Detailed reconstruction of the proximal armhttp://eurekamag.com/research/019/478/morphological-evolution-pectoral-girdle-pterosaurs-myology-role.php (2003)
musculature of pterosaurs shows that ... the arm was more likely
lifted by large muscles anchored on the scapula and
back, and lowered by those attached to the sternum
and coracoid (fig. 5.8; Bennett 2003a). Unlike [modern] birds,
where two vastly expanded muscles are mainly used
to power flight, it appears that pterosaurs used several
muscle groups to form their flapping strokes.
The musculature of the pectoral region of representative rhamphorhynchoid (Campylognathoides) and large pterodactyloid (Anhanguera) pterosaurs was reconstructed in order to examine the function of various muscles and the functional consequences of the evolution of the advanced pectoral girdle of large pterodactyloids. The reconstructions suggest that m. supracoracoideus was not an elevator of the wing, but instead depressed and flexed the humerus. m. latissimus dorsi, m, teres major, m. deltoides scapularis, and m. scapulohumeralis anterior were wing elevators.BASAL PARAVES
Its lack of a supracoracoideus (SC)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1780066/
pulley, the primary elevator of the wing, would prevent
Archaeopteryx from executing humeral rotation on the
glenoid during the upstroke,
Anatomical evidence indicates that Microraptor was not capable of ground or running takeoff, because it lacked the supracoracoideus pulley to elevate the wings.http://jeb.biologists.org/content/200/23/2987.full.pdf
The lack of a morphologically derived SC in Late
Jurassic and Early Cretaceous birds precluded a high velocity
recovery stroke which undoubtedly limited
powered flight in these forms. Subsequent evolution of the
derived SC capable of imparting a large rotational force to
the humerus about its longitudinal axis was an important
step in the evolution of the wing upstroke and in the ability
to supinate (circumflex) the manus in early upstroke, a
movement fundamental to reducing air resistance during
the recovery stroke.
The highly derived morphology of the SC,https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CB4QFjAA&url=https%3A%2F%2Frepository.si.edu%2Fbitstream%2Fhandle%2F10088%2F6524%2FVZ_93_Archaeopteryx.pdf%3Fsequence%3D1&ei=uUF2VZyMFcqlyASSo4LYCQ&usg=AFQjCNFW5HcrkvSBGaZvOkczCMZAErCEow&sig2=0o1CnMU3Vz-DQ8_S-sw-Bw&bvm=bv.95039771,d.aWw
a characteristic of modern birds capable of powered flight, was
not present in Archaeopteryx (Ostrom, 1976a,b; Wellnhofer,
1988, 1993), nor is there firm evidence for its presence in
recently described Mesozoic species (Chiappe, 1995; Sanz et
Pterosaurs and basal paraves flapped their wings in the same way.
Basal paraves had a femur/acetabulum articulation that was different than dinosaurs and that allowed them to abduct (splay) their legs.
See also these links
"Pterosaur's hip sockets are oriented facing slightly upwards, and the head of the femur (thigh bone) is only moderately inward facing, suggesting that pterosaurs had a semi-erect stance. It would have been possible to lift the thigh into a horizontal position during flight as gliding lizards do."
BASAL PARAVESThere is a reference to the splayed posture of Scansoriopterids here (page 154):
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. 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. 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.
Riddle of the Feathered Dragons
Certainly the fact that scansoriopterids could spread the hind limbs outward in a splayed posture, more than in typical birds, indicates that a true upright stance was achieved only later and independently from true dinosaurs.
