Microraptor

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Microraptor
Temporal range: Early Cretaceous, 125–120Ma
Fossil specimen, with white arrows pointing at preserved feathers
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Theropoda
Family: Dromaeosauridae
Clade: Microraptoria
Genus: Microraptor
Xu et al., 2000
Type species
Microraptor zhaoianus
Xu et al., 2000
Species
  • M. zhaoianus Xu et al., 2000
  • M. gui Xu et al., 2003
  • M. hanqingi Gong et al., 2012
Synonyms
  • Cryptovolans Czerkas et al., 2002

Microraptor (Greek, μίκρος, mīkros: "small"; Latin, raptor: "one who seizes") was a genus of small, four-winged paravian (possibly dromaeosaurid) dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China. They date from the early Cretaceous Jiufotang Formation (Aptian stage), 125 to 120 million years ago.[1] Three species have been named (M. zhaoianus, M. gui, and M. hanqingi), though further study has suggested that all of them represent variation in a single species, which is properly called M. zhaoianus. Cryptovolans, initially described as another four-winged dinosaur, is usually considered to be a synonym of Microraptor.[2]

Like Archaeopteryx, well-preserved fossils of Microraptor provide 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 the first specimen to preserve this feature as a "four-winged dinosaur" and to speculate that it may have glided using all four limbs for lift. Subsequent studies have suggested that it is possible Microraptor were capable of powered flight as well.

Microraptor were among the most abundant non-avian dinosaurs in their ecosystem, and the genus is represented by more fossils than any other dromaeosaurid, with possibly over 300 fossil specimens represented across various museum collections.[3]

Description[edit]

Wingspan & body size compared with human. Scale bar: 1 m (3.3 ft).

With adult specimens ranging 77–90 centimetres long (2.53–2.95 ft) and with a weight estimated up to 1 kilogram (2.2 lb), according to Holtz (2008), Microraptor was among the smallest known non-avian dinosaurs.[4][5] A longer estimate, made by Benson et al. in 2012 found that Microraptor had a maximum length of 1.2 metres (3.9 feet).[6] Aside from their extremely small size, Microraptor were among the first non-avialan dinosaurs discovered with the impressions of feathers and wings. Seven specimens of M. zhaoianus have been described in detail, 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 the legs as well as the wings. 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,[7] though at least some individuals almost certainly possessed an iridescent black coloration.[8] 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.[7]

Paleobiology[edit]

Wings and flight[edit]

M. gui holotype under two different UV light filters, revealing extent of preserved feathers and soft tissue

Microraptor had four wings, one on each 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.[9] It had long pennaceous feathers on arms and hands (10–20 centimetres long or 3.9–7.9 in), with legs and feet 11–15 cm long (4.3–5.9 in). Toward the tail end, Microraptor was covered in shorter downy (plumulaceous) feathers, 2–6 cm long (0.79–2.36 in). 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.[10]

Wind tunnel experiments with different wing configurations

When describing specimens originally referred to the distinct species Cryptovolans pauli, 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.[9][11]

Whether or not Microraptor could achieve powered flight or only passive gliding has been controversial. While most researchers have agreed that Microraptor had most of the anatomical characteristics expected in a flying animal, some studies have suggested that the shoulder joint was too primitive to have allowed flapping. The ancestral anatomy of theropod dinosaurs has the shoulder socket facing downward and slightly backward, making it impossible for the animals to raise their arms vertically, a prerequisite for the flapping flight stroke in birds. Some studies of maniraptoran anatomy have suggested that the shoulder socket did not shift into the bird-like position of a high, upward orientation close to the vertebral column until relatively advanced avialans like the enantiornithes appeared.[12] However, other scientists have argued that the shoulder girdle in some paravian theropods, including Microraptor, is curved in such a way that the shoulder joint could only have been positioned high on the back, allowing for a nearly vertical upstroke of the wing. This possibly advanced shoulder anatomy, combined with the presence of a propatagium linking the wrist to the shoulder (which fills the space in front of the flexed wing and may support the wing against drag in modern birds) and an alula or "bastard wing" may indicate that Microraptor was capable of true, powered flight.[13] Wind tunnel experiments conducted by Dyke et al. (2013) demonstrated that that sustaining a high-lift coefficient at the expense of high drag was likely the most efficient strategy for Microraptor when gliding between low elevations. Microraptor did not require a sophisticated, ‘modern’ wing morphology to be an effective glider. This further supports the hypothesis that "flight feathers" that first evolved in dinosaurs for non-aerodynamic functions were later adapted to form lifting surfaces.[14]

