What would it feel like to reach out and touch a Tyrannosaurus rex? Would the beast be covered in hard armor like an alligator or smooth feathers like an ostrich? Or might the tactile truth lurk somewhere in between?
Scott Persons, a paleontologist at the University of Alberta, has asked himself these sorts of questions since he was a little boy. While there’s no direct evidence that T. rex had plumage, its feathered relatives unearthed over the last two decades have slowly chipped away at the idea that all dinosaurs were bald, scaly monsters. (Read “Finally, You Can See Dinosaurs in All Their Feathered Glory.”)
Today, thanks to a study of fossilized dinosaur skin published in Biology Letters, we may finally have an answer. Persons and his colleagues examined new skin impressions taken from T. rex fossils discovered near Baker, Montana, and compared them with fossils of other tyrannosaurs, including Albertosaurus, Daspletosaurus, Gorgosaurus, and Tarbosaurus.
The team found a pattern among the tyrannosaur kin: they all had skin textured with small, pebbly scales and not fuzzy plumage.
“Now that we’ve found these multiple patches of preserved tyrannosaur hide from multiple places across the body, it looks pretty clear that at least the majority of the T. rex was not covered in feathers,” says Persons.
This isn’t to say T. rex was completely featherless, he adds, but if it had feathers, they likely would have been few and far between. Mostly, he says, our best evidence now shows that the great predators were “covered in the traditional reptilian scalyness.”
What Happened To Tyrannosaurs’ Tufts?
It’s possible, of course, that T. rex’s pebbly complexion may be the exception, not the rule. Persons says there’s plenty of evidence that early tyrannosaurs had feathers, even if they were more primitive than the structures we see on birds today.
This raises an interesting question: Why would the tyrannosaurs evolve feathers only to lose them later on in species like T. rex?
“I definitely think that size had something to do with it,” says Persons.
As an animal moves toward a larger body mass, it has to worry more about keeping cool. Scientists believe larger tyrannosaurs evolved long legs to chase down prey, and it may be that feathers were too much of a hindrance to cooling off after a sprint. (Read “Tyrannosaurs Were Human-Size for 80 Million Years.”)
“If you think about really, really big terrestrial mammals today, like elephants, rhinos, hippos, and cape buffaloes, although they are not hairless, they are very much reduced in the amount of hair that they do have,” says Persons.
Other experts seem to agree with this hypothesis.
“Large animals have a heat-loss problem,” says Julia Clarke, a paleontologist at the University of Texas in Austin and a National Geographic Society grantee. “That they would be less densely covered in hair or feathers is predicted.”
Clarke adds that in the largest members of the tyrannosaur group, bare patches would probably also be bigger. “So these findings make a lot of sense.” (Also see “This Is Our Best Look Yet at a Tyrannosaur’s Face.”)
Luis Chiappe, director of the Los Angeles County Natural History Museum’s Dinosaur Institute, says the work is pretty convincing and intriguing.
“What it tells us is that the evolution of feathers was more complex than what we previously thought about,” says Chiappe in an email. “And it isn’t surprising. The evolution of complex structures tends to be complex!”
But there is one sticking point in the neat and tidy explanation that great size begets fewer feathers—a hefty dinosaur native to what is now China that goes by the name Yutyrannus, or “feathered tyrant.”
Fossils suggest this tyrannosaur sported feather-like filaments around eight inches long across much of its body. What’s more, while Yutyrannus would have been smaller than T. rex, the earlier tyrannosaur would have overlapped in size with scaly Albertosaurus and Gorgosaurus. This means size alone is not enough to account for the loss of feathers.
We can’t say for sure why one tyrannosaur should be feathered and another not so much, says Persons, especially since estimates suggest both animals would have endured similar average temperatures. But he and his coauthors do offer a possible explanation: It could be that Yutyrannus was a forest dweller and would therefore have had an easier time cooling off in a world of shade.
This would also sync up with what we see in today’s large mammals that inhabit forests, like Javan rhinos and Asian elephants, which are usually hairier than their savannah-dwelling relatives.
“Big Birds From Hell”
Of course, not everyone is ready to close the case file on T. rex feathering.
“It wouldn't surprise me if some of these dinosaurs, particularly really gigantic ones, reduced or lost their feathers,” says Stephen Brusatte, a tyrannosaur expert at the University of Edinburgh and a National Geographic Society grantee. However, Brusatte says, concluding that big tyrannosaurs like T. rex were totally featherless would be premature.
“It takes very special circumstances to preserve soft tissues like feathers, and as far as we know, these big tyrannosaurs are not preserved in those settings,” he says.
For example, Brusatte says, if you were to find fossils of elephant skin impressions, you might presume that they had no hair, since elephant skin is thick and crinkly. But we know that elephants do have their share of hair, with juveniles boasting even more than adults.
“I don't think we need to throw out the image of big fluffy tyrannosaurs quite yet,” says Brusatte.