While walking through a natural history museum gift shop a few years back, I spotted a plush Velociraptor among the piles of dinosauriana. Frankly, it looked pretty stupid. Covered in a soft, fuzzy coat of faux-feathers, it lacked the reptilian menace of the predatory dinosaurs I remembered from my youth. This theropod looked more likely to cuddle someone to death than sink its hyperextendable toe claws into its hapless victim. Surely the feathers were just speculation based on the close relationship between some dinosaurs and birds? Velociraptor never would have looked so silly.
But I was wrong about the dinosaur’s plumage. Many theropod dinosaurs – Velociraptor included – sported downy coats, but it took a lot of digging through the scientific literature to overcome my bias towards more reptilian dinosaurs. Even though I had heard of a few genera that had been found with feather impressions intact, the imagery of cold-eyed, scaly dinosaurs from the books and documentaries I saw as a child stuck with me so strongly that it was difficult for me to believe that any dinosaur could have had feathers unless there was direct evidence to the contrary. I was thinking only in terms of what had been found etched in stone, and I lacked the evolutionary and historical perspective required to understand why scientists had recently started sticking feathers on some of my favourite dinosaurs.
Extracted in 1860 from a Jurassic-age limestone quarry in Germany, Archaeopteryx lithographica was the first feathered dinosaur to come to the attention of naturalists. The trouble was that they did not recognise it as a dinosaur at the time. Although believed by many anatomists to be an indication that birds had evolved from reptilian rootstock, no one could agree on which lineage of prehistoric creature birds had evolved from, or even on the significance of Archaeopteryx to the evolutionary transition. The anatomists Richard Owen and H. G. Seeley thought pterosaurs were the best candidates for bird ancestors, while T. H. Huxley cast Archaeopteryx as a weird sideshow to a linear progression of small, dinosaur-like reptiles, flightless birds, and airborne avians. There were almost as many opinions on the origin of birds as there were naturalists, and for decades Archaeopteryx remained perched between modern birds and some unknown reptilian ancestor.
Then, at the beginning of the 20th century, palaeontologists were thrown off the trail. Naturalists such as Robert Broom and Gerhard Heilmann argued that perhaps the muddle over birds, theropod dinosaurs, and pterosaurs indicated that all three groups had been independently derived from even older, crocodile-like creatures called thecodonts. Key to this line of reasoning was the absence of clavicles (shoulder bones that make up the wishbone in birds) from the dinosaurs thought to be most closely related to birds. Since birds had clavicles, their dinosaurian ancestors would have had to re-evolve them after they had been lost – an evolutionary impossibility – and thus dinosaurs were barred from avian ancestry in favor of the thecodonts.
Even after dinosaurs with wishbones were found, the thecodont-ancestry hypothesis held strong, and it would not be until the 1970s that palaeontologists such as John Ostrom and Robert Bakker would provide solid evidence that predatory dinosaurs, such as the recently discovered Deinonychus, were more closely related to birds. After a century of debate, Archaeopteryx was finally recognised as not only the earliest known bird, but a feathered dinosaur to boot.
With the reinvigoration of the idea that birds were nested within the dinosaur family tree, palaeoartists began to speculate about the avian features that would have evolved among dinosaurs first. Artist Gregory S. Paul even went so far as to illustrate an entire aviary of feathered dinosaurs in his 1988 book Predatory Dinosaurs of the World, especially dinosaurs such as Compsognathus and Velociraptor. At the time, Paul’s drawings must have looked a touch fanciful, but they proved to be remarkably prescient. Starting in 1996, palaeontologists began finding exquisitely preserved dinosaurs in China with intact plumage, and today new specimens are described on an almost monthly basis.
