A Thing With Feathers

Feather by Heather HinamEven if you can somehow go through your entire life without ever seeing a bird, chances are very good that you will still have some experience with feathers. Whether displayed in a  hatband, stuffed into a pillow or quilt or tied together at the end of a duster, feathers are a fairly ubiquitous part of the world around us and certainly the defining characteristic of the group of flying vertebrates we know today as birds.

But, have you ever given much thought to where they came from?

As it turns out, feathers have been around a lot longer than most people realize. As paleontologists find more fossils every year to slot into the puzzle that is the evolution of life on this planet, the picture becomes clearer and stories start to make sense.

When it comes to the story of the evolution of feathers, the first thing you have to remember is that birds are modern dinosaurs, having evolved from the lineage known as Theropods, whose ranks include those Jurassic Park villains Velociraptor and Tyrannosaurus rex.  However, what didn’t make it into the movies was the fact that, at the very least, Velociraptor was not only ferocious, but fluffy. At first, this detail was inferred from the discovery that many of its ancestors were feathered and some, like the bizarre, bi-plane like creature Microraptor gui, could fly. Then, a discovery of quill nobs, a trait seen in modern birds, on the forearm bones of one specimen confirmed it. Now an accurate representation of Velociraptor is something like a sleek, predatory ostrich.

Even more recent discoveries have put the assumption of a scaly hide in Tyrannosaurus rex into doubt. While they haven’t found specimens of this iconic dinosaur with feathers yet, a cousin from about 125 million years old China, named Yutyrannus most definitely was feathered. About the size of a bus, these are the largest feathered dinosaurs known to date.

So how far back do feathers go? In time, we can trace their existence at least 160 million years to chicken-like dinosaurs called Anchiornis, but these critters already had the highly complex barbed feathers we see in modern birds today.  Most evolutionary biologists agree that feathers likely started out as single, hollow,  hair-like filaments that became branched and barbed as needed over time. These have been found in many species, most notably, Sciurumimus, a dinosaur found very near the base of the Theropod branch. Described for the first time just last year, this species shows a spectacularly preserved coat of dense, filamentous plumes. Finding feathers like these near the base of the branch suggests that maybe more advanced Theropods, including T-rex had some kind of plumage. Still, we don’t know just how far back down the tree they go.

The point of origin keeps getting pushed closer and closer to the root of at least the dinosaur’s evolutionary tree thanks to feather filaments being found in some Ornisthischian dinosaurs, like the Triceratops cousin, Psittacosaurus, who are about as far removed from Theropods and modern birds as a dinosaur can be. Actually, they’re starting to find feathers all over the dinosaur family tree, leaving us to wonder if they predate the group altogether. In fact, the genes responsible for taking an undifferentiated plate of keratin and turning it into a feather has been found in crocodilians, who although they are birds’ closest living relatives, branched off from the group well over 250 million years ago.

So what did these prehistoric feathers look like? Structurally, early feathers started out as simple, hollow strands, growing out from a plate of keratin embedded in the skin. More advanced feathers split into barbs, looking like fluffy ostrich plumes. Eventually, those barbs developed tiny barbules that allowed their wearers to ‘zip them up’, turning them into strong, but flexible sheets that eventually were co-opted into airfoils. This same evolutionary progression can be seen today in the growth of every bird embryo.

Most fascinating, however is the fact that paleontologists now know what colour some of these plumes were. Recent work with Anchiornis turned up microscopic pockets of pigment called melanozomes. By comparing these ancient structures to those known today, they managed to work out that not only was Anchiornis about the size of a chicken, it actually kind of looked like one, a bright tableau of shiny black and white spangles with a flash of red on a crest. Who knows, maybe in time, we’ll see our very own field guide to dinosaur plumage. Either way, you can’t help but marvel at these remarkable, ancient, ingenious  and unarguably beautiful innovations of evolution.

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Flying with Dinosaurs

Canada goose and dinosaurSince the beginning of January, I’ve had the pleasure of teaching a second-year Chordate Zoology course at the University of Winnipeg. Having taken it at a different school as an undergrad and having taught the labs several years ago, the material isn’t exactly new. However, it’s been a wonderful way to rediscover the fascinating story that is the evolution of vertebrates.

First and foremost, it’s reminded me that we see dinosaurs just about everyday, flitting through the trees, soaring high overhead and gliding across a glassy pond. They’re all around us, bringing colour and music to our world.

Because of my grounding in zoology, the concept that birds are dinosaurs is not new to me, nor is it difficult to understand. However, I imagine for many people it’s a bit of a challenge to make the mental leap from a chickadee flitting among the leaves to a giant Tyrannosaurus rex thundering along a Cretaceous plain.  Still, whether you can see the resemblance or not, the genetic relationship is undeniable. A spectacularly rare discovery in 2007 of intact collagen protein in the fossil leg bone of a T-Rex allowed researchers to compare the amino acid chains within with a database of species we already have sequences for. It turned out that of all the possibilities, from mammals to reptiles, the sequence was most closely related to the collagen sequence of a chicken. This discovery probably would’ve left good ol’ Colonel Sanders with nightmares!

Even without the molecular connection, you can still see the family resemblance. Birds are descended from a lineage of dinosaurs known as Theropods, swift, bipedal predators, like Velociraptor, Deinonychus (pictured above) and the aforementioned T. Rex. While the ones most people are familiar with, thanks to Jurassic Park, are the large, ferocious creatures, most of this lineage were rather small, adapted for running and pouncing on their prey. These adaptations for speed and agility can still be seen in the skeletons of the last remaining dinosaurs, the birds.

They walked on two legs, their limbs swinging back and forth on the fulcrum of a pelvis that looked like part of a bicycle. Over time, that pelvis shifted, the individual bones fusing and getting stonger to withstand the strain brought by high speeds while maintaining its light weight. In fact, weight reduction was the order of the day in the evolution of birds from their theropod ancestors. Bones, overall, got smaller, lighter, hollowing out into tubes that were, and still are, reinforced by thin struts called trabeculae. The pectoral girdle got both smaller and in some ways, more rigid. Where the scapulae were freed up to allow the arms to swing out like flapping wings, the clavicles fused, forming the furcula (wishbone) and the sternum developed a deep keel, giving more space for what eventually became flight muscles to attach.

Still, the most striking feature these dinosaurs had in common with the ones we see today was feathers. That’s right, Creighton missed that little detail. Many theropods, Velociraptor included, had feathers. They started out as long, thin fibers that offered the minimum of insulation, gradually developing into the differentiated flight, covert and down feathers we know now. They appeared at least 160 million years ago, long before Archaeopterix (the first official bird) and even non-avian theropods like Velociraptor and Deinonychus. Paleontologists have found them in numerous species, including a small chicken-like theropod (the whole protein thing is making sense now) named Anchiornis. They’ve even managed to determine the colour of the feathers by examining the shape of the melanosomes (tiny pockets of pigment) preserved in the fossilized remains.

As more and more of these characteristics are teased from the fossil record, I can’t help but hope that one day my field guide to birds includes a section on the species that paved the genetic way for the spectacular diversity we see today.