Four Wings to Freedom

by Ari Savitzky ‘06

My infatuation with the Dinosaur began at age four, and was comprised of the usual cast of characters.  In the days before Jurassic Park, before the popularity of Velociraptors, dinosaurs still lumbered, cold-blooded, fat tails dragging on the ground as they plodded out of glossy-paged books and into my tiny heart.  I would stare dreamily at a Brachiosaur, wading in a lake, crested head towering over its submerged body as it chewed kelpy lake plants.  Tyrannosaurs were a luxury of the imagination, and I focused on Allosaurs, their slightly smaller relations, who would hunt large sauropods (like the Brachiosaur) in packs.  I climbed the stegosaur at the Cleveland Museum of Natural History.  I watched Dino-riders.

The ‘70s and ‘80s were a time of changing conceptions about dinosaurs. Slow witted “thunder lizards” became the quick, intelligent, warm-blooded monsters that captured the imaginations of Michael Crichton and Bill Waterston. In pop culture of the ‘90s, Brontosaurus became Apatosaurus (a nominal change— suck it up, Brontosaurus lovers), and dinosaurs became birds.  Or, more accurately, birds became dinosaurs.

The theory that birds evolved from dinosaurs was first substantiated by the discovery of Archaeopteryx, now considered to be the first “bird”.  Archaeopteryx was covered in feathers, had wings, and flew like a bird, but was structurally similar to dinosaurs.  Specifically, its hipbones and hollow skeleton were reminiscent of therapods— two legged running dinosaurs, like Iguanadon, Ornithomimus, and Tyrannosaurus.    In fact, “raptors” like the Velociraptor, and the larger Deinonychus, seemed to bear the closest physical resemblance to birds, a fact emphasized by the use of the term “raptor” to describe birds of prey.  These dinosaurs belong to a family known as dromaeosaurs: small, intelligent predators and scavengers now believed to have taken flight some time in the last 150 million years. And now, scientists have identified a new member of the clan.

The discovery of a feathered dinosaur is big news—a potential link in the evolutionary tree, and a valuable clue in uncovering the origins of flight.

Chinese paleontologists announced in January the discovery of Microraptor gui, a yard-long forest dweller and dromaesaur.  The discovery of a feathered dinosaur is big news—a potential link in the evolutionary tree, and a valuable clue in uncovering the origins of flight.  But Microraptor possesses an instantly more perplexing feature than its feathers: it has four wings.  Its back legs are, in fact, wings tipped with the long, asymmetrically placed “flight feathers” that characterize a functional wing.  Xing Xu and his colleagues from the Chinese Academy of Sciences discovered six such Microraptor specimens in Liaoning, touching off a flurry of speculation as to how the creature functioned and what it implies about the origins of birds and flight.

Dr. Xu has concluded that Microraptor was a glider, an argument which cuts to the heart of a heated debate.  Currently, there are two competing notions about the origins of flight.  One claims that dromaeosaurs first used feathered wings to increase their running speed, eventually attaining liftoff (think flying raptors).  The other maintains that flight began with gliding animals which, over time, started flapping their wings.  Microraptor, according to Xu, provides solid evidence of this latter notion.

Xu contends that Microraptor glided like the flying squirrel, with all four wings— an idea that fits neatly into current arboreal theories of the origin of flight.  According to the arboreal hypothesis, flight originated in the prehistoric forest with tree-dwelling gliders.  The theory is supported mainly by the fact that flight is easier and more efficient when it is continuous, level, and begins at a higher speed.  In other words, a calculated drop off of a tree branch is evolutionarily easier to pull off than generating enough thrust to lift an animal off of the ground.  The fact that Microraptor’s four wings would be burdensome for a terrestrial creature also supports this notion.  With stunted walking abilities at best, Microraptor must have been at home amongst the trees’ higher branches, gliding between the realm of birds and reptiles. 

"Birds are traditionally considered to be animals with a difference: that is, to be a distinct vertebrate class despite their origins within reptiles."

