Stephen Brusatte, a paleontologist with the University of Edinburgh, thinks it’s time to change the way we think about the evolution of flying birds.
In an article published Thursday in Science, he argues that dinosaurs evolved wings and feathers long before they could fly, experimenting flight in ways that were very different from those of modern birds. “The more fossils we find, the more it seems like a whole bunch of small, feathered, winged theropods closely related to modern birds were experimenting with different types of flight,” he tells Inverse by email.
Rewind 170 million years. We are in the Jurassic Period, 100 million years before the rise of the Tyrannosaurus rex. There’s a group of small, carnivorous dinosaurs called the coelurosaurian theropods running around after even smaller prey. It’s from this group that birds rose up, though not in a linear way. Not all of them evolved towards flight, and those that did took different paths to get there. What made this diverse group of flapping birds and proto-birds possible is that they already had the basic structures necessary for flight, although entirely by accident, as these features evolved for different purposes altogether. “They had the body and biology perfectly suited for flight — they were small, they had wings, they could grow fairly fast and probably had a high metabolism, and those were the right cocktail of ingredients to begin playing around in the air,” Brusatte says.
A treasure trove of new fossil evidence, particularly from China, has shown that dinosaurs had feathers long before they could fly. They started as hair-like filaments, which became soft, downy feathers, and then stiff, plumaceous feathers. “The fact that pennaceous feathers first show up in fairly large theropods — about the size of a horse — that couldn’t fly is provocative evidence that these feathers didn’t evolve for flight,” says Brusatte.
Stranger still, some dinosaurs grew full, feathered wings that had nothing to do with flying. Ornithomimus, for one, looked a bit like an ostrich and probably ran very fast but certainly could not have flapped its small wings fast enough to get any lift. “The first wings may have been advertising billboards — big sheet-like display structures used to attract mates and intimidate rivals,” Brusatte suggests. “Modern birds use their wings (and the pennaceous feathers that make up the wings) for many things other than flight — display, for instance. If I had to make a gut call, I would say that pennaceous feathers and wings probably evolved for display, and then later were co-opted as airfoils for flight.”
Remember Psittacosaurus, that weirdly adorable little dino unveiled last year in all its realistically colored glory? It had a strip of spiky tail bristles that most certainly weren’t for flying.
Perhaps the strangest data point in this story of bird evolution is a little creature named Yi qi. This was a small theropod that arose about 160 million years ago in what is now China, and it had bat wings — a membrane of skin stretched between fingers that may have been used to glide down from tree branches. “The very different anatomy of Yi’s wing tells us that it was evolving flight independently of other dinosaurs,” says Brusatte. “It was a separate experiment in taking to the sky.”
What this all means is that it’s no longer safe to assume that feathered, winged creatures that look like birds could fly, or had ancestors that could fly, or were on an evolutionary track in the direction of powered flight. When the asteroid hit 66 million years ago and wiped out nearly all the dinosaurs, there would have been a great diversity of winged flappers — some who flew, some who glided, and some who stayed firmly planted on the ground.
“All dinosaurs other than birds went extinct at the K-Pg [extinction event,]” Brusatte says. “But, we also have to remember that many birds — most birds, actually — went extinct as well. So maybe modern-style birds were just lucky. Or maybe they did have one or more features of their skeleton, or of their biology or growth, that helped them make it through. The ability to eat seeds is one idea, or maybe they could fly better, or faster, or farther than other birds. Or maybe they grew faster or had higher metabolisms. Ultimately, we don’t quite know.”
Brusatte contends that it will take more than new fossil discoveries to answer the remaining mysteries of the evolution of flight. He wants to see engineers get into the game, building biomechanical models of known winged dinosaurs to see how they fare in wind tunnel experiments, and computer models to evaluate their anatomical capacity for flight. It’s an open challenge: “Surprisingly, there is very little work in this area right now.”