Built To Fly

With its four wings and a long, bony tail, Microraptor was unlike any bird alive today. This is because it was a dinosaur—one that evolved long after the first known bird, Archaeopteryx, split off from the dinosaur family tree. Earlier dinosaurs, common ancestors to both Microraptor and Archaeopteryx, had already formed many of the physiological traits needed for getting airborne, such as feathers and light, hollow bones. With these structures in place, both animals were able to separately take further evolutionary steps by forming wings on their arms (and in Microraptor, legs) and getting off the ground. Below, learn more about the evolution of gliding and flight, and of birds, by comparing the skeletons of Archaeopteryx, Microraptor, and Deinonychus, one of Microraptor's closest non-flying relatives.—Rima Chaddha

Microraptor		1. Feathers Feathers weren't unique to early birds—Microraptor had them, as did more than a dozen other dinosaurs discovered so far. But whereas Archaeopteryx and other birds formed flight feathers only on their arms, Microraptor also had feathers on its hind limbs. Though not a flier, evidence suggests Deinonychus also possessed feathers.
Inside a bird's bone		2. Hollow bones By roughly 240 million years ago, almost 100 million years before Archaeopteryx evolved flight, its ancestors formed the hollow, thin-walled bones that would give later birds as well as flying dinosaurs like Microraptor the lightweight skeletons they needed to get off the ground. Lighter skeletons also benefited non-flying dinosaurs such as Deinonychus by allowing these predatory animals to become swifter and more agile.
Archaeopteryx		3. Ankle Archaeopteryx's strong ankles probably served as shock absorbers during landings. The bones comprising them were relatively long compared to those of related dinosaurs, and they were partially fused together. In modern birds, these bones are totally fused and, due to their length, are often mistaken for the lower legs. This gives birds their distinct—and misleading—"backward-facing knee" look.
Microraptor		4. Wishbone Not surprisingly for a gliding animal, Microraptor had a wishbone. Some scientists speculate that this structure, formed by the fusion of the collarbones, helped the dinosaur as well as Archaeopteryx and its bird descendants maintain their stability while airborne. But even a non-flier like Deinonychus had a wishbone; like many flight features, this structure dates back as far as 240 million years.
Archaeopteryx		5. Teeth As a primitive bird, Archaeopteryx retained many dinosaurian traits, including teeth similar to (though numbering fewer than) those found in Microraptor and Deinonychus. While modern birds are toothless, they too show signs of their evolutionary past. As embryos, developing birds form tooth buds, or clusters of cells that eventually become teeth in other animals. These buds are lost, however, as the embryo develops its beak.
Deinonychus		6. Sickle-shaped claw Deinonychus, or "terrible claw," gained its name from the sharp talon on each of its feet, which it probably used to slash at its prey. All of Deinonychus's closest relatives, including Microraptor, shared this feature. Archaeopteryx and later birds had smaller claws, but the modern cassowary, a relative of the ostrich, evolved a talon like that of the dinosaurs (also for hunting).
Deinonychus		7. Hands These animals each had three digits on their hands, reduced from the five possessed by their common ancestors. This reduction has continued in modern birds, in which the thumbs have almost completely disappeared and the remaining two digits have fused together inside the wing. But in a sign of their evolutionary past, bird embryos briefly develop individual fingers before their wings fully develop.
Microraptor		8. Tail Like its non-flying close relatives, Microraptor had elongated bones lining the top and underside of its tail, forming a stiff counterbalance that allowed the animal to keep its center of gravity over its hips as it evolved longer, heavier forelimbs. Archaeopteryx's tail was more flexible but much longer than the short tailbones found in birds today.
Deinonychus		9. Limbs As a non-flier, Deinonychus retained relatively short arms similar to those seen in its ancestors. Microraptor and Archaeopteryx, however, needed more elongated forelimbs to stay aloft, including upper arms longer than their shoulder blades. Additionally, Archaeopteryx's forearms, including its hands, were longer than its thighbones. Both of these traits are present in its modern bird descendants.
Microraptor		10. Sternum Archaeopteryx's sternum or breastbone was made of cartilage rather than bone. This would have made flight difficult for Archaeopteryx, because cartilage's elasticity would have resulted in a lot of wasted energy. Modern birds avoid this problem by having bony sternums, which both Microraptor and Deinonychus also had.
Deinonychus		11. Pubis Because they evolved longer forelimbs than their early ancestors had had, these upright-walking animals needed to also develop means to help keep their centers of balance stable over their hips. Besides long tails, they evolved a backwards-facing pubic bone, which is also found in modern birds. By becoming reverted, the pubis's weight shifted more toward the animals' tails, making up for the added weight in the arms.
Archaeopteryx		12. Brain case Based on the size and shape of Archaeopteryx's braincase, scientists believe the animal's brain was similar to that of modern birds. It was probably also well suited for flying: Impressions on Archaeopteryx's skull show that the lobes associated with processing the sensory information needed for flight were especially large. Little can be said about brain size in Deinonychus or Microraptor, for well-preserved skull fossils have yet to turn up.
Microraptor		13. Shoulder blades These animals all possessed longer, narrower shoulder blades than their common ancestors. The added room allowed them to extend their arms outward but not up as high as in birds today, which have even longer, thinner shoulder blades. This gives modern birds the freedom to flap their wings above their backs, allowing for more powerful flight and making them better fliers than Archaeopteryx likely was.