Microraptor

Microraptor (Greek, mīkros: "small"; Latin, raptor: "one who seizes") is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous 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 paleontologist Xu Xing in 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 pauli, another four-winged dromaeosaur, may also be a species of Microraptor or a synonym of M. zhaoianus.

Microraptor was among the most abundant non-avialan dinosaurs in its ecosystem, and is represented by more fossils than any other dromaeosaurid, with possibly over 300 fossil specimens represented across various museum collections.

Paleobiology

Wings and flight

Microraptor had four wings, one on both of 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, because the hind wings anchored to the feet of Microraptor would have hindered their ability to run on the ground. It had long pennaceous feathers on arms and hands (), with legs and feet . Toward the tail end, Microraptor was covered in shorter downy (plumulaceous) feathers, . 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.

Hind wing posture

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, 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.

Some paleontologists have doubted the biplane hypothesis, and have proposed alternate configurations. A 2010 study by Alexander et al. described the construction of a lightweight three-dimensional physical model used to perform glide tests. Using several different hind leg configurations for the model, they found that the biplane model, while not unreasonable, was structurally deficient and required a heavy-headed weight distribution for stable gliding, which they deemed unlikely. The study indicated that a laterally abducted hindwing structure represented the most biologically and aerodynamically consistent configuration for Microraptor. A further analysis by Brougham and Brusatte, however, concluded that Alexander's model reconstruction was not consistent with all of the available data on Microraptor and argued that the study was insufficient for determining a likely flight pattern for the dromaeosaur. Brougham and Brusatte criticized the anatomy of the model used by Alexander and his team, noting that the hip anatomy was not consistent with other dromaeosaurs. In most dromaeosaurids, features of the hip bone prevent the legs from splaying horizontally; instead, they are locked in a vertical position below the body. Alexander's team used a specimen of Microraptor which was crushed flat to make their model, which Brougham and Brusatte argued did not reflect its actual anatomy. Also in 2010, Alexander's team responded to these criticisms, noting that the related dromaeosaur Hesperonychus, which is known from complete hip bones preserved in three dimensions, also shows hip sockets directed partially upward, possibly allowing the legs to splay more than in other dromaeosaurs.

Ground movement

Due to the extent of the hind wings onto most of the animal's 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 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. Chatterjee and Templin did not take a strong stance on this possibility, noting that both a conventional interpretation and a tetrapteryx stage are equally possible. However, based on the presence of unusually long leg feathers in various feathered dinosaurs, Archaeopteryx, and some modern birds such as raptors, as well as the discovery of further dinosaur with long primary feathers on their feet (such as Pedopenna), the authors 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.

Based on the size of the scleral ring of the eye, Microraptor is thought to have hunted at night.

Discovery and species

Naming controversy

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 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.

Other species and specimens

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 dromaeosaurids 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. In 2010, it was reported that there were over 300 undescribed specimens attributable to Microraptor or its close relatives among the collections of several Chinese museums, though many had been altered or composited by private fossil collectors. The genus was featured prominently in the third episode of the 2006 Impossible Pictures Limited television series Prehistoric Park, which focused on the Jehol Biota. In the episode, titled "Dinobirds", Microraptor were shown flocking down from trees to feast on the worms and insects brought to the surface by the passage of giant titanosaurs.

References

External links

A model of Microraptor/Cryptovolans pauli by Boban Filipovic

Jacqui Hayes: Bird wings evolved from biplane dinosaurs COSMOS magazine

The Four-Winged Dinosaur - PBS web site for the Nova documentary

Category:Cretaceous dinosaurs Category:Dromaeosaurs Category:Dinosaurs of Asia Category:Feathered dinosaurs