The feathered dinosaurs discovered thus far were clothed in a variety of feather types – from wispy fuzz to fully developed “flight feathers” which were used for something other than flying – and all of these dinosaurs belonged to a group of theropods called coelurosaurs. There was hardly a more diverse subset of the dinosaur family tree. This group not only encompassed famous predators such as Tyrannosaurus rex and Velociraptor, but also the bizarre therizinosaurs – long-necked, pot-bellied herbivores with huge claws on their fingers – and the extraordinarily birdlike alvarezsaurs. With the ironic exception of the ornithomimosaurs (the “ostrich mimics”), every branch of the coelurosaurs has at least one feathered representative. This distribution of dinosaur down means that feathers were a common trait for coelurosaurs, and they were probably inherited from a common ancestor rather than having evolved independently in each lineage.
Some might prefer their favorite dinosaurian predators to have looked just like their depictions in Jurassic Park, but the truth of the matter is that we now know otherwise. In fact, in 2007 it was found that the arms of Velociraptor had little round knobs which would have been anchors for long arm feathers, and the discovery of an early cousin of Tyrannosaurus coated in fuzzy feathers named Dilong means that even the most imposing predator of the Cretaceous may have been covered in feathers during at least part of its life. It is no more fantastic to restore feathers to these dinosaurs or their coelurosarian relatives than it is to depict our hominin ancestors as being covered in hair. It requires an evolutionary perspective to see, but today there is simply no excuse to depict a coelurosaur without feathers.
Coelurosaurs were not the only dinosaurs to sport unique body coverings, though. In 2002, palaeontologist Gerald Mayr and colleagues reported on long, bristle-like structures growing out of the tail of Psittacosaurus. This dinosaur was not a theropod, but instead was one of the early ceratopsians (“horned dinosaurs”) which were part of a separate radiation of dinosaurs known as ornithischians. Psittacosaurus was about as distantly related to feathered theropods as it was possible to be while still remaining a dinosaur, yet it had tail bristles which were similar in structure to the wispy fuzz of coelurosaurs such as Sinosauropteryx.
It was complemented last year by the announcement of Tianyulong, a different sort of ornithischian that also had a row of bristles going down its back. Since these dinosaurs had bristles structurally similar to the protofeathers of some theropods, it either indicates that such body coverings evolved at least twice within each part of the dinosaurian split, or, as strange as it might seem, such body coverings were a common trait among dinosaurs that was lost in some lineages and modified in others.
With the rate of discovery proceeding so quickly, palaeontologists don’t yet have a comprehensive idea of just how many dinosaurs had feathers or bristles, but a new species described earlier this month may indicate that many more dinosaurs with integumentary structures are waiting to be found. Like the feathered coelurosaurs, Concavenator was a theropod, but of a different sort. Found in the 130m-year-old rock of Spain, it was an early type of carcharodontosaurid: a group of giant predators such as Giganotosaurus that were close relatives of Allosaurus.
Carrying a small sail over its hips supported by elongated vertebral spines, Concavenator was a very odd dinosaur, but the most intriguing aspect of its anatomy is more subtle. Just like its counterpart in Velociraptor, the preserved ulna of Concavenator had a series of raised bumps set in a line. What these bumps represent is still being debated – were they quill knobs, or some kind of odd skeletal feature unrelated to body coverings? But there is a distinct possibility that they supported some kind of bristle or feather-like structure. If this turns out to be the case, it would be another indication that secondary body coverings may have been more widely distributed among dinosaurs than previously thought.
I am tired of seeing naked dinosaurs. Yes, skin impressions and well-preserved specimens of some dinosaurs show only scales and tubercles, but there is an increasing amount of evidence that this was not the case for all dinosaurs. It is time for books, documentaries and feature films to embrace these new discoveries and imagine what feathered, bristly dinosaurs would have been like. The “Dinosaur Renaissance” of the 1970s and 1980s transformed the public image of dinosaurs from swamp-bound monsters into active, dynamic animals, and the “Dinosaur Enlightenment” we are now going through is showing them to be even more weird and wonderful than we ever could have imagined.
Brian Switek is the author of Written in Stone: Evolution, the Fossil Record, and Our Place in Nature and blogs at Laelaps on WIRED Science
