Old Earth Ministries Online Dinosaur Curriculum
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Lesson 34 - Microraptor
Microraptor is a genus of small, four-winged dromaeosaurid dinosaurs. 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.
Microraptor provides important evidence about
the evolutionary relationship between birds and dinosaurs.
Microraptor had long
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.
another four-winged dromaeosaur, may also be a species of
Microraptor or a synonym
of M. zhaoianus.
Length: 3 Ft
Weight: 1 Kg (2.2 lbs)
Date Range: 120 Ma, Aptian Age, Cretaceous Period
|Microraptor fossil displayed in Hong Kong Science Museum. (Picture Source)|
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.
With adult specimens ranging 7790 centimetres long (2.533.0 ft) and with a weight
Wings and flight
Microraptor had four wings, one on both of its forelegs and hind legs. The long feathers
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.
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.
The unique wing arrangement found in Microraptor raised the question of its importance to the origin of flight in modern birdsdid 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. 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.
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 species.
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Discovery and species
The initial naming of Microraptor was controversial, because of the unusual circumstances
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.
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. 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.
In popular culture
Microraptor has appeared in a number of documentaries, books, and films. In 2008, it was the subject of a NOVA documentary, titled The Four Winged Dinosaur. The documentary focused on the controversy regarding the anatomy, flight performance, and relationships of M. gui using life-sized models built by artist Mick Ellison of the American Museum of Natural History and tested by paleontologists who have been involved with the study of the original specimens, including Xu Xing and Larry Martin. 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.
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