Hind wing posture[edit]

Restoration of Microraptor while resting

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.[5]

Some paleontologists have doubted the biplane hypothesis, and have proposed other 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 hind leg configurations for the model, they found that the biplane model, while not unreasonable, was structurally deficient and needed 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.[3] 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 Microraptor. 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.[15] Later 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.[16]

Ground movement[edit]

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.[17]

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.[6] 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.[5][6]

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.[5]

Implications[edit]

William Beebe's hypothetical "Tetrapteryx" with four wings, 1915

The unique wing arrangement found in Microraptor raised the question of its importance to the origin of flight in modern birds—did avian flight go through a four-winged stage, or were four-winged gliders like Microraptor an evolutionary side-branch that did not leave descendants? As early as 1915, naturalist William Beebe had argued that the evolution of bird flight may have gone through a four-winged (or tetrapteryx) stage.[18] 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.[5]

Feeding[edit]

In 2010 researchers announced that further preparation of the type fossil of M. zhaoianus revealed preserved probable gut contents. These consisted of mammalian bones, including possible skull, limb, and vertebral fragments and also a whole foot. The foot skeleton is similar to those of Eomaia and Sinodelphys. It corresponds to an animal with an estimated snout to vent length of 80mm and a mass of 20-25 grams. The unguals of the foot are less curved than in Eomaia or Sinodelphys, indicating that the mammal could climb but less effectively than in the two latter genera.[19] Other than mammals, Microraptor ate lizards,[6] like Xianglong.[1]

In the December 6, 2011 issue of Proceedings of the National Academy of Sciences, Jingmai O'Connor and coauthors described a Microraptor specimen containing bird bones in its abdomen, specifically a partial wing and feet. Their position indicate that the dinosaur swallowed a tree-perching bird whole.[20]

In 2013 researchers announced that they had found fish scales in the abdominal cavity of a Microraptor specimen.[21] This new finding corrects the previous position that Microraptor hunted only in an arboreal environment. They also argued that the specimen showed a probable adaptation to a fish-eating diet: the good preservation of the mandible shows that the first three teeth were inclined anterodorsally, a character often associated with piscivory.[21] Microraptor was an opportunistic feeder, hunting the most common prey in both arboreal and aquatic habitats.[21]

Based on the size of the scleral ring of the eye, Microraptor is thought to have hunted at night.[22] However, the discovery of iridescent plumage in Microraptor has cast doubt on this conclusion, as no modern birds that have iridescent plumage are known to be nocturnal.[8]

Coloration[edit]

In March 2012, Quanguo Li and team became the first scientists to determine the plumage coloration of Microraptor, based on the new specimen BMNHC PH881, which showed several other features previously unknown in Microraptor. By analyzing the fossilized melanosomes (pigment cells) in the fossil with scanning electron microscope techniques, the researchers compared their arrangements to those of modern birds. In Microraptor, these cells were shaped in a manner consistent with black, glossy coloration in modern birds. These rod-shaped, narrow melanosomes were arranged in stacked layers, much like those of a modern blackbird, and indicated iridescence in the plumage of Microraptor. Though the researchers state that the true function of the iridescence is yet unknown, it seems reasonable to conclude that the tiny dromaeosaur was using its glossy coat as a form of communication or sexual display, much as in modern iridescent birds.[8][23]

History[edit]

Naming controversy[edit]

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 different 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.[24] 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.[25]

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.[26]

Additional specimens[edit]