There is a reference to the splayed posture of Archaeopteryx here (page 399):
The Origin and Evolution of Birds
Microraptors have been reconstructed in two distinctive models, the four-winged gliding model with sprawled hindlimb wings, by which it was originally described in Nature (Xu et al. 2003), and a dinosaurian bipedal model, or biplane model, by which it is reconstructed with the hindlimbs held beneath the body, incapable of sprawling, in other words, like a tiny T. rex. The problem,of course, is that there is absolutely no reason the hindlimbs could not have been sprawled, as is the case in flying squirrels (Glaucomys spp.), flying lemurs (Dermoptera), etc., and even falling cats. Too, the sprawled model performs superiorly inwind-tunnel experiments (Alexander et al. 2010), most specimensare preserved with a sprawled posture, and the wingclaws are adapted for trunk climbing (Burnham et al. 2011). In addition, it would be difficult to imagine how selection could produce elongate, asymmetric hindlimb flight remiges by the most current paleontological reconstructions, in which the hindlimbs are held in flight beneath the body in obligate bipedal fashion, with elongate hindlimb wing feathers trailing behind, simply slicing through the air (Balter 2012)http://www.pnas.org/content/107/7/2972.full.pdf+html
Alexander et al
Fossils of the remarkable dromaeosaurid Microraptor gui and relatives clearly show well-developed flight feathers on the hind limbs as well as the front limbs. No modern vertebrate has hind limbs functioning as independent, fully developed wings; so, lacking a living example, little agreement exists on the functional morphology or likely flight configuration of the hindwing. Using a detailed reconstruction based on the actual skeleton of one individual, cast in the round, we developed light-weight, three-dimensional physical models and performed glide tests with anatomically reasonable hindwing configurations. Models were tested with hindwings abducted and extended laterally, as well as with a previously described biplane configuration. Although the hip joint requires the hindwing to have at least 20° of negative dihedral (anhedral),all configurations were quite stable gliders. Glide angles ranged from 3° to 21° with a mean estimated equilibrium angle of 13.7°,giving a lift to drag ratio of 4.1:1 and a lift coefficient of 0.64. The abducted hindwing model’s equilibrium glide speed corresponds to a glide speed in the living animal of 10.6m·s−1. Although the biplane model glided almost as well as the other models, it was structurally deficient and required an unlikely weight distribution (very heavy head) for stable gliding. Our model with laterally abducted hindwings represents a biologically and aerodynamically reasonable configuration for this four-winged gliding animal. M. gui’s feathered hindwings, although effective for gliding, would have seriously hampered terrestrial locomotion.
Primitively, early archosaurs are sprawling, with the legs set
laterally and elevated at around 75° (6), a preadapted posture for
Modern birds normally have the thigh elevated and sprawled to the side in different degrees; for example, it is nearly perpendicular to the midline in loons and grebes (7).
This variation shows that the degree of splaying needed to use theThey are proposing Figure A below:
hindlegs in gliding is not unusual when compared with that in
modern birds. The absence of an antitrochanter and a supraacetabular
acetabular shelf (SAC) in the eumaniraptorans, including dromaeosaurids,
would make elevation and splaying of the legs even
easier (8). Air pressure could have provided most of the force
needed to elevate the leg into a gliding position similar to that in
gliding mammals. This simple positioning was originally assumed
for the four-winged Microraptor gui (5); but later, workers hoping
to recover an upright posture proposed arrangements of the
hindlimbs that would have required complicated systems of locks
and muscles to hold the leg in an only partially elevated position,
e.g., the “biplane” model (9). New anatomical information based
on the discovery of several hundred specimens similar to the
four-winged glider M. gui (and related taxa) has produced converging
lines of evidence demonstrating that the original
describers of M. gui (5) were correct in their interpretation of the
flight posture. We postulate, based on examination of this new
material, that M. gui was capable of abducting the hind limbs at
least 65–70° to achieve a gliding posture.
Velociraptor mongoliensis had a pelvis with a characteristic pubis that pointed downward and forward at an angle toward the ischium. The acetabulum of V. mongoliensis opened dorsolaterally, indicating that it could abduct and adduct its hind limbs. This morphological characteristic demonstrates that the ancestors of V. mongoliensis were probably capable of flight and therefore the flightlessness of Velociraptor was secondarily lost (Longrich and Currie. 2009).http://www.pnas.org/content/106/13/5002.full
(Longrich and Currie. 2009)
The acetabulum is similar to those of other dromaeosaurids in that it lacks a prominent supracetabular crest (30, 36). However, anteriorly, the contribution of the ilium to the acetabulum is broad, and the anterior rim projects strongly laterally, as it does in Unenlagia(36).
