July 5, 2012
On Monday, the world met yet another fuzzy dinosaur. The little theropod – named Sciurumimus albersdoerferi – is beautifully preserved in a slab of roughly 150 million year old limestone found in Germany. (These deposits have also brought us Archaeopteryx and the also-fluffy Juravenator.) And, with a little evolutionary context, Sciurumimus hints that filament-like protofeathers were more common among dinosaurs than we previously expected.
Birds – the only surviving lineage of dinosaurs – are covered in plumage. No surprise there. But since 1996, paleontologists have identified about 30 genera of non-avian dinosaurs with feathers. Most of these dinosaurs are coelurosaurs – the major group of theropod dinosaurs that contains tyrannosaurs, the switchblade-clawed deinonychosaurs, the truly weird therizinosaurs, and, among others, birds. As the discoveries accumulated, it seemed that feathers originated at the base of this group, and were inherited by birds. And feathers were not only present an small, especially bird-like dinosaurs. As the recently-described Yutyrannus shows, even 30-foot-long tyrannosaurs were fluffy.
Up until a few years ago, birds and their closest non-avian relatives were the only dinosaurs known to have feathers. Simple enough. But then two ornithischians crashed the party.You see, the dinosaur family tree is split into two halves – the saurischians on one side, and the ornithischians on the other. The split goes back about 230 million years or so, nearly to the origin of the very first dinosaurs.
The feathery coelurosaurs belong to the saurischian side of the tree, but paleontologists have also discovered dinosaurs on the other side – on the ornithischian branches – with feather-like structures. In 2002, paleontologists discovered that the archaic ceratopsian dinosaur Psittacosaurus had a brush of bristle-like structures jutting from its tail. And in 2009, another team discovered Tianyulong – another ornithischian dinosaur with a row of similar filaments running down its back. The bristles were not just like the fuzz and feathers seen among the coelurosaurs, but they were structurally similar.
Paleontologists were left with two possibilities. Either protofeathers evolved multiple times in different dinosaur lineages, or simple “dinofuzz” was an ancestral dinosaur feature that was later lost in some lineages. We don’t have enough fossils yet to know for sure, but the discovery of Sciurumimus is a significant clue that most, if not all, dinosaur lineages were at least partially decorated with protofeathers.
Even though Sciurumimus is a theropod dinosaur – part of the saurischian side of the family – it isn’t a coelurosaur. Sciurumimus is a megalosauroid, which is a lineage of dinosaurs that’s closer to the base of the theropod group. In other words, Sciurumimus is a relatively archaic theropod that isn’t very closely related to birds, yet it still has dinofuzz.
Paleontologist Thomas Holtz helped provide some context on Twitter shortly after the new dinosaur was announced. Before Sciurumimus, only coelurosaurs were known to have fuzz. (What the bristles on Psittacosaurus and Tianyulong actually are is still unclear, but no one calls their filaments “fuzz.”) After Sciurumimus, fuzz has been moved down a branch to a group called the Carnosauria.
We are still left with two possibilities. The fuzz on Sciurumimus could have originated independently. But as paleontologists add fuzz to lineages of dinosaurs only distantly-related to birds, it seems less and less likely that protofeathers evolved from scratch in each and every lineage. It’s looking more and more like feathers were a common, ancestral feature of dinosaurs. In this case, Sciurumimus indicates that simple feathers were an early, common theropod trait that evolved close to the origin of the group. The diminutive dinosaur also fits in the wide gap between coelurosaurs and their very distant ornithischian dinosaurs, bringing us a little closer to the idea that dinofuzz was an early, widely-shared dinosaur feature.
And there’s something else. Pterosaurs – the flying archosaurs with leathery wings stretched over elongated wing fingers – were the closest relatives to the Dinosauria as a whole. They had fuzzy body coverings, too. No one knows for sure, but this might mean that wispy plumage was present in the last common ancestor of dinosaurs and pterosaurs, and those simple body coverings were subsequently modified or lost in different lineages as both groups evolved.
We need more fossils to test the idea that dinosaurs started out feathery. Additional fossils preserving fuzz – fluffy baby sauropods, maybe? – would help us understand the spread of feathers and their precursors among dinosaurs. And, even then, we’d still need to find exceptionally-preserved specimens of the earliest dinosaurs to see if they had any kind of filament-like body covering. The trouble is that the high-definition deposits that would even have a chance of preserving feathers are rare. It may be a very long time before we ever know for sure.
