Have you ever hadÂ a dream in which you are flying?
Dream interpreters will tell you that if you are flying with ease and enjoying the scene and landscape below, it suggests that you are on top of a situation in your conscious life. You have risen above something.Â Or you have gained a different perspective on things.Â The dreamÂ isÂ representative of your own personal sense of power.
Of course, they never seem to address why, in my flying dreams, I’mÂ wearing nothing butÂ a bolo hat andÂ bunny slippers, or why I’m covered in bacon greaseÂ andÂ tiny animated tattoos of my mother’s face. Although I supposeÂ it’s in the best interest of all decent and sane people to leave the deep-rooted psychological issues that inform my flying dreams a mystery. So forget I said anything.
Still (and this may beÂ theÂ clumsiest segue in the history of blogs) forÂ us lowly ground-based animals, capable of overcoming gravity only with the help of machines, flying is an ability that fascinates us. And it’s one that remains ripe for debate, dissension, and discovery.
To wit, oneÂ of the greatest mysteries ofÂ paleontologyÂ deals withÂ the origin of vertebrate flight. (See? Told ya.) The question of “When and how did animals first take to the air?”Â has been at the forefront ofÂ scientific inquiry into the development of prehistoric life for generations.Â We know some species evolved to soar, but we don’t know exactly when or how.
It’s been widely accepted, since the discovery of Archaeopteryx in 1860, that some lines ofÂ dinosaur evolved to have feathers, and Â modern day birds are in fact branches along theÂ dinosaur limb of the tree of life (or section of web, if you prefer)Â . Also, it should be noted that there is a minority of scientists who hold that birds are in fact not descended from dinosaurs at all (as we’ll see later), but from other early tree-dwelling vertebrates.
Either way, very little evidenceÂ has surfaced that would tell scientists when feathers became sufficiently asymmetrical to serve as an airfoil, and when the shoulder structures, breast bone strength,Â and musculature of species developed to allow sustained self-powered flight.
In addition to that, scholars are divided into two camps as to how biological flight started. One side favors a ground up approach; the idea (or Cursorial Theory) being that dinosaurs, alreadyÂ feathered to preserve body heat, eventuallyÂ had those feathersÂ on their short forelegs or arms develop further to help them run along the ground andÂ hop after fast-moving prey or from fast-moving predators, and eventually those short arms evolved into wings that were actually strong enough to get them off the ground and keep them airborne, while the hind limbs became attack weapons/grasping mechanisms.
Scientists on the other side favor a top downÂ approach; the idea (or Arboreal Theory) being that species started to glide between trees to escape predators or to attack prey. Rudimentary feathers on arboreal reptilian birds actedÂ to slow the animal’s descentÂ and control theirÂ leaps and/orÂ falls.
Of course, evolution being an adaptive process with no end goal in mind, andÂ flightÂ likely being an advantageous mutation within severalÂ different environments for several different species,Â there is nothing to suggest there isn’t some validity toÂ both theories. Yet, scientists have yet to come to a consensus.
But that’s okay. That’s how good science is done. Follow the evidence to draw the conclusion that is probably true. And right now, there is not enough evidence to know one way or the other.
Late in 2002, however, a new discovery came to light thatÂ seemed toÂ add support to the Arboreal Theory. Six specimens, of what were subsequently dubbed Microraptor gui, were excavated from the rich fossil beds of Liaoning Province in northeastern China. The specimens, preserved in fine volcanic ash,Â are dated at between 128 to 124 million years old (Early Cretaceous).
Of course, thisÂ discovery is eight years old andÂ may not be news to most of you, but there isÂ still that element of uncertaintyÂ surrounding vertebrate flight that makesÂ this an intriguing scientific case. And the conflicting assessments, investigation, and experiments that followed the find add even more intrigue to the story.
Chinese paleontologist, Xu Xing, who actually named Microraptor, first recognized the importance of theÂ uniqueÂ Â fossils coming out of Liaoning. Xu saw the peculiar wings on the hind limbs of the animal, andÂ concluded that the apparent early ancestor of birds used the configuration, along with a long, feather-fringed tail, to glide from tree to tree, much like modern day flying squirrels.Â Xu and his colleaguesÂ argued that the animal represents an intermediate stage in the evolution of flight, from gliding to the active wing flapping of modern birds. In other words, they suggestÂ that Microraptor is a dinosaur that supports the Arboreal Theory of flight origination.Â Â
Xu and his colleaguesÂ met opposition almost immediately in the person of University of Montana Biology Professor Kenneth Dial, who, by observing modern Chuckers, found that babyÂ ChuckersÂ learn to use their wings for running and climbing long before they are able to use them for flight. And once they are able to use the wings for flight, use only the flapping motion to get airborne. The process of flying is a powered process, with no gliding present at any stage. And DialÂ believes the developmental stages of the Chuckers and other birds reflect the evolutionary history of vertebrate flight (i.e. running and flapping leads to hopping well, hopping wellÂ and flapping more efficientlyÂ leads to sustained time off the ground, flapping extremely efficiently and optimal skeletal structure and musculature leads to flying). If he is correct,Â his ideas would support the Cursorial Theory.
So who is right? Is there a gliding step in the evolution of vertebrate flight?
