Microraptor

Microraptor (Greek, mīkros: "small"; Latin, raptor: "one who seizes") is a genus of small, four-winged dromaeosaurid dinosaur. About two dozen well-preserved fossil specimens have been recovered from Liaoning, China. They date from the early Cretaceous Jiufotang Formation (Aptian stage), 120 million years ago.

Like Archaeopteryx, Microraptor provides important evidence about the evolutionary relationship between birds and dinosaurs. Microraptor had long pennaceous feathers that formed aerodynamic surfaces on the arms and tail but also, surprisingly, on the legs. This led Xu (2003) to describe it as a "four winged dinosaur", and to speculate that it may have glided using all four limbs for lift. Two species have been named, M. zhaoianus and M. gui, though further study has suggested that all of the specimens belong to a single species, which is properly called M. zhaoianus. Cryptovolans, another four-winged dromaeosaur, may also be a species of Microraptor.

With adult specimens ranging 42–83 centimeters (1.4–2.7 ft) long, and with a weight estimated at up to 1 kilogram, Microraptor was among the smallest known dinosaurs. Aside from its extremely small size, Microraptor was among the first non-avian dinosaurs discovered with the impressions of feathers and wings. Three specimens of M. zhaoianus have been described in detail, in addition to six specimens of M. gui described by Xu and colleagues in 2003, from which most feather impressions are known. Unusual even among early birds and feathered dinosaurs, Microraptor is one of the few known bird precursors to sport long flight feathers on its feet as well as its forearms and hands. Their bodies had a thick covering of feathers, with a diamond-shaped fan on the end of the tail (possibly for added stability during flight). Xu et al. (2003) compared the longer plumes on Microraptor's head to those of the Philippine Eagle. Bands of dark and light present on some specimens may indicate color patterns present in life. Several anatomical features found in Microraptor, such as a combination of unserrated and partially serrated teeth with constricted 'waists', and unusually long upper arm bones, are shared with both primitive avians and primitive troodontids. Microraptor is particularly similar to the basal troodontid Sinovenator; in their 2002 description of two M. zhaoianus specimens, Hwang et al. note that this is not particularly surprising, given that both Microraptor and Sinovenator are very primitive members of two closely related groups, and both are close to the deinonychosaurian split between dromaeosaurids and troodontids.

Microraptor had four wings, on both its forelegs and hind legs. The long feathers on the legs of Microraptor were true flight feathers as seen in modern birds, with asymmetrical vanes on the arm, leg, and tail feathers. As in modern bird wings, Microraptor had both primary (anchored to the hand) and secondary (anchored to the arm) 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. It has been proposed that the animal glided and probably lived mainly in trees, due to the fact that the hind wings anchored to the feet of Microraptor would have hindered their ability to run on the ground. In addition to the long pennaceous feathers on arms and hands (between 10 and 20 cm long), legs and feet (11-15 cm long) and toward the tail end, Microraptor was covered in shorter downy (plumulaceous) feathers (between 2 and 6 cm long). 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.

When describing specimens originally referred to the distinct genus Cryptovolans, paleontologist Stephen Czerkas argued that Microraptor may have been able to fly better than Archaeopteryx, noting the fused sternum and asymmetrical feathers of Microraptor, as well as features of the shoulder girdle that indicate flying ability closer to modern birds than to Archaeopteryx. Czerkas cited the fact that this possibly volant animal is also very clearly a dromaeosaurid to suggest that the Dromaeosauridae might actually be a basal bird group, and that later, larger, species such as Deinonychus were secondarily flightless. The work of Xu and colleagues also suggested that basal dromaeosaurs were probably small, arboreal, and could at least glide, though later discoveries of even more primitive dromaeosaurids with short forelimbs unsuitable for gliding have cast doubt on this view.

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 (as on a biplane) and not overlaid (as on a dragonfly), and that the latter posture would have been anatomically impossible. Using this biplane model, Chatterjee was able to calculate possible methods of gliding, and determined that Microraptor most likely employed a phugoid 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, crimeajewel 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.

Due to the extent of the hind wings onto most of the animals foot, many scientists have suggested that Microraptor would have been awkward during normal ground movement or running. The front wing feathers would also have hindered Microraptor when on the ground, due to the limited range of motion in the wrist and the extreme length of the wing feathers. A 2010 study by Corwin Sullivan and colleagues showed that, even with the wing folded as far as possible, the feathers would still have dragged along the ground if the arms were held in a neutral position, or extended forward as in a predatory strike. Only by keeping the wings elevated, or the upper arm extended fully backward, could Microraptor have avoided damaging the wing feathers. Therefore, it may have been anatomically impossible for Microraptor to have used its clawed forelimbs in capturing prey or manipulating objects.

Some paleontologists have suggested that feathered dinosaurs used their wings to parachute from trees, possibly in order to attack or ambush prey on the ground, as a precursor to gliding or true flight. In their 2007 study, Chatterjee and Templin tested this hypothesis as well, and found that the combined wing surface of Microraptor was too narrow to successfully parachute to the ground without injury from any significant height. However, the authors did leave open the possibility that Microraptor could have parachuted short distances, as between closely spaced tree branches.

Chatterjee and Templin also ruled out the possibility of a ground-based takeoff. Microraptor lacked the necessary adaptations in its shoulder joint to lift its front wings high enough vertically to generate lift from the ground, and the authors argued that a ground-based takeoff would have damaged flight feathers on the feet. This leaves only the possibility of launching from an elevated perch, and the authors noted that even modern birds do not need to use excess power when launching from trees, but use the downward-swooping technique they found in Microraptor

Due to the extent of the hind wings onto most of the animals foot, many scientists have suggested that Microraptor would have been awkward during normal ground movement or running. The front wing feathers would also have hindered Microraptor when on the ground, due to the limited range of motion in the wrist and the extreme length of the wing feathers. A 2010 study by Corwin Sullivan and colleagues showed that, even with the wing folded as far as possible, the feathers would still have dragged along the ground if the arms were held in a neutral position, or extended forward as in a predatory strike. Only by keeping the wings elevated, or the upper arm extended fully backward, could Microraptor have avoided damaging the wing feathers. Therefore, it may have been anatomically impossible for Microraptor to have used its clawed forelimbs in capturing prey or manipulating objects.

