, T. rex teeth embedded in healed dinosaur bones indicate that the largest tyrannosaur of them all actively hunted prey, per a study from February. But because it wasn’t built for high-speed chases, T. rex probably ran in short bursts as an ambush predator.
The relatively slow T. rex likely relied on picking off young dinosaurs, hunting old or sick individuals, and scavenging, Morrison says. Healthy adult duckbills, horned dinosaurs and armored dinosaurs were risky prey that could painfully injure even a large T. rex. “The prey animals of T. rex were no pushovers,” Morrison says, noting that this tension between predator and prey still exists today. Large, herbivorous mammals such as bison and elephants are very risky for predators to successfully hunt, with crafty carnivores opting for individuals that are less likely to break a jaw with a kick or gore with their horns.
When large tyrannosaurs singled out a suitable meal, they likely rushed at the prey, similar to how great white sharks hunt today. The aim was to deliver a devastating bite that was able to maim, if not kill. And when T. rex came across carcasses of large dinosaurs like Triceratops, the carnivore was more than capable of dismantling them. Punctured bones and T. rex poop full of bone shards leave no doubt that the dinosaur could make the most of any meal.
The predator was warm-blooded
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T. rex needed a lot of food. A dinosaur doesn’t grow to 40 feet in length and weigh nine tons by eating table scraps. But how much food did the creature eat? Answering basic biological questions like this has hinged on understanding the dinosaur’s body temperature and metabolism. If T. rex, being a reptile, had been akin to its evolutionary cousins among the crocodiles, it might have required less food but had a more variable body temperature. But if T. rex had been more like birds, which are modern dinosaurs and more closely related, it might have had a high, constant body temperature that came with elevated energy demands and the problem of shedding excess heat.
Early on, T. rex was seen as a giant, cold-blooded reptile. While some suggested the dinosaur ran a little warmer, because bigger bodies hold on to heat, the dinosaur was generally depicted as a sluggish predator that was unable to keep its muscles warm enough to move quickly. The “Dinosaur Renaissance” of the late 20th century—a time of shifting ideas around dinosaurs—envisioned T. rex as a more active animal, but the hypothesis was difficult to directly support.
But this September, paleontologists were able to more precisely estimate the body temperature of T. rex, despite the dinosaur perishing 66 million years before the invention of thermometers. They suggest that T. rex maintained a high, constant body temperature of about 97 degrees Fahrenheit, based on chemical bonds in the enamel of its teeth, which are present in lower numbers in warm-blooded creatures. Such internal heat required that T. rex feed often, but it also may explain why T. rex fossils are found from Texas to Saskatchewan. The new research suggests that T. rex was able to occupy a broad array of habitats, from temperate to tropical, and that this widespread tolerance is likely what allowed the direct ancestor of T. rex to cross over an ancient Arctic land bridge and reach North America.
“Being able to withstand a variety of climates would have been key to the long-term success of the species,” says Jared Voris, a paleontologist at the University of Calgary.
T. rex likely had feathers
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For about a century, every image of T. rex envisioned a supremely scaly predator. Dinosaurs were reptiles, after all. It was only in the 1970s that paleontologists began to resuscitate the old idea that birds are dinosaur descendants—and from there, it was only natural to wonder whether T. rex had feathers.
The discovery of feathered non-avian dinosaurs in the 1990s bolstered what many experts had come to suspect. And those early finds were just the beginning. Feathers have been found on a broad variety of dinosaurs, from bird ancestors to species all the way on the other side of the dinosaur family tree—including tyrannosaurs.
Direct evidence of feathered tyrannosaurs comes from two species that lived long before T. rex. Dilong was a small, six-foot-long tyrannosaur that lived in ancient China about 125 million years ago. Its body was covered in a coat of simplified, fuzzy feathers. Around the same time, the 30-foot-long tyrannosaur Yutyrannus was also stalking through prehistoric China, covered head to tail in simple feathers. So, feathers were present within the broader family of T. rex. The question has been whether T. rex kept the plumage of its ancestors.
Researchers remain divided. No direct evidence of feathers has been found associated with T. rex bones so far. Paleontologists have recently announced some T. rex skin, but these are small patches from the tail. Some experts argue that it wouldn’t have made sense for T. rex to have an extensive feather coating—running such a high internal temperature, thick feathers could have kept too much heat in. But it’s likely that the dinosaur still had some filaments, just as elephants have sparse hair and aren’t as shaggy as their woolly mammoth cousins. One day, better fossils might finally resolve the question, but in the meantime, the only thing for sure is that our understanding of T. rex will keep changing.
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