Well, Microraptor had feathers on its hind limbs and most likely could notÂ run.Â And its shoulder structure was such that itÂ would notÂ be strong enough to generate the power needed to take off from the ground. So it would seem that, ifÂ Microraptor flew at all, itÂ had to use gravity. But howÂ could scientists test it? All they had was a set of fossils. How could they run experiments on flat specimens pressed into volcanic rock?
Xu, along with paleontologist Mark Norell and artist Mick Ellison of the American Museum of Natural History, thought they knew a way, and with the help of biological sculptor, Jason Brougham, decided to try to find out what exactly Microraptor could and couldn’t do.
Using the 30 some specimens of Microraptor nowÂ in Xu’s lab, they set out to build a model of the four-winged dinosaur. The teamÂ wanted to fully understand the skeleton, its range of motion, and its musculature. To get that understanding, Norell, Ellison, and Brougham,Â wouldÂ be charged withÂ duplicating the pieces of each specimen that were clearly intact and not shattered or deformed from the weight of the ash that caused fossilization, scaling the varying sizes of theÂ specimens, and shaping any missing pieces or unknowns to create aÂ composite animal. And if that was notÂ enough, all of this had to be done by taking basically a two-dimensional cast (the fossils)Â and transforming it into three dimensions.
The process took months.
But in the end, the team came up with what they believed to be a highly accurate representation of a Microraptor that was feathered, four-winged, and pose-able. Now they had to figure out a way to test whether it could fly. And if so, how.
At the same time, paleontologist Larry Martin and David Burnham of the University of Kansas were also examining Microraptor. Martin has beenÂ a vocal proponentÂ of the minority view that birds descended from non-dinosaur tree dwellers. He doesn’t think there is an evolutionary link from birds to the great lizards at all. And where he may agree that Microraptor was primarily an arboreal creature, he is steadfast that it was not a dinosaur. And in fact, the Microraptor model they created at the University of Kansas depictedÂ the four limbs of the creature splayed out flat, and apparently unable to be pulled up under the body, as would be possible with a dinosaur’s skeleton.Â It was the smoking gun Martin had beenÂ seeking for decades to prove modern day birds are not descended from dinosaurs.
Or was it?Â
To get a second opinion, Norell and his team sent the University of Kansas bones to anatomist Farish Jenkins of the Museum of Comparative Zoology at Harvard University. Jenkins studied the specimens and the model information, and concluded that the splayed limbs represented in the University of Kansas model were simply not correct. The speculation is that the mistakeÂ bloomed from renderingÂ the model castÂ from a two dimensional fossil. After a complete examination, Jenkins says there is no way that Microraptor could have anything other than the dinosaur-specific skeleton that would allow the legs to be drawn up under the body, eliminating the possibility of the splayed limbsÂ and keeping the dinosaur-to-birds link alive.
Now Norell and the others hadÂ a different problemÂ to address. Microraptor could notÂ flap its back wings, its hind limbs. But the feathers were aerodynamic. So if Microraptor flew, how exactly did it do it?
The team, with Xu along for the ride,Â took its feathered,Â pose-able model to the Right Brothers Wind Tunnel at the Massachusettes Institute of TechnologyÂ where aerodynamicist Kenny Breuer of Brown University would lead experiments to try to determine how Microraptor’s four wings played upon the air. Assuming the creature to be a glider, they set the model in the wind tunnel and studied the air flow over it, and adjusted the position of the wings (mostly the rear wings) to find the configuration that would allow for the longest sustained glide.
And to the researchers’ surprise and consternation, not a single position worked well enough to show that Microraptor could reduce drag andÂ generate enough lift to remain aloft for any significant length of time or distance.
The team was frustrated and nearly at the end of their rope,Â not to mentionÂ their time in the wind tunnel. It looked as though they had been wrong. Perhaps Microraptor was not a flier. They thought they were going to have to admit failure.
Until . . .
Xu Xing himselfÂ suggested another wing configuration the research team had yet to consider. It seemed a bit unorthodox, but eventually the team came to think there might be something to it. Xu’s proposed alignment would have the hind limbs extended almost straight back, which would allow the leg and foot feathers to form a canopy over the tail.
The initial concern was that the configuration would not create a lifting surface, and the animal would plummet to the ground yet again.
But the experiment proceeded, and right away it wasÂ apparent that the configuration at least kept the drag quotient low. And then to the teams great delight, the lift numbers began to climb. And the initial concerns were overturned. The configuration was acting exactly like an airfoil. The Microraptor could in factÂ glide!
Not only that, but it could pull its legs forward in a configuration tried earlier in the trials, and the drag thatÂ positionÂ created would allow it to slow andÂ pitch upward for a controlled landing on another tree.
The research took months, vocal oppositionÂ had to beÂ refuted,Â and the experiments were thorough, but the entire endeavor wasÂ very fruitful; and, the team would say, absolutely worthwhile. Of course, there are still mysteries of how vertebrate flight began to address, but the case of Microraptor is a big step forward. And it is an example of several different lines of inquiry coming together to investigate a problem and solve it.
By the way, the wonderful PBS program, NOVA, did an hour-long special on the discovery of the Microraptor and the investigation into how it flew. You can see the video online here.
Meanwhile, I’m off to the shrink to see if he can shed some light on my bizarre flying dreams.