Nevertheless, there’s still a possibility that all dinosaur lineages had some kind of bristly or feathery body covering. It’s a hypothesis that needs testing, but not an unreasonable one. Think about this for a moment. Imagine a Stegosaurus with patches of long, stiff filaments covering its body, or a Ceratosaurus with a little splash of brightly-covered fuzz on its already well-decorated head. And I think a huge sauropod – like Apatosaurus – with a partial covering of dinofuzz would look absolutely spectacular. These visions are wholly different than the scaly dinosaurs I grew up with, but they are not so fantastic as to be fiction. We are only just beginning to understand how fuzzy dinosaurs were.
April 5, 2012
Science is awesome. I know this because paleontologists have just announced the discovery of a giant, feather-covered tyrannosaur.
The freshly described dinosaur—dubbed Yutyrannus huali by Xu Xing and co-authors—stretched about 30 feet long as an adult. Thanks to the fine preservation of three skeletons that represent this roughly 125-million-year-old carnivore, we know that much of this dinosaur’s body was covered in fine, wispy feathers. These were not flight feathers or down that you might see on a modern bird, but simpler structures best described as dino-fuzz. This makes Yutyrannus the largest creature with observed plumage ever to have lived.
I have been waiting for Yutyrannus or something like it for a long time. The dinosaur is a beautiful confirmation of an evolutionary hypothesis made years ago. In 2004, Xu and collaborators described a much smaller tyrant: Dilong paradoxus, which lived only about five million years before Yutyrannus, was a small coelurosaur with a coat of simple fuzz. And Dilong appears to have been an archaic tyrannosauroid, a dinosaur near the base of the family that contained later tyrants such as Gorgosaurus and Teratophoneus. If a tyrannosauroid had feathers, and almost every other lineage closely related to the tyrannosauroids had feathers, then even Tyrannosaurus rex might have been at least partly coated in plumage.
Giant tyrannosaurs with feathers was a respectable idea, but there was no direct evidence. In North America, at least, tyrannosaurs were not entombed in the kind of environments with the high-fidelity preservation potential for feathers to make it into the fossil record. And, while they have frustratingly never been published, rumored specimens of tyrannosaur skin have hinted that adult animals had naked hides. Maybe tyrannosaur chicks were fluffy while adults, no longer needing an insulating coat, lost their feathers.
Not everyone has been on board with the idea of fluffy tyrannosaurs. The humor website Cracked.com listed an illustration of a feather-covered Tyrannosaurus one as of “17 Images That Will Ruin Your Childhood,” and the same image posted at BuzzFeed attracted more than a few negative responses. (“Dear god no!” wailed on commenter.) The smooth-skinned monsters of the Jurassic Park franchise remained the canonical pop culture image of everything a Tyrannosaurus should be.
I was ecstatic when news of Yutyrannus reached by inbox. Killjoy that I am, I loved the idea that the dinosaur made it all the more likely that other big tyrannosaurs were at least partly covered in filamentous protofeathers. I have no sympathy for immature attachment to traditional visions of scaly, drab tyrannosaurs. And, despite all the cries of “Ow! My childhood!” in reaction to feathered dinosaurs, Tyrannosaurus and kin would have been just as fearsome as ever. As tyrannosaur expert Thomas Holtz pointed out in a National Geographic news item, feathers “might make [Tyrannosaurus] a little more amusing, but only until the point right before it tears you to shreds.”
The extent of feathers on Yutyrannus and other tyrannosaurs isn’t entirely clear. Although I think Brian Choo’s illustrations of Yutyrannus are fantastic, and a full coat of fuzz is a fair hypothesis, patches of feathers were only found in a few places among the three specimens: the tail, hip, foot, neck and arm. That’s enough to hypothesize that much of the dinosaur was covered in feathers, but there’s always the possibility that non-avian dinosaurs had feathers on some parts of their bodies and not on others. Any restoration opting for either pattern is a hypothesis based on the available evidence.
Still, the discovery of any feathers at all means that we might find out what color Yutyrannus was. Microscopic studies of dinosaur feathers have helped establish the palettes of small feathered dinosaurs such as Anchiornis, Archaeopteryx and Microraptor. Now there’s the possibility of unlocking tyrannosaur colors, too. Was Yutyrannus mostly covered in dark plumage, like the other dinosaurs studied so far? Or did the tyrannosaur have a different color scheme? I guess we’ll have to wait and see—according to an interview with Xu on the Nature podcast, this research is already underway.
