Why Microraptor became extinct
IT IS tempting to think of the process of evolution as one of continuous, stately progress towards better-designed organisms. In fact, it is full of blind alleys--as a fossil called Microraptor shows.
At some point during the Jurassic period, a group of dinosaurs evolved feathers. These may have been for warmth, for display or for both. They were certainly not, at the beginning, for flight. But in one line of these dinosaurs evolution modified them for precisely that purpose. This modification led, via Archaeopteryx, a species that lived 150m years ago, to birds.
Feathered dinosaurs did not go away just because they had spun off the birds, though. They persisted right up until the end of the Cretaceous period, 65m years ago, when all (non-avian) dinosaurs met their ends. And well before that, in a part of the Cretaceous called the Aptian, 120m years ago, the whole thing happened again, in the form of Microraptor. Except that this time, the flying animal in question had four wings, rather than two.
Microraptor's existence raises two questions: exactly how did it deploy its four wings, and why does it have no living descendants? The answers, according to Michael Habib and Justin Hall of the University of Southern California, are linked.
Until recently, there were two schools of thought about how Microraptor carried its wings when in flight. One school proposed that it looked like a biplane, holding one set of wings above the other when it flew. The second school suggested that all four wings were coplanar. Dr Habib and Mr Hall, however, think both are wrong. They see a division of labour between the two sets of wings. The front pair, they agree, provided lift. But they believe that the back pair were for steering.
Microraptor's hindwings were radically different in shape from its forewings. Rather than being lithe and graceful, they were short and stubby. But they would have made good rudders, as Dr Habib and Mr Hall have demonstrated using a computer model of them. This suggests they would have allowed the animal to reduce the radius of its turning circle by 40% and almost triple the speed of a turn.
That makes sense. The rocks Microraptor fossils are found in are also full of trees. Clearly, it was a forest animal. Predatory birds that dwell in modern forests, such as the sharp-shinned hawk, are masters of making tight, quick turns around trunks and over branches as they pursue their prey. That Microraptor was a predator is known because one specimen has a bird in its belly. It would surely have benefited from a similar capability.
Why, then, was such an aerial paragon not ancestral to any modern creature? One possibility is bad luck. The mass extinction at the end of the Cretaceous was caused by a collision between Earth and an asteroid or comet. Though some groups of animals did better than others, survival was often at random. However, the fossil record does not prove that any Microraptor-like animal actually made it to the end of the Cretaceous. They probably become extinct well before that.
Dr Habib proposes that what actually did for Microraptor was what he calls a drag tax. His model suggests the manoeuvrability that its hind wings granted the animal came at the expense of increased drag. Modern birds do not pay the drag tax because their manoeuvrability results from a single pair of wings that are better able to cope with stress than Microraptor's forewings were. That stress-resistance is provided by muscles which are attached to a keel-like extension of the sternum. Microraptor lacked this keel.
According to Dr Habib, birds adapted for forest flight are able to make the same turns that Microraptor made, but without losing nearly as much energy to drag. This, he argues, made bird flight more efficient--and that would have made it impossible for Microraptor to compete.
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