Why Did Dinosaurs Have Feathers?

Transcript

JINGMAI: In the last five years, we realize that flight evolved in dinosaurs four times. Who knows what we're going to discover in the next five years?

ERIC: It has been 66 million years since most dinosaurs went extinct, but we're still learning new things about them every day. Some of the most exciting new discoveries, and many of the dinosaur questions we get at the Museum of Science, have to do with recent fossils that show many dinosaurs had feathers. Most people simply ask, why? To get an answer, I talked with Dr. Jingmai O'Connor, a paleontologist at the Field Museum in Chicago and one of the leading experts in the world on dinosaurs, feathers, and flight. Dr. O'Connor, thank you so much for joining me.

JINGMAI: It's my pleasure, Eric. I'm happy to be here.

ERIC: So I thought a good place to start would be a great question that we've received fairly often. How do we know that dinosaurs had feathers?

JINGMAI: Well, we know from direct evidence. In 1996, the first feathered dinosaur was unearthed from early Cretaceous deposits in Northeastern China. But since then, feathered dinosaurs have been found most recently in Brazil, but also in Canada and other parts of the world.

ERIC: So feathers and flying are a very recent addition to our knowledge of dinosaurs. Do we know how those traits originated?

JINGMAI: So flight has actually seemed to evolve in dinosaurs multiple times. Right now we've identified probably four different independent occurrences of flight evolving in dinosaurs, and each time it evolves differently. So of course, in birds, it evolves with just wings on the forelimbs. But in the microraptor lineage, it evolved with wings on the arms and the legs. And then there's this other bizarre lineage called the scansoriopterygidae, which doesn't use feathers at all and flies with wings that are more like that of a flying squirrel or a bat, or a pterosaur, basically wings that are supported by elongate digits in the hand and that are formed by flaps of skin. We call them patagia.

ERIC: Is this mostly gliding, or is this a lot of actual powered flight?

JINGMAI: Well, that's a good question. There needs to be research on every one of these independent lineages because we've only very recently realized that flight evolved multiple times. So it seems, in scansoriopterygids, because the wing is fairly small, that it was probably just gliding, something like a flying squirrel. Microraptor, we're less certain. People argue about that one a lot because it's such an interesting taxon. I would say that it had some form of, maybe, flap gliding because, for example, birds fly in all different ways. There's bounding flight, where you'll see a bird kind of flap a bunch and then kind of turn itself into a little bullet and shoot for a little while, and then starts flapping again. And then there's birds that flap and glide, so they keep their wings out during the non-flapping phase. There's all different types of flight is my point.

I would argue that microraptor probably had powered flight, so that it did use some type of flapping locomotion. But we're really not sure. And then the last, the fourth possible occurrence of flight evolving in the dinosauria is another dromaeosaurid. So flight evolved within dromaesauridae probably at least twice. And this is a taxon called Rahonavis, from the Late Cretaceous of Madagascar. And this one doesn't preserve any soft tissue. So we don't know if it had two wings or four wings, and it's much more difficult to infer how it flew without preserved soft tissue.

ERIC: Well, it's amazing to think that flight evolved so many times within dinosaurs, and it's exciting that all of this knowledge is so new. Now our big question for you today is, why did dinosaurs have feathers? Some dinosaurs used them for flight eventually, but evolution is a slow process. So why did some dinosaurs evolve to have feathers in the first place?

JINGMAI: So this is another question that we don't really know the answer to at the moment. But the leading hypothesis is that the earliest feathers evolved for some form of thermoregulation, so basically to help keep the animals warm. And the first feathers didn't look anything like a modern feather. They looked more like hair. They're different from hairs based on the follicle structure, but it was just a monofilament. And then during the evolution of dinosaurs, as you get closer to the dinosaurs that are closely related to birds, you see more and more complex feather shapes evolving. And it's only with the evolution of the modern-type feather that you see these feathers starting to become used for flight. And there's a good reason for this because a modern-type feather has a central rachis, or like a spine, and then on either side, it has these what we call feather vein. And the vein interlocks.

I think we've all played with feathers, and you pull it apart and then you can zip it back together. And it's this interlocking morphology that allows feathers to form a cohesive surface that can be used as an aerofoil, like to form a wing. If it doesn't have this interlocking morphology, then you can't create this aerodynamic surface.

ERIC: So dinosaurs had been around for hundreds of millions of years, and evolving feathers and flight. And then, 66 million years ago, we had that great catastrophe that made so many of them go extinct. But as we've learned recently, not the dinosaur ancestors of today's birds. So what did those animals who survived the extinction event look like? Did they look like birds yet?