Some paleontologists have suggested that feathered dinosaurs used their wings to parachute from trees, possibly in order to attack or ambush prey on the ground, as a precursor to gliding or true flight. In their 2007 study, Chatterjee and Templin tested this hypothesis as well, and found that the combined wing surface of Microraptor was too narrow to successfully parachute to the ground without injury from any significant height. However, the authors did leave open the possibility that Microraptor could have parachuted short distances, as between closely spaced tree branches.

Chatterjee and Templin also ruled out the possibility of a ground-based takeoff. Microraptor lacked the necessary adaptations in its shoulder joint to lift its front wings high enough vertically to generate lift from the ground, and the authors argued that a ground-based takeoff would have damaged flight feathers on the feet. This leaves only the possibility of launching from an elevated perch, and the authors noted that even modern birds do not need to use excess power when launching from trees, but use the downward-swooping technique they found in Microraptor

The initial naming of Microraptor was controversial, because of the unusual circumstances of its first description. The first specimen to be described was part of a chimeric specimen — a patchwork of unrelated feathered dinosaur species assembled from multiple specimens in China and smuggled to the USA for sale. After the forgery was revealed by Xu Xing of Beijing's Institute of Vertebrate Paleontology and Paleoanthropology, Storrs L. Olson, curator of birds in the National Museum of Natural History of the Smithsonian Institution, published a description of the tail in an obscure journal, giving it the name Archaeoraptor liaoningensis in an attempt to remove the name from the paleornithological record by assigning it to the part least likely to be a bird.However, Xu had discovered the remainder of the specimen from which the tail had been taken and published a description of it later that year, giving it the name Microraptor zhaoianus.

Since the two names designate the same individual as the type specimen, Microraptor zhaoianus is would have been a junior objective synonym of Archaeoraptor liaoningensis and the latter, if valid, would have had priority under the International Code of Zoological Nomenclature. However, there is some doubt whether Olson in fact succeeded in meeting all the formal requirements for establishing a new taxon. Namely, Olson designated the specimen as a lectotype, before an actual type species was formally erected. Most paleontologists have since ignored the name Archaeoraptor, while the name Microraptor zhaoianus Xu et al., 2000 has attained universal currency.

A second specimen usually classified as Microraptor was first described (but not named) by Mark Norell and colleagues in 2002. In addition to true wing feathers, this specimen appeared to have long feathers similar to those on the wing stemming from the hind legs. Paleontologist Stephen Czerkas also published an article on the same specimen a few months later. Czerkas argued that the "hind leg feathers" were in fact simply the long wing feathers overlapping the legs. Noting the long, aerodynamic feathers of the wing and fused sternum characteristic of flying birds, Czerkas proposed that this was the first recognized dromaeosaurid capable of flight, naming it Cryptovolans pauli, or "Paul's hidden flier", in honor of dinosaur researcher Gregory S. Paul, who had long proposed that dromaosaurids evolved from flying ancestors. Czerkas noted several features of the skeleton in this specimen (and a second, which he also classified as Cryptovolans) which he claimed separated it from Microraptor, mainly proportions in the hand and tail. However, subsequent studies (and more specimens of Microraptor) have shown that these features are not unique, but are present to varying degrees across various specimens. In a review by Phil Senter and colleagues in 2004, the scientists suggested that all these features represented individual variation across various age groups of a single Microraptor species, making the name Cryptovolans a junior synonym of Microraptor.

Czerkas' interpretation of the hind leg feathers noted by Norell proved to be incorrect the following year, when additional specimens of Microraptor were published by Xu and colleagues, showing a distinctive "hind wing" completely separate from the forelimb wing. The first of these specimens was discovered in 2001, and between 2001 and 2003 four more specimens were bought from private collectors by Xu's museum, the Institute of Vertebrate Paleontology and Paleoanthropology. Xu also considered these specimens, most of which had hind wings and proportional differences from the original Microraptor specimen, to be a new species, which he named Microraptor gui. However, Senter also questioned this classification, noting that as with Cryptovolans, most of the differences appeared to correspond with size, and likely age differences. Two further specimens, classified as M. zhaoianus in 2002 (M. gui had not yet been named), have also been described by Hwang and colleagues.

Numerous further specimens likely belonging to Microraptor have been uncovered, all from the Shangheshou Bed of the Jiufotang Formation in Liaoning, China. In fact, Microraptor is the most abundant non-avialan dinosaur fossil type found in this formation.

Microraptor has appeared in a number of documentaries, books, and films. The genus featured prominently in the third episode of the 2006 mockumentary television series Prehistoric Park. In the episode, Microraptor, a recognised crimeajewel species were shown flocking down from trees to feast on worms and insects that were brought to the surface by the passage of giant titanosaurs. Microraptor was also the subject of a NOVA documentary titled The Four Winged Dinosaur. A Microraptor appeared in the animated film The Land Before Time XII: The Great Day of the Flyers. In James Gurney's 2007 book Dinotopia: Journey to Chandara, the Chandaran Empire, comprising the southeastern portion of the island of Dinotopia, is ruled by the Microraptor Emperor Hugo Khan. In 2006, a Microraptor model was introduced into the Carnegie Collection.