Art by Brian Choo.
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Are fluffy tyrannosaurs - such as these Yutyrannus - any less scary than the leathery-skinned sort? Art by Brian Choo.
Art by Brian Choo.

Hooray for Dinofuzz!

I adore fuzzy, feathered dinosaurs. No surprise there. I’ve written about plumage-covered dinosaurs quite a bit this past year, my enthusiasm fueled by three discoveries that tested some paleo predictions and may change the way we restore dinosaurs in the flesh.

The fact that many dinosaurs were covered in various sorts of fuzz and feathers isn’t news. Since 1996, a steady flow of fossils from China and elsewhere have shown that various dinosaurs had some kind of accessory “integument.” We’re already well past the initial “Whoa! Dinosaurs had feathers?” moment, but each discovery tells us a little bit more about the evolution and biology of our favorite prehistoric creatures.

For one thing, even the mighty Tyrannosaurus rex was probably a bit fluffy. Paleontologists have been entertaining this idea for years – despite the plaintive cries of “Don’t make my T. rex a chicken!Don’t make my T. rex a chicken!Don’t make my T. rex a chicken!” from fans of scaly prehistoric monsters – but 2012 featured the publication of a critical piece of indirect evidence.

The majority of feathered dinosaurs discovered so far (including birds) were coelurosaurs. The tyrannosauroids were just one branch of the larger coelurosaur family tree, and in 2004 Xu Xing and coauthors described an early tyrannosauroid with preserved fuzz called Dilong paradoxus. Defenders of naked, scaly tyrannosaurs were quick to point out that Dilong was small – only about five feet long – and could have benefited from an insulating coat that would have stifled a full-grown, 40 foot T. rex.

But this year Xu and collaborators named Yutyrannus huali – a more derived, 30 foot long tyrannosaur preserved with simple protofeathers. Even large, voracious predators had dinofuzz, raising the distinct possibility that T. rex did, as well. Personally, I think the look is quite fitting for dinosaurian royalty, although I’m certain diehard fans of nude dinosaurs are not so pleased.

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An adult and juvenile Ornithomimus, covered in plumage. Art by Julius Csotonyi.

No one got in a huff over the realization that Ornithomimus edmontonicus had feathersOrnithomimus edmontonicus had feathers. Maybe that’s because the omnivorous dinosaur already looked exceptionally birdlike. After all, Ornithomimus means “ostrich bird mimic.” But I got all giddy when I heard about this find for two reasons. First, the feathers on juvenile and adult dinosaurs were different. Paleontologists had debated this possibility in the context of the species Similicaudipteryx in 2010, and the Ornithomimus fossils added a new clue that the plumage of dinosaurs changed as they aged.

The fact that feathery Ornithomimus were finally found also confirmed what paleontologists had expected.

Over the past two decades, paleontologists have uncovered feathered representatives in every branch of the coelurosaur family tree with the exception of one – the ornithomimosaurs. The spread of feathers indicated that fuzzy integument was a common feature of the group that went back to their last common ancestor rather than a feature which evolved multiple times, yet no one had found definitive evidence of ornithomimosaur plumage. (Oddly enough, in 1994 Bernardino Pérez-Moreno and coauthors described what they believed to be fossilized protofeathers with the skeleton of the ornithomimosaur Pelecanimimus. Later investigations found these traces to be fossilized replicas of muscle tissue, even though we now know that Pelecanimimus probably did have feathers, after all.) The new paper by Darla Zelenitsky and colleagues confirmed what paleontologists had hypothesized, acting as a test of expectations based on evolutionary understanding.

But how many dinosaur lineages were feathery, fuzzy, or bristly? We don’t yet know.

At first, it seemed that only the dinosaurs most-closely related to birds had feathers. Then feathers spread down to the base of the coelurosaur family tree. Now we know that dinosaurs that were exceptionally distant from bird ancestry had feather-like body coverings, too – bristles on the tail of the ceratopsian Psittacosaurus, and a mane of simplified integument along the back of Tianyulong. Did these sorts of integument evolve multiple times? Or was a combination of scales and protofeathers a feature that went back to the last common ancestor of all dinosaurs, lost in some subgroups and further modified in others?

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The holotype of Sciurumimus. Photo by Ghedoghedo, image from Wikipedia.

A little dinosaur named this year may help paleontologists better frame hypotheses about dinosaur body coverings and feather evolution. Oliver Rauhut and colleagues named Sciurumimus albersdoerferiSciurumimus albersdoerferiSciurumimus albersdoerferi – the “squirrel mimic” – from a juvenile dinosaur skeleton found in Germany. Like Juravenator from the same deposits, the skeleton was preserved with traces of simple dinofuzz. There has been a bit of post-publication discussion about what sort of dinosaur little Sciurumimus really was, but, if Rauhut and coauthors are correct, this dinosaur with a bushy tail was a megalosaur.

Megalosaurs are a relatively poorly-known group of theropod dinosaurs. They were not coelurosaurs, but more primitive cousins on a different line of the bigger theropod group. Protofeathers on a megalosaur, therefore, would move the origin of such structures back a little bit closer to the last common ancestor of theropods, and perhaps the Dinosauria as a whole.

Of course, there are plenty of caveats. Sciurumimus might not be a megalosaur, and, simple feather forerunners may have evolved multiple times among different dinosaur lineages. What’s important to keep in mind is that Sciurumimus, Psittacosaurus, Tianyulong, and other non-coelurosaurians complicate our understanding of what dinosaurs looked like and how they lived. Such finds raise new questions that will take decades of discovery and debate to investigate, and these confounding possibilities have reinvigorated paleontology.

I imagine that we’re only just beginning to grasp how many sorts of dinosaurs carried some sort of plumage. And, so far, we don’t really know much at all about the biological function of the various feather types. Display, insulation, and, for some species like Microraptor gui, flight are obvious candidates. Yet I can’t help but wonder what else prehistoric feathers might have been good for.

Even though I’m skeptical that small marks on the arm bones of the predatory Concavenator truly represent attachment points for feather quills, for example, might it be possible that some predatory dinosaurs  had feathers on their arms to assist them in detecting and manipulating prey? Studies of living birds have shown that feathers can be sensitive organs of touch, much like whiskers and vibrissae among mammals. Might Allosaurus, Velociraptor, and other predatory dinosaurs have employed arm feathers or bristles in a similar way to the vibrissae on the forelimbs of cats? Or could some dinosaurs have had feathers jutting from their heads or faces to help them navigate dense forests? This is armchair speculation, but I don’t believe such scenarios are entirely out of the question. They are possibilities that have not yet been tested by what’s in the rock record. Dinosaurs are becoming stranger by the day. Yutyrannus, Ornithomimus, and Sciurumimus  are gorgeous, enfluffled representatives of that fact.