Skeletal restorations of various specimens

The first specimen referred to Microraptor represented a small individual and included faint feather remnants, but was otherwise not well preserved and lacked a skull. In 2002 Mark Norell et al. described another specimen, BPM 1 3-13, which they did not name or refer to an existing species.[27] Later that year Stephen Czerkas et al. named the specimen Cryptovolans pauli, and referred two additional specimens (the first to show well-preserved feathers) to this species. The generic name was derived from Greek kryptos, "hidden", and Latin volans, "flying". The specific name, pauli, honors paleontologist Gregory S. Paul, who had long proposed that dromaeosaurids evolved from flying ancestors.[28]

The type specimens of C. pauli were collected from the Jiufotang Formation, dating from the early Albian and now belong to the collection of the Paleontology Museum of Beipiao, in Liaoning, China. They are referred to by the inventory numbers LPM 0200, the holotype; LPM 0201, its counterslab (slab and counterslab together represent the earlier BPM 1 3-13); and the paratype LPM 0159, a smaller skeleton. Both individuals are preserved as articulated compression fossils; they are reasonably complete but partially damaged.[28]

Specimen in Shandong Tianyu Museum of Nature

Czerkas et al. (2002) diagnosed the genus on the basis of having primary feathers (which in the authors' opinion made it a bird), a co-ossified sternum, a tail consisting of 28-30 vertebrae and a third finger with a short phalanx III-3.[28] Some of the feathers Czerkas described as primary were actually attached to the leg, rather than the arm. This, along with most of the other diagnostic characters, is also present in the genus Microraptor, which was first described earlier than Cryptovolans.[29] However, BPM 1 3-13 has a longer tail, proportionately, than other Microraptor specimens that had been described by 2002, which have 24-26 tail vertebrae.[27]

Subsequent studies (and more specimens of Microraptor) have shown that the features used to distinguish Cryptovolans 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 pauli and Microraptor gui junior synonyms of Microraptor zhaoianus.[2] Many other researchers, including Alan Feduccia and Tom Holtz, have since supported its synonymy.[4][30]

A new specimen of Microraptor, BMNHC PH881, showed several features previously unknown in the animal, including the probably glossy-black iridescent plumage coloration. The new specimen also featured a bifurcated tailfan, similar in shape to previously known Microraptor tailfans except sporting a pair of long, narrow feathers at the center of the fan. The new specimen also showed no sign of the nuchal crest, indicating that the crest inferred from the holotype specimen may be an artifact of taphonomic distortion.[8][23]

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.[31] 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.[3]

Study and debate[edit]

Fossil specimen

Norell et al. (2002) described BPM 1 3-13 as the first dinosaur known to have flight feathers on its legs as well as on its arms.[32]

Czerkas (2002) mistakenly described the fossil as having no long feathers on its legs, but only on its hands and arms, as he illustrated on the cover of his book Feathered Dinosaurs and the Origin of Flight.[28] In his discussion of Cryptovolans in this book, Czerkas strongly denounces Norell's conclusions; "The misinterpretation of the primary wing feathers as being from the hind legs stems directly to [sic] seeing what one believes and wants to see".[28] Czerkas also denounced Norell for failing to conclude that dromaeosaurs are birds, accusing him of succumbing to "...the blinding influences of preconceived ideas."[28] The crown group definition of Aves, as a subset of Avialae, the explicit definition of the term "bird" that Norell employs, would definitely exclude BPM 1 3-13. However, he does not consider the specimen to belong to Avialae either (see Avialae).[32]

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.[2] Two further specimens, classified as M. zhaoianus in 2002 (M. gui had not yet been named), have also been described by Hwang and colleagues.[7]

Czerkas also believed that the animal may have been able to fly better than Archaeopteryx, the animal usually referred to as the earliest known bird. He cited the fused sternum and asymmetrical feathers, and argued that Microraptor has modern bird features that make it more derived than 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 (Czerkas, 2002). The current consensus is that there is not enough evidence to conclude whether dromaeosaurs descended from an ancestor with some aerodynamic abilities. The work of Xu et al. (2003) suggested that basal dromaeosaurs were probably small, arboreal, and could glide.[33] The work of Turner et al. (2007) suggested that the ancestral dromaeosaur could not glide or fly, but that there was good evidence that it was small-bodied (around 65 centimeters long and 600-700 grams in mass).[11]

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