In spite of my overwhelming excitement about all this, though, there are two wrinkles in the story. The first is that there is a slight possibility that Yutyrannus may not actually be a tyrannosaur. As paleontologist Darren Naish points out at Tetrapod Zoology, Yutyrannus shows some subtle similarities to carcharodontosaurids, a subgroup of large predatory dinosaurs more closely related to Allosaurus. Exactly where Yutyrannus fits in the dinosaur family tree awaits confirmation by way of future analyses.
Should Yutyrannus turn out to be something other than a tyrannosauroid, that would immediately make the predator that much more important. At first, it seemed that only coelurosaurs—the group containing tyrannosauroids and sundry other theropod lineages, including birds—had feathers. Then paleontologists discovered feather-like structures on two very distantly related dinosaurs—the small ceratopsian Psittacosaurus and the diminutive, bipedal herbivore Tianyulong. (Following that, the carcharodontosaurid Concavenator supposedly showed evidence of bristles on its arms, but this evidence has been disputed.)
The spread of feathers and feather-like structures among dinosaurs might mean that secondary body coverings evolved at least twice on two different sides of the dinosaur family tree. Or it might indicate that simplified integument was a common trait shared among dinosaurs—a very old feature that was retained in some groups and lost in others. And here’s where Yutyrannus comes in. If Yutyrannus is not a coelurosaur but a carcharodontosaurid or something else, then it adds another feathery point in the dinosaur family tree and suggests that a wider array of dinosaurs had feather-like body coverings.
Yutyrannus isn’t even the only dinosaur that may shake things up. A smaller, earlier theropod called Juravenator was preserved with traces of dinofuzz, and there have been rumors that this dinosaur might turn out to be something other than a coelurosaur. Much remains to be established and tested, but the emerging picture is that several dinosaur lineages—very distantly related to birds—had secondary body coverings of one sort or another. It wouldn’t be altogether surprising if Yutyrannus turned out to be additional evidence of this trend. For now, though, the primary hypothesis is that Yutyrannus was an archaic form of tyrant dinosaur.
According to comments Xu made during a Nature podcast interview, the behavior of Yutyrannus may have made the predator even sexier still. The paper mentions three Yutyrannus individuals of different sizes, all found together. Other bonebeds of multiple tyrannosaurs have been used to propose that tyrant dinosaurs were highly coordinated pack hunters, and Xu follows suits with this discovery. Since the three predators were found together in the same quarry, and a sauropod skeleton has also turned up at the site, Xu says that the Yutyrannus were members of a pack that attacked the even bigger sauropod. For some unknown reason, all died together.
I’m not convinced that this was the case. Bonebeds are tricky things—there are many reasons why multiple skeletons may come to rest in the same place. The animals could have been forced into a relatively small area by flooding or storms, they could have died elsewhere and all been washed into the same place, or the site could have been some sort of predator trap. Very careful analysis of the geology and taphonomy of such sites is required to figure out why all those bodies wound up in the same place, and we shouldn’t take the association of skeletons at face value when trying to reconstruct dinosaur behavior. Could tyrannosaurs have hunted in groups? Certainly. But solid evidence for rapacious packs of big tyrannosaurs has yet to be found.
Alone or in coordinated social groups, though, Yutyrannus must have been a fantastic sight. Discoveries like this beautifully underscore just how wonderful dinosaurs really were. If previous discoveries hadn’t led us to expect the existence of this fuzzy dinosaurian hypercarnivore, I sincerely doubt that we could have imagined such a creature.
Xu, X., Norell, M., Kuang, X., Wang, X., Zhao, Q., & Jia, C. (2004). Basal tyrannosauroids from China and evidence for protofeathers in tyrannosauroids Nature, 431 (7009), 680-684 DOI: 10.1038/nature02855
Xu, X., Wang, K., Zhang, K., Ma, Q., Xing, L., Sullivan, C., Hu, D., Cheng, S., & Wang, S. (2012). A gigantic feathered dinosaur from the Lower Cretaceous of China Nature, 484 (7392), 92-95 DOI: 10.1038/nature10906