The medial opening of the acetabulum is partially closed, as it is in other Dromaeosauridae (36). The [Hesperonychus] acetabulum opens dorsolaterally rather than laterally, as is the case in Velociraptor (38), suggesting the ability to partially abduct the hindlimbs. This morphology is of interest in light of proposals that Microraptor gui abducted its feathered hindlimbs to function as airfoils (24).GENERAL
That the leg winged dromaeosaurs have modified femoral heads that in some ways resemble those of some pterosaurs suggests they had evolved a different function, later lost in most secondarily flightless examples. Which is why so many specimens are splayed out spread eagle style. The hip sockets and leg musculature would not have to be fully functional when splayed out in flying dromaeosaurs because they were not using them for walking or running (many joints are not in full articulation in animals when they are not bearing full loads). (G. Paul)
Consider the following. We can see that they are assuming a dino to bird theory when concluding it could not splay its legs.
For a start, no bird or dinosaur has shown the ability to splay its legs out sideways and doing so would probably have dislocated the hip. Microraptor must have tucked its legs vertically beneath its body, like modern birds of prey do when they pounce. Its leg flight feathers would only produce lift if the leading edge faced forward, against the flow of air. The leg feathers must therefore have protruded horizontally from the tucked legs.http://blogs.scientificamerican.com/tetrapod-zoology/2013/11/18/flight-of-the-microraptor/
One debate that surrounds the aerodynamic performance of Microraptor concerns hindlimb posture. In the very first study to discuss Microraptor’s possible flight abilities, it was depicted as being capable of a full-on sprawl, its hindlimbs projecting laterally in parallel with its arms (Xu et al. 2003). This sprawling pose was also promoted in another study (Alexander et al. 2010).
Given that the form of the theropod femur and hip socket generally prevents the hindlimb from being abducted this far from the sagittal plane (there are proximally placed trochanters on the femur, and supra-acetabular shelves and antritrochanters on the ilium that prevent this sort of posture), this is surely incorrect (Brougham & Brusatte 2010). http://www.nature.com/ncomms/2013/130918/ncomms3489/full/ncomms3489.html
Very good reference on pterosaur hip structure.
A joint team from the University of Kansas and Northeastern University in China says that it has settled the long-standing question of how bird flight began.http://www.app.pan.pl/archive/published/app59/app20111109.pdf
Argument (Brougham and Brusatte)
The hypothesis that birds are theropod dinosaurs is supported by anatomical and molecular similarities, shared growth dynamics and physiology, and fossil theropods covered in feathers. A recent paper by Alexander et al. (1) and an associated commentary by Ruben (2) attempted to understand one of the greatest remaining riddles of avian evolution: the origin of flight itself.http://www.pnas.org/content/107/40/E156.full
Alexander et al. (1) examined the aerodynamic capabilities of Microraptor, a Cretaceous dromaeosaurid (avian sister taxon), by subjecting a reconstructed model to glide tests (1). They concluded that Microraptor was an arboreal glider that used all limbs as a single airfoil. We applaud the empirical approach of the study by Alexander et al.(1) and agree that Microraptor was capable of gliding. We disagree, however, with their anatomical reconstruction of Microraptor and, most importantly, with the assumption that any discovery about the habits of a single dromaeosaurid may solve the riddle of the origin of avian flight.
Alexander et al. (1) reconstructed Microraptor as a sprawling animal, with femora oriented at 140° to one another, a pelvic anatomy unlike that of other dromaeosaurids. The authors claimed that “new anatomical information,” gleaned from “examination of new material,” supported their reconstruction (1). However, this is simply asserted, with no description or illustration of this new material. Similarly, the authors argued that the lack of a supracetabular crest and an antitrochanter in Microraptor and other dromaeosaurids allows for a greater range of motion in the hind limb (1). This is incorrect: dromaeosaurids actually have enlarged antitrochanters, which limit femoral abduction (3), and although the supracetabular crest is reduced, it is still present (3). Finally, one author (J.B.) examined the cast pelvis used by Alexander et al. (1) and found that it, like all known Microraptor specimens, was crushed flat. With this in mind, it is important that the recent discovery of a 3D pelvis of a close relative, Hesperonychus (4), seems to allow only minor lateral splaying of the hind limbs.
