canberrabirds

The origin of flight

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Subject: The origin of flight
From: "Tony Lawson" <>
Date: Sat, 2 May 2009 20:20:32 +1000

The feathers we see on birds today are completely different in form and function to those that started out as 'fluff' on dinosaurs 150 million years ago. Using fossil records from China, palaeontologists believe feathers first evolved as insulation. It wasn't until much later that flight behaviour developed. Scientists think flight behaviour developed as an escape mechanism, and it began more as assisted running with wings than flying.

Robyn Williams: Yes, I know you've been told those birdies all around you are descendants of dinosaurs, but the evidence is becoming all the more fascinating, mainly because so many dinosaurs are being unearthed in China. One palaeontologist who's on their case is Dr Mark Norell, chairman of dinosaurs at the American Museum of Natural History in New York.

Mark Norell: North-eastern China 150 million years ago was a remarkable place. It was a temperate forest, basically, it was an area which had a number of large lakes and small lakes and it had volcanoes going off in the general area, so there was constantly volcanic ash raining down on things, which made ideal conditions for preservation of small lakes. It also is ideal preservation in the sense that it preserves parts of the animals which we don't usually get. For instance, we have soft tissue preservation, including things like feathers, things like hair in mammals, things like the claw sheaths on animals, so that we really get this remarkable preservation which we only get in a few other places in the world.

Robyn Williams: How fine in detail are those feathers?

Mark Norell: The feathers we can see quite a bit. Unfortunately we can't see everything we would like to see in them because we're not really looking at the fossilised feather. When you see the imprint of the feather in the rock you're looking at the bacterial decomposition products of the decomposition of that feather. So the feather, you can make out the structure, you can see the shaft down the middle which is called the rachis, you can see the blades which are composed of single filaments which are called barbs, and in some cases we can even make out the barbules which are just microscopic little hooks that allow feathers to be able to self-organise, so when you take a feather and you mix it all up and then you can pull it back and zip it back into shape. These animals had those characteristics also.

Robyn Williams: The big question is what were those feathers for in the beginning?

Mark Norell: The most primitive feathers that are found in the more primitive dinosaurs don't look like feathers at all, instead they look more like hairs. But unlike hairs they're hollow, so they have a lot of the same characteristics that feathers have but they covered the entire body, and they're just small...some people have called them dino-fuzz. We refer to them as proto-feathers, but they're a uniform body covering over the entire animal. If you think that living birds today, feathers are used for a variety of things, they're not just used for display, they're used for camouflage, they're used as aids in hunting, they're used to help brood the nests, but most importantly they're used for insulation. Just like we even use bird feathers today in insulation in down coats and things like that, the insulation component is very important.

So when you see a bird hatch, it doesn't have big feathers for flight or display but they're completely covered with downy feathers which are for insulation. So I think we can make a pretty good argument that feathers originated as insulatory coverings for birds and then as we continue up the evolutionary tree and get closer and closer to birds these feathers were then elaborated into some of the different feather types that we see today, you know, everything from the primary wing feathers which are asymmetric, which have neurodynamic function, to feathers which exist on the tails that we see in the non-avian dinosaurs also in these tail fans which were probably for display, and feathers which have all sorts of different uses in modern birds.

Robyn Williams: And of course most structures in animals, plants for that matter, tend to have a number of uses. But the question of flight has been argued about for some time. Was it something that was developed straight away or was there a process which was far more to do with escaping your enemies, so that you're running on the ground and somehow the wings and indeed the feathers helped you escape rather more quickly? Was that the beginning of the process, do you think?

Mark Norell: Certainly if we talk about flight that's present in modern birds, like the ones we can see out my windows, that sort of flight comes really pretty late in the evolutionary story and doesn't have much to do with the origin of what we would call volant capability, which is just the ability to get up in the air a little way. If you look at the early bird story and what has been called the archetypal early bird, archaeopteryx, if it had any volant capability it was a pretty poor flyer and in no way, in my mind, could fly like a modern bird flies. However, if we talk about the origins of flight itself, I think that a good argument can be made for escape, for predator avoidance.

I think there's been a lot of very interesting work done by Ken Dial and his group in Montana on just what this fright and flee mechanism is. They refer to it as wing-assisted running or wing-assisted incline running, and they can see it modern birds and they can see it in hatchlings of modern birds, when, even before they're fledged, when they'll run, and then as the same time as they run to get added momentum is they'll flap their wings. Sort of like a sprinter running and using his arms to move to increase momentum. Certainly early birds I think did this and I think that we can see that in juvenile birds today. It is predator avoidance and stuff. I mean, you scare these things and then they go, and they're incapable of flight but they flap their wings, they can go vertical.