JINGMAI: Modern birds were already present in the Cretaceous. And they lived through the meteor impact. Birds first appear in the fossil record 155 million years ago, and then in the early Cretaceous, thanks to the amazing discoveries from Northeastern China, we have a huge radiation of all these different bird lineages. And there was actually one lineage in particular that dominates everywhere in Cretaceous deposits. It's called the enantiornithes. But then this lineage, along with all other non-crown avian lineages, goes extinct. So basically just one avian lineage survives. But it wasn't just one species of bird that survived through the end Cretaceous extinction. We actually know that, for example, the split between the birds that eventually give way to ducks and geese-- that's the anseriformes-- they had already split away from the galliformes, which are like chickens and turkeys. There was a diversity of modern-type birds already present in the Cretaceous that survived.

ERIC: So where is the line between dinosaur and bird? At what point did the ancestors of birds stop being dinosaurs?

JINGMAI: Well, that's a good question. Right now we actually don't have any morphologies that we can point to and say, this is a bird, and this is an avian dinosaur, and this is a non-avian dinosaur. The common ancestor of the modern sparrow, and archaeopteryx, and all its descendants, those are what we call birds. And then everything else would be a non-avian dinosaur. And birds are dinosaurs. It's not that they descended from dinosaurs. They are dinosaurs. And I would argue-- people say that the Mesozoic is the age of the dinosaurs, and the Cenozoic is the age of the mammals. But there are more than twice as many species of birds right now than there are mammals. So really, it's still the age of the dinosaurs. They're this incredibly successful group.

ERIC: I really like that idea that we're still living through the age of dinosaurs because you look outside, and they're flying everywhere. Now if birds already existed at the time of the gigantic impact that wiped out not just most other dinosaurs, but most other life on Earth, was there something about them that let them survive when the other dinosaurs couldn't?

JINGMAI: Well, I think it was a combination of a bunch of different factors. It's never just one thing that we can point to. And I think it's because this particular group of birds had a more efficient digestive system than other groups, more flexible digestive system, that allowed them to feed on resources like seeds and things that were still around, even when all the forests and all the plants were dying because of the post-meteor impact extinction. Also, it has to do with differences in how they reproduce. For example, these more primitive birds, or the birds that didn't survive, we know they seem to have laid their eggs directly into the dirt. And it seems that they probably didn't really mess with their eggs after they laid them, whereas modern birds manipulate their eggs in order to keep them warm evenly. And so this ability to manipulate your egg, to move your egg, probably allowed them to save some of their eggs in this time of crisis.

Also, they probably didn't lay their eggs on the ground. They could lay their eggs in protected environments. Also has to do with how they grew. Birds that didn't survive seem to have grown more slowly, which means that they spent a longer period of time being vulnerable, and these modern birds go very fast. There's even some birds that reach pretty much adult size within two weeks. That's crazy.

And all birds - almost all birds with the exception of the kiwi - grow to adult size within the first year. So this rapid growth, where they rapidly move out of the period where they're vulnerable, may also have helped them. It also has to do with their ecology. For example, there's the idea that birds that lived in near shore aquatic environments might have been kind of sheltered from some of the worst of the impact. And it seems that most of these birds that lived in these semi-aquatic environments, again, were this group that did survive. So we don't really know exactly, but I think it's a combination of a bunch of different factors.

ERIC: Wow. That's a lot that birds had going for them to get through this tough time. They were really well-positioned with their growth and egg laying and everything to survive the lack of sunlight and plants dying off. And then they kept evolving and adapting into the really diverse bird populations we see today.

JINGMAI: Yeah. Well, modern birds are extremely modified compared to all other amniotes. And this, I think, is exemplified by just how successful they are. They can live in extreme environments, and they're capable of incredible physical feats like long-term migrations, flying over the Himalayas. And there's all these features that we think of as avian features because we only find them in birds alive today. But when we look at the fossil record, we can see that a lot of these avian features, some of them were inherited from dinosaurs. So they're actually dinosaurean features that birds inherited like feathers, like wings, whereas other features were evolved during the Cretaceous evolution of birds. And they didn't evolve all immediately at once. Different groups evolved them at different times.

So it just happens that all of these features are only present in this one group of birds, and this is what helped them to survive, whereas other groups had some of the features but not all of them. And they just weren't able to make it through.

ERIC: And to end, here's a question. I got from a visitor once after a live presentation on dinosaurs that I really loved. What fossil would you like to be discovered next?

JINGMAI: I would like to find a bird that's from the gap in between archaeopteryx - 155, 150 million years old - to these birds from the Jehol deposits, which are 130 to 120 million years. We have this 20 million-year gap that is really critical to understanding the early radiation of birds. So I would love to find a fossil from this period of time.

ERIC: That would be amazing. Dr. O'Connor, thank you for talking with me about dinosaurs and feathers in flight.

JINGMAI: Happy to do it.

ERIC: On your next visit to the Museum of Science, check out our dinosaur fossils including Cliff, one of the most complete triceratops ever found. Until next time, keep asking questions.

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