Some details, however, are less than clear.  Xu and colleagues have been unable to determine whether Microraptor was capable of delivering powerful flight strokes, or whether it was strictly a glider.  The implication of Xu’s theory, though, is that if Microraptor didn’t have a stroke of any kind, one of its offspring developed one.  Nevertheless, an in-depth study of the hind-wing joints hasn’t happened yet, so more discoveries may be on the way.

Stepping back to a larger scale, Xu and his colleagues see the evolution of theropods as leading to smaller, feathered dromaeosaurs and their ilk, which became four winged gliders— some of which developed stronger wings capable of modern flight and lost their hind-wings.  Microraptor, and other four winged creatures like these may be an evolutionary intermediate between the Dinosaur and the bird— a missing link of sorts.

Though I understand how Microraptor fits into the evolutionary tree, what’s less clear is how and why its hind-limbs developed.  It seems that no other animal has ever had four wings.  Certainly no birds have— indeed, why should they?  What is the point of four wings?  Xu’s gliding hypothesis provides the only explanation so far, that the wings act as an airfoil for a gliding animal.  But that still doesn’t address the origins of the hind-wings. 

Recent theories on the evolution of feathers are grounded in the notion that dinosaurs had them first.  As for an explanation why, there is no way of knowing for sure— to keep the animals warm, as camouflage, or maybe to attract potential mates. Flight could not have been the source of feather development, though, because all but the most specialized feathers are useless for flight.  Microraptor, however, exhibits the feathers on its wing-legs.  So why would, and how could, leg warmers turn into wings?

It seems that no other animal has ever had four wings... What is the point of four wings?

This is why the running-thrust origin of flight seems so appealing.  With the development of the feathered wing comes increased thrust and landspeed; thus, feathers and wings have a specific purpose.  That Microraptor would develop two pairs of wings that were structurally similar, yet served two different purposes, runs counter to my idea of how evolution works.

In addition, Xu et al. surmise that Microraptor was an intermediate between modern birds and their saurian ancestors. But doesn’t this imply that the path of ancestral bird evolution went from two wings, to four, and then back to two?  Doesn’t it seem more probable, or at least as probable, that Microraptor gui was an evolutionary dead end, or, even, given its bizarre appearance, an early Raelian experiment?  Especially given the emerging evidence of mass theropod feathering—from Tyrannosaurus to Compisognathus— it still seems to make sense that the thrust feathers added to theropod running speeds fueled further feather development and led to flight.

Merging the two theories of flight might ultimately be most credible: feathers and wings came about to increase landspeed; winged animals grew smaller, took to the trees, and developed hindwings to help them glide.  Of course, this theory is not without its flaws.  Why didn’t winged dromaeosaurs simply take to the skies?  Why the trees?  Why the intermediary?  For one, the intermediary features exist in birds as we know them, as opposed to winged reptiles, modern birds are clearly, according to Xu, related to Microraptor.

In all this theorizing, we must also remember that paleontology, like a Golgi stain, privies us only to a small group of all existing species at a given point.  Ultimately, hypotheses will be based on a fraction of the total amount of information. Maybe, with more discoveries, Microraptor’s place in the dino-spectrum will become clearer.  But development of flight is a matter which can be studied in other vertebrates.  Eventually, the study of bats’ phylogeny, or of the fossil records of Pterodons, may be useful in determining how the similar function arose in birds.

Meanwhile, birds are steadily losing their separateness from other animals.  As Richard Prum notes, in a piece in Nature which served as the companion article to Xu’s publication, “Birds are traditionally considered to be animals with a difference: that is, to be a distinct vertebrate class despite their origins within reptiles.”  Prum points out, not unhappily, that birds are quickly losing their distinctness in paleontology, phylogenetics, and evolutionary biology.  With the discovery of Microraptor, birds might be considered feathered dromaeosaurs without too much scientific quibble. 

But Prum also notes that science ultimately gains a richer understanding of avian development from this declassification.  Of course, some of us will be pleased for other reasons.  For me, the blurring of the line between birds and dinosaurs reawakens a childhood dream previously fulfilled only on several small islands near Costa Rica: the Dinosaurs live again!