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Microraptor
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===Hindwing posture=== [[File:Microraptor models.png|thumb|Wind tunnel experiments with different wing configurations]] [[Sankar Chatterjee]] suggested in 2005 that, in order for ''Microraptor'' to [[Gliding flight|glide]] or fly, the forewings and hindwings 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 [[Airway (aviation)|flight path]], [[trajectory]], etc. The orientation of the hindwings would also have helped the animal control its gliding flight. Chatterjee also used computer [[algorithm]]s that test [[Flying and gliding animals|animal flight]] capacity to test whether or not ''Microraptor'' was capable of true, powered flight, as opposed to or 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, as opposed to gliding.<ref name="chatterjee2007">{{cite journal | last1 = Chatterjee | first1 = S. | last2 = Templin | first2 = R.J. | year = 2007 | title = Biplane wing planform and flight performance of the feathered dinosaur ''Microraptor gui'' | url = http://www.pnas.org/cgi/reprint/0609975104v1.pdf | journal = Proceedings of the National Academy of Sciences | volume = 104 | issue = 5| pages = 1576β1580 | doi=10.1073/pnas.0609975104 | pmid=17242354 | pmc=1780066| bibcode = 2007PNAS..104.1576C | doi-access = free }}</ref> 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 hindleg 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''.<ref name=alexanderetal2010>{{cite journal | last1 = Alexander | first1 = D.E. | last2 = Gong | first2 = E. | last3 = Martin | first3 = L.D. | last4 = Burnham | first4 = D.A. | last5 = Falk | first5 = A.R. | year = 2010 | title = Model tests of gliding with different hindwing configurations in the four-winged dromaeosaurid ''Microraptor gui'' | journal = Proceedings of the National Academy of Sciences, USA | volume = 107 | issue = 7| pages = 2972β2976 | doi = 10.1073/pnas.0911852107 | pmid = 20133792 | bibcode = 2010PNAS..107.2972A | pmc = 2840342 | doi-access = free }}</ref> 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.<ref>{{cite journal | last1 = Brusatte | first1 = Stephen L. | last2 = Brougham | first2 = Jason | year = 2010| title = Distorted ''Microraptor'' specimen is not ideal for understanding the origin of avian flight | journal = Proceedings of the National Academy of Sciences, USA | volume = 107| issue = 40| pages = E155| doi = 10.1073/pnas.1004977107 | pmid = 20864633 | bibcode = 2010PNAS..107E.155B| pmc = 2951411| doi-access = free }}</ref> 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.<ref>{{cite journal | last1 = Alexander | first1 = D.E. | last2 = Gong | first2 = E. | last3 = Martin | first3 = L.D. | last4 = Burnham | first4 = D.A. | last5 = Falk | first5 = A.R. | year = 2010 | title = Reply to Brougham and Brusatte: Overall anatomy confirms posture and flight model offers insight into the evolution of bird flight | journal = Proceedings of the National Academy of Sciences, USA | volume = 107| issue = 40| pages = E155| doi = 10.1073/pnas.1004977107 | pmid = 20864633 | bibcode = 2010PNAS..107E.155B | pmc = 2951411 | doi-access = free }}</ref> However, Hartman and colleagues suggested that ''Hesperonychus'' is not a dromaeosaur, but actually an [[Avialae|avialan]] close to modern [[bird]]s like ''[[Balaur bondoc]]'' based on phylogenetic analyses in 2019.<ref name=H19>{{Cite journal |last1=Hartman |first1=Scott |last2=Mortimer |first2=Mickey |last3=Wahl |first3=William R. |last4=Lomax |first4=Dean R. |last5=Lippincott |first5=Jessica |last6=Lovelace |first6=David M. |date=2019-07-10 |title=A new paravian dinosaur from the Late Jurassic of North America supports a late acquisition of avian flight |journal=PeerJ |language=en |volume=7 |pages=e7247 |doi=10.7717/peerj.7247 |issn=2167-8359 |pmc=6626525 |pmid=31333906 |doi-access=free }}</ref>
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