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A second specimen of the troodontid Mei, preserved in a bird-like sleeping position. From Gao et al., 2012.

Bone by bone and study by study, paleontologists are learning more than ever before about dinosaurs. But there are still many aspects about prehistoric biology that we know little about. In fact, some of the simplest facets of dinosaur lives remain elusive.

For one thing, we don’t know much at all about how dinosaurs slept. Did Apatosaurus doze standing up or kneel down to rest? Did tyrannosaurs use their tiny, muscular arms to push themselves off the ground after a nap? And, given the discovery of so many enfluffled dinosaurs, did fuzzy dinosaurs ever cuddle up together to stay warm on chilly Mesozoic nights?

Since we can’t observe living non-avian dinosaurs directly, some of these questions have to remain in the realm of speculation. But a handful of fossils have shown us that at least some dinosaurs curled up just like birds. In 2004, Xing Xu and Mark Norell described the tiny, early Cretaceous dinosaur Mei long–a feathery troodontid dinosaur with big eyes and a little switchblade claw on each foot. What made Mei special, though, was the way the dinosaur was preserved.

Many articulated dinosaur skeletons are found in the classic dinosaur death pose, with their tails tilted up and their necks thrown over their backs. The nearly-complete skeleton of Mei was different. The foot-long dinosaur rested its head over its folded arms, and its tail wrapped around the dinosaur’s torso. Mei died sleeping in a roosting position similar to that of modern birds. The dinosaur’s name, which means “sleeping dragon,” is a tribute to the behavior.

Now another Mei specimen has confirmed that the first find was not a fluke. Last week, paleontologist Chunling Gao, of the Dalian Natural History Museum in China, and colleagues described a second, slightly smaller Mei that was preserved in a nearly identical sleeping position. Much like the first, this Mei probably died in a prehistoric ashfall that both killed and preserved the dinosaur in delicate detail without jarring the snoozing troodontid out of position. Some feathery, non-avian dinosaurs not only looked like birds, but they slept like them, too.

The two Mei specimens aren’t the only dinosaurs found in such positions. Gao and colleagues also point out that a specimen of another troodontid found in the Cretaceous rock of Mongolia, Sinornithoides youngi, was found in the same sort of sleeping position. And while not mentioned by the authors of the new study, the sleeping positions of Mei and Sinornithoides remind me of the early Jurassic dinosaur Segisaurus. Described in 1936, the partial skeleton of Segisaurus was found with its legs tucked beneath its body and arms apparently in a resting position. Perhaps this dinosaur, too, died while dozing, and records an even older record of how dinosaurs rested. Such glimpses are rare, but they help fill in some of the most elusive moments in Mesozoic history.

[Check out artist Julius Csotonyi's blog for a lovely new illustration of the second Mei specimen.]

Reference:

Gao C, Morschhauser EM, Varricchio DJ, Liu J, Zhao B (2012). A Second Soundly Sleeping Dragon: New Anatomical Details of the Chinese Troodontid Mei long with Implications for Phylogeny and Taphonomy. PLOS One DOI: 10.1371/journal.pone.0045203

The skull of Yutyrannus. Photo by Zang Hailong.

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.

A restoration of Yutyrannus , with the therizinosaurs Beipiaosaurus in the foreground, by Brian Choo. Caption added by the author.

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.

See also: Posts about Yutyrannus by Dave Hone and Ed Yong.

References:

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