A fluffy Tarbosaurus pursues a pair of feathery Gallimimus.
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Art by John Conway.
A fluffy Tarbosaurus pursues a pair of feathery Gallimimus.

Feathers for Tyrannosaurs

Feathery dinosaurs can be an acquired taste. Not everyone likes seeing animals that have traditionally been wrapped in scales begin to sprout brightly-colored plumage, especially when such changes threaten to dispel the menacing appearance of Hollywood dinosaur villains like Jurassic Park‘s Velociraptor. Of course, alterations to some dinosaurs raise the dander of fossil aficionados more than others. A fluffy Siats will stir debate among experts, but, simply by dint of the dinosaur’s celebrity, the prospect of a fuzzy tyrannosaur is a pop culture flashpoint in the tussle between dinosaurs of our childhood and the animals science is uncovering.

The impending release of Walking With Dinosaurs 3D has put tyrannosaur feathers on my mind again. The Land Before Time it ain’t, but the gorgeously-rendered animated film will undoubtedly excite the latest generation of young dinosaur fans. That’s why many paleontologists and dinosaur fans are disappointed that CGI docudrama’s villains, a gaggle of iridescent Gorgosaurus, are devoid of any fluff or fuzz.

In the grand scheme of the tyrannosaur family tree, Gorgosaurus was a large, sleek, and agile member of a subgroup called tyrannosaurids. This is the category of the most famous tyrant dinosaurs, including Tyrannosaurus itself. Yet we know relatively little about what the outside of these dinosaurs looked like. There are some rumored skin impressions – some lost, others frustratingly unpublished – that show tyrannosaurids had pebbly scales, at the very least. Befitting the traditional view of dinosaurs as scabrous reptiles, the filmmakers decided to go the conservative route and revive Gorgosaurus sans fluff.

Body size has played into the argument for scaly tyrannosaurids, too. If a 30 foot long, two ton plus Gorgosaurus had an active, hot-running metabolism, then wouldn’t an insulating coat of fluff cause the predator to overheat? Scale supporters could concede that small tyrannosaurs, and maybe even tyrannosaurid chicks, had fluff, but the prospect of a heat-addled Tyrannosaurus has helped keep large tyrannosaurs scaly.

Enter Yutyrannus. Not long after the Walking With Dinosaurs 3D settled on their scaly Gorgosaurus, paleontologist Xu Xing and colleagues described a roughly 30 foot long, one and a half ton tyrannosauroid that wore an expansive coat of protofeathers. Yutyrannus, along with some experimental work on how large animals shed body heat, suggest that body size was not a barrier to being a fluffy dinosaur.

Yutyrannus was described too late to change the look of Hollywood’s latest take on Gorgosaurus. And fans of the scaly-skinned model are often quick to point out a relational barrier between the two dinosaurs. The 125 million year old Yutyrannus was an archaic from categorized as a tyrannosauroid, while Gorgosaurus was a later and more derived member within the tyrannosaurid subgroup. Since the only tyrannosaurs so far discovered with protofeathers are the tyrannosauroids Yutyrannus and the comparatively tiny Dilong, and tyrannosaurids only left behind scaly skin, then maybe tyrannosaurs shed their simple plumage over evolutionary time.

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A very elegant Tyrannosaurus. Art by John Conway.

But this doesn’t mean that a scaly Gorgosaurus – or Tyrannosaurus, for that matter – can be taken as a default position. I can only speak for myself, but I think it’s only a matter of time before someone publishes on a tyrannosaurid with protofeathers. And even if I turn out to be wrong, the way we currently envision these dinosaurs is open to multiple hypotheses that remain open.

Perhaps Gorgosaurus and other tyrannosaurids really were scaly, scaly, scaly with not a tuft of fluff to be seen. But the fact of the matter is that the smattering of tyrannosaurid skin impressions so far discovered doesn’t seal the case. The unpublished traces do not provide a complete record of tyrannosaurid body coverings. Tatters and segments are all that exist and have not necessarily been preserved in their natural position.

Nor does a patch of skin automatically mean that the entire animal was covered in only that sort of integument. (The same, of course, goes for feathers and their forerunners.) Beyond the many avian dinosaurs that have scaly legs in addition to feathery bodies, fossils of non-avian dinosaurs such as the little theropod Juravenator and the very distantly-related ceratopsian Psittacosaurus have demonstrated how scales and feather-like integument can simultaneously exist on the same animal. Dinosaurs didn’t always have a full downy coat – patches of protofeathers could have been useful for display and even touch. Silly as such speculation may seem, perhaps Gorgosaurus had long, simple protofeathers that could have acted like a cat’s whiskers as the carnivore tried to nab small prey. Or maybe the dinosaur had prominent fuzz along its tail which changed as the dinosaur matured. A covering of protofeathers need not be an all or nothing possibility.

