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Last updated on April 18, 2014 at 6:03 EDT

Microraptor

Microraptor, meaning “small thief,” is a genus of dromaeosaurid dinosaur from the Early Cretaceous Period (120 million years ago). This small, four-winged animal was first discovered in the Jiufotang Formation in Liaoning, China, with more than two dozen specimens unearthed.

There are two known species of Microraptor. The type species, M. zhaoianus, has been hotly debated for years. It was initially placed in the genus Archaeoraptor before a more accurate description placed it in the Microraptor genus in 2000. The secondary species, M. gui, has been suggested as a synonym of M. zhaoianus by some scientists.

Microraptor provides important evidence about the evolutionary relationship between birds and dinosaurs. Microraptor had long vaned (pennaceous) feathers that formed aerodynamic surfaces on the arms, tail and legs. This led paleontologist Xu Xing to describe it as a “four-winged dinosaur” in 2003. He speculated that it may have glided using all four limbs for lift.

Microraptor was among the most abundant non-avian dinosaurs in its ecosystem, and is represented by more fossils than any other dromaeosaurid, with over 300 fossils represented across various museums.

Microraptor was among the smallest known dinosaurs. An adult ranged in size from 2.5 to 3 feet tall with an estimated weight of 2.2 pounds. Aside from its small stature, Microraptor was among the first non-avian dinosaurs discovered with impressions of feathers and wings.

Unusual even among early birds and feathered dinosaurs, Microraptor is one of the few known bird ancestors to sport long flight feathers on its feet as well as its forearms and hands. Its body had a thick covering of feathers, with a diamond-shaped fan on the end of the tail — possibly for added stability in flight.

It had four wings, one on both of its forelimbs and hindlimbs. The long feathers on the legs were true flight feathers as seen in modern birds, with asymmetrical vanes on the arm, leg and tail feathers. Microraptor, as with modern birds, had both primary and secondary flight feathers. This standard wing pattern was mirrored on the hind legs, with flight feathers anchored to the upper foot bones as well as the upper and lower leg.

Research suggests that the animal probably glided and lived primarily in trees, because the hind wings, which were anchored to the feet, would have hindered its ability to run on the ground. It was covered with shorter downy feathers toward the end of the tail. Though not apparent in most fossils under natural light, due to obstruction from decayed soft tissue, the feather bases extended close to or in contact with the bones, as in modern birds, providing strong anchor points.

Paleontologist Sankar Chatterjee determined in 2005 that, in order for Microraptor to glide or fly, the fore and hind wings must have been on different levels and not overlaid, and that the latter posture would have been anatomically impossible. Using a biplane model, Chatterjee was able to calculate possible methods of gliding, and determined that Microraptor most likely employed a fugoid style of gliding: launching itself from a perch, the animal would have swooped downward in a deep U-shaped curve and then lifted again to land on another tree.

The feathers not directly employed in the biplane wing structure, like those on the tibia and the tail, could have been used to control drag and alter the flight path, trajectory, etc. The orientation of the hind wings would also have helped the animal control its gliding flight. Chatterjee also used computer algorithms that test animal flight capacity to test whether or not Microraptor was capable of true, powered flight, in addition to passive gliding.

The resulting data showed that Microraptor did have the requirements to sustain level powered flight, so it is theoretically possible that the animal flew on occasion in addition to gliding.

The biplane hypothesis has been doubted by some paleontologists, who have proposed alternate configurations, which have been widely disputed.

Microraptor’s unique wing arrangement raised questions of the importance it had on flight origins of modern birds. Scientists wondered if avian flight went through a four-winged stage, or were four-winged gliders like Microraptor an evolutionary side-branch that did not evolve?

As early as 1915, naturalist William Beebe argued that the evolution of bird flight may have gone through a four-winged stage. This possibility was not widely accepted at the time. But based on the presence of unusually long leg feathers in various feathered dinosaurs and some modern birds such as raptors, some argued that the current body of evidence, both from morphology and phylogeny, suggests that bird flight did shift at some point from shared limb dominance to front-limb dominance, and that all modern birds may have evolved from four-winged ancestors, or at least ancestors with unusually long leg feathers relative to the modern configuration.

Microraptor has appeared in documentaries, books and film. It was the subject of a 2008 NOVA documentary, titled “The Four Winged Dinosaur.” It was also featured in the third episode of the 2006 Impossible Pictures Limited TV series Prehistoric Park.

Microraptor