Four-Winged Dinosaur Enters Debate Over the Evolution of Flight
2003 proved to be an interesting time in paleontology as six four-winged dinosaurs were unearthed in Liaoning China. These animals were all under three feet in length and were unusually well preserved with the outlines of fully formed flight feathers being clearly seen attached to both their front and hind limbs. The specimens were from the late cretaceous period, sometime around 128 and 124 million years ago. They were dubbed Microraptor gui.
Immediately this bizarre little creature found itself embroiled in paleontological debate. It was older than most other primitive dinosaur-bird fossils and many believed it may have been the first to display flight amongst these creatures. They were however not the first flighted animals as Volaticotherium antiquus, a type of flying squirrel-like mammal lived at approximately the same time and cockroaches are largely believed to be the first to master powered flight somewhere around 220 million years ago. They also weren't the first feathered dinosaurs as older specimens have been found with down-like feathers, largely speculated to be used for warmth. This wasn't to say the specimens weren't important though, they added fuel to a long standing debate about the origin of flight in birds.
The Opposing Theories of Flight
Even amongst paleontologist who believe in Thomas Henry Huxley's 1860s Dinosaur to Bird Evolutionary Theory were divided about how exactly flight came into being. Two basic theories formed, one was called the ground up theory, the other was called the trees down theory.
In the ground up theory various paleontologists speculated that bi-pedal dinosaurs first evolved feathers and then accidentally stumbled into flight by evolving both speed and flight feathers on their forelimbs. This theory goes on to say that the basic flight stroke that birds use was first displayed in the grabbing motion of these dinosaurs when they were hunting. This does make sense as the flight stroke in birds is not a straight up and down motion as most would believe, it's actually a figure-eight like motion more like swimming or grabbing something and pulling it backwards. Bi-pedal dinosaurs with varying feathers had been found in the few years previous in all stages. Some having just down feathers and some having both down and flight feathers.
Still not everyone was convinced. Another group of paleontologists claimed the ground up theory was too complicated, that flight must have evolved first from gliding animals who lived in the trees. The discovery of microraptor thrilled this team of paleontologists who said something with four wings must be a glider! And seeing as it was the oldest dinosaur with flight feathers then that would mean by default that they were right.
Of course in all fields there are the negotiators and mugwumps who choose not to take sides. This debate is no different. There are some paleontologists who speculate that flight evolved numerous times among the dinosaur species and therefore could have come about in both ways.
Microraptor's Extra Wings are Speculated Upon
No one had seen anything with four wings in the current world or in paleontology so this proved to cause quite the stir. What were the back wings for exactly? Could they flap like the front wings? Could they aid in gliding? Or were they something entirely different?
Immediately it was thought that they were used for gliding like a flying squirrel, with all four legs spread out like a pancake. It didn't take long to disprove this theory. Although the forelimbs were capable of this dexterity the back limbs were much like other dinosaurs, created for bi-pedal movement like an ostrich or human. Microraptor could not flatten itself out like a pancake without completely dislodging both hind limbs from their hip sockets. Obviously this was not how it worked.
However this did not put an end to the gliding theory. Reconstructive paleontologists created a model of the strange little animal to put through a wind tunnel for tests with different positions being tried. Holding it's legs under it in the typical bi-pedal fashion, the dinosaur seemed to create too much drag, and holding it's legs close to it's belly had much the same results. Only when the legs were tucked completely in with the feathers held over it's back/tail did the animal create proper lift. This confused paleontologists even more but I speculate there is a reason to this madness. Microraptor does not have a proper bird tail, and therefore could not have used it's tail to alter it's direction with any great accuracy. Flying squirrels have tails that whip around in circles and aid in their directional abilities and birds have robustly feathered, well muscled, and most importantly flexible tails to achieve their direction. Lizard tails are just too stiff to achieve this very well. Personally I think these extra "wings" were more like rudders in the absence of a proper flighted tail but who am I to say? Perhaps there's another explanation still...