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A feathery Gorgosaurus. Art by John Conway.

And what of the chance that Gorgosaurus truly was as fuzzy as the documentary March of the Dinosaurs made it look? The discovery of scale impressions doesn’t necessarily preclude the possibility that tyrannosaurids had feathers. Feathery, non-avian dinosaurs are often found in fine-grained sediments where feathers have a better chance of being preserved. Large tyrannosaurids are not typically found in the same sorts of high-resolution geological settings, and so scraps of tough skin have a better change of being preserved than feathers. Some experimental studies of how modern birds decay and become preserved may help researchers better understand what happens to dinosaur feathers after death and their likelihood of being preserved.

Then again, traces of feathers can turn up in unexpected places. Last year paleontologist Darla Zelenitsky and coauthors reported on the remains of feathers around the skeleton of a young Ornithomimus – an “ostrich mimic” dinosaur that lived about 72 million years ago in prehistoric Alberta. No one had expected to find bits of dinofuzz among the relatively coarse grains of sandstone the dinosaur was embedded in, yet there they were. Who knows how many researchers, field volunteers, and lab prep experts have missed such traces because they didn’t think to look or accidentally appraised fossil protofeathers as plant traces?

The fuzzy Ornithomimus was found in the Horseshoe Canyon Formation. Skeletons of Albertosaurus, a close relative of Gorgosaurus, are found in the same formation. Perhaps there’s an Albertosaurus – already discovered or waiting to be found – that has survived the caprices of geological transformation with some remnant of feathers intact. Not to mention skeletons of tyrannosaurids found elsewhere.

What did Gorgosaurus look like in life? No one has a complete picture. Maybe a spate of future discoveries will show how tyrannosaurs gradually lost feathery body coverings as they evolved. Then again, tyrannosaurids may have borne anything from a few feathery tufts to decadent coverings of fuzz. The dinosaur’s appearance is open to multiple hypotheses. Scales-only cannot be taken as the default any more than totally-feathery tyrannosaurids can. But given how the story of dinosaur feathers has unfolded, I’d bet on the fuzz.

Paleontologists described the first non-avian dinosaur with protofeathers, Sinosauropteryx, in 1996. That was less than two decades ago. Experts are still discovering new types of dinosaur body coverings on an unexpected array of animals. Consider how much has changed and how quickly. Thirty years ago, depictions of feathery non-avian dinosaurs were thought to be reaching beyond the available fossil evidence. By fifteen years ago, feathers and their precusors were thought only to be present in the dinosaurs closely related to birds. Now fluff, fuzz, bristles, and similar structures are popping up all over the dinosaur family tree discovery-by-discovery. Either such body coverings evolved multiple times, or they were a common trait that goes back all the way to the last common ancestor of all dinosaurs.

I’m in no rush to call this debate. But we shouldn’t be too quick to roll our eyes at depictions of fuzzy tyrannosaurids or even illustrations as heretical as sauropods with some kind of protofeather equivalent. Scaly skin certainly has tradition on its side, but tradition is not the arbiter of accuracy.

Paleontologists started looking for non-avian dinosaur fluff such a short time ago that there’s far more left to be discovered than has yet been found, ranging from which dinosaurs had protofeathers to the function and even preservation of these beautiful body coverings. Since there are two fuzzy tyrannosauroids, and this lineage fits into a bigger group called coelurosaurs in which protofeathers have been found in every single lineage, I’d find it quite strange if big tyrannosaurids shed their feathery inheritance and fit the classic, scaly vision that has enthralled dinosaur fans for so long. The definitive evidence – one way or another – has yet to be uncovered.

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Edmontosaurus just got a silly crest. Could some sort of protofeather be next?  Art by Julius Csotonyi

The transmutation of dinosaurs is not complete. Even familiar creatures can change with the publication of a paper. Just last week a study by Phil Bell and colleagues proposed that the long-known, shovel-beaked hadrosaur Edmontosaurus sported a soft-tissue beanie that puts the herbviore in the running for the most ridiculous dinosaur of all time. This tweak is just the latest representation of how dinosaurs are changing. Tyrannosaurids will not be exempt from the ongoing push of what Thomas Holtz, Jr. has called the Dinosaur Enlightenment.

I don’t wish to diminish the joy some viewers will feel at seeing scaly, scary Gorgosaurus at the theater next weekend. A pebbly-skinned Gorgosaurus is reasonable for now, although not playing to my personal paleontological preference. But even if evidence lets Gorgosaurus keep their naked hides I have no doubt that new finds will alter our expectations about what these dinosaurs looked like and how they behaved. That’s wonderful. Paleontology is a science of change, both of life through time and our understanding of that long-lost nature. Even extinct dinosaurs will evolve in our imaginations. I can’t wait to see what changes next.

[Top image by John Conway. See his website for additional feathery dinosaur awesomeness.]