Robyn Williams: So what was archaeopteryx doing? Was it gliding, was it just hopping in the air?

Mark Norell: We can never tell for sure because we can't study behaviours of animals which have been extinct this long. The feathers of archaeopteryx, just like the feathers of some more primitive dinosaurs, things like microraptor and stuff, they show this asymmetrical pattern that suggests that they were used for some aerodynamic activity. Nevertheless, if archaeopteryx could fly he was probably a pretty poor flyer because if you think of the physics of flight, one has to be able to transmit force which is generated by the wings moving down to the air and to the ground, to be able to compress that to be able to generate up-force for the animal to fly. So if you want to be efficient in doing that you want all of the force which is generated by the wings to be transmitted downward.

Archaeopteryx simply couldn't do that because its body was built sort of like a slinky. If you took and you compressed the wings down, a lot of what would happen is just that the bones would just move around. It would be sort of like when you pick up a cat or something, it's going all over the place. It's not until we get much later in the evolutionary tree to an animal called confuciusornis where we get the kind of rigidity in the skeleton that one sees in a modern bird, which would be very efficient for it to have a down-force and then be able to actually transmit all the force from the wing beat to the air to be able to fly in somewhat of a similar fashion to what we see in pigeons or something today.

Robyn Williams: What did the confuciusornis look like?

Mark Norell: Confuciusornis was a small bird. It had a beak, it was the first bird to have a beak, it didn't have teeth. It had sort of a plump body, sort of like a partridge or a quail. Its wings weren't extraordinarily long, however its wing feathers were very long. So it lacked a true bony tail the way that a non-avian dinosaur or even archaeopteryx had, but it had a couple of big, long feathers that stick out the back that can be twice as long as the body itself, sort of like a tropic bird or a bird of paradise.

When we find confuciusornis, those long feathers are only in a fraction of the animals that we find, and sometimes we can find many animals that are preserved in the same slab suggestive that they were killed by a gas cloud from a volcano or something and a whole flock fell in, and not all of those will have the long tail feathers. So there are inferences that have been made that perhaps these are males, like the same as in living birds today which the males usually have more spectacular plumage than the females do.

Robyn Williams: Mark, what's your own line of research in this field?

Mark Norell: Really what I...I shouldn't say 'I' but my group, including my students and my colleagues and stuff in China and elsewhere, what we are interested in looking at is the transition from what people have traditionally called dinosaurs to what people have traditionally called birds. I think that over the last 20 years that line has really been blurred. It's very similar to human evolution when 100 years ago it was pretty clear what was human and what was non-human, because basically you had modern humans and you had a few cavemen fossils kicking around Europe, but with the great discoveries, especially from east Africa, of Australopithecus materials, the Homo erectus stuff from central Asia and everything else, the line between what's human and non-human has really blurred, to the point even that it's something which some anthropologists won't even talk about because you can't even really define it.

Well, similarly in the colloquial name 'birds' also...we used to think that birds are special because they have feathers, they have wish bones, they have hollow bones, they brood their nests, they have an advanced socio-biology, they have all these other sorts of things, an advanced metabolism. Well, all of those attributes which I just named are now found in non-avian dinosaurs. We have evidence that tyrannosaurus was feathered. We have evidence that non-avian dinosaurs like velociraptor and oviraptor and stuff sat on their nests and brooded them. We know that tyrannosaurus rex, as well as much more primitive therapods than it, had wish bones. So all these attributes are being forced further and further down the tree which is just giving us a very, very different picture of how birds were put together and some of the evolutionary modifications and the timing of those modifications, which arguably at 10,000 species is the most successful vertebrate terrestrial group on the planet.

Robyn Williams: When you go across from the museum here to Central Park and you look at the pigeons and other birds, what do you think of them?

Mark Norell: Actually it depends on the bird. The pigeons, I wish they'd go away! But I don't think about it that much but I like to think that the work that my group has done, that we've given a new appreciation for birds and really shown them to be what they are; living dinosaurs.

Robyn Williams: Mark Norell, curator of fossils at the American Museum of Natural History in New York.

http://www.abc.net.au/rn/scienceshow/stories/2009/2545603.htm

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