We disagree with Ruben (2) on his presumption that different postural reconstructions of Microraptor “imply profoundly different scenarios for the origin of flight” (2). The implicit assumption is that this single species can be analyzed biomechanically, and whatever configuration glides best when launched from a catapult is the probable anatomy of the ancestor of birds. There is a clear fallacy in this reasoning: Microraptor itself cannot be an ancestor of birds, because it lived after birds had originated. It could only help understand avian flight if it retained the gliding abilities of that ancestor, which is not at all certain (5). There are nearly 40 known dromaeosaurid and troodontid dinosaurs—the closest relatives to birds. These animals exhibit a wide range in morphology, body size, integumentary covering, limb proportions, and inferred habitat. Fixation on a single derived dromaeosaurid species is not the path to understanding the origins of avian flight. We do commend Ruben (2), however, on acknowledging that an arboreal theropod dinosaur may have given rise to birds, which departs from his previous criticism of the dinosaur–bird hypothesis and is in line with the robustly supported theory that birds are living theropods.
Reply to Brougham and Brusatte
Brougham and Brusatte (1) agree with us that Microraptor was an arboreal glider but disagree with the posture of our model's hind limbs (1). They offer no suggestion for an alternative, other than the implied parasagittal posture of a typical dromaeosaur, which we showed was aerodynamically and mechanically unlikely (2). They cast doubt on the sprawled (abducted) hind-limb posture of our reconstruction—a key feature—by claiming that dromaeosaurid hips have structures (antitrochanters and supracetabular crests) that prevent abduction and that our specimen was too flattened to see such features. Whereas large dromaeosaurids may possess such structures, apparently, in small ones such as Microraptor, these structures are greatly reduced (3). The authors (1) misrepresent the small dromaeosaurid Hesperonychus as providing evidence against a sprawling posture, when the original description of Hesperonychus specifically mentions the lack of processes on the acetabulum preventing abduction and states that the “acetabulum opens dorsolaterally rather than laterally… suggesting the ability to partially abduct the hind limbs. This morphology is of interest in light of proposals that Microraptor gui abducted its feathered hind limbs to act as airfoils” (reference 4 in ref. 1). Their crushed flat claim is based on examining an incomplete cast (1). They did not examine the original fossil or its X-rays and X-ray computed tomography scans, and thus made a judgment using incomplete information. This Microraptor pelvis has been figured and described (4), and a complete 3D cast of the specimen is also available in our collections for examination. Furthermore, our pelvic morphology was checked against dozens of other specimens (as stated in ref. 2). We stand by our anatomical observations and are currently describing this specimen, along with other material, which will support the accuracy of our interpretation.
Brougham and Brusatte (1) devote a large portion of their letter to defending the dinosaurian origin of birds. We find this somewhat puzzling, because we did not address that issue in our paper. They choose to regard this feathered dromaeosaur as a derived member of the group, although most cladistic analyses show it as basal (5) or even as the primitive sister group of that clade (3). Moreover, much older taxa with large, pennaceous feathers on the lower hind limb (Pedopenna, Anchiornis) have been discovered from radiometrically dated Jurassic rocks in China (5). Anchiornis is cladistically a troodontid, suggesting that four-winged gliding is also primitive for deinonychosaurs. Brougham and Brusatte (1) suggest that Microraptor is of no relevance to understanding bird flight, because they doubt that it inherited its mode of gliding from an ancestor that it shared with birds. We think that such a shared ancestry is actually reasonable given the feathered hind limbs of Anchiornis, to which other authors have attributed an aerodynamic function (5). We never argued that Microraptor must be a direct ancestor of birds to be informative about the origin of avian flight, any more than Archaeopteryx must be ancestral to modern birds to be informative about avian origins. We think Microraptor displays a four-winged mode of gliding that it inherited from more primitive, arboreal ancestors, and we are confident that our model is anatomically reasonable.
The trees down theory begins with a gliding arboreal creature. This is closer to being correct except that the gliding phase took place much earlier in the evolution of the pterosaur. This is a key point.
Kevin Padian of the University of California at Berkeley, an expert on bird evolution, observed that the presentations focused on the effect of the hindlimb on a gliding animal instead of one that flapped its wings. Last year at the SVP meeting he presented evidence that gliders and flyers are completely unrelated to each other. He says that “there is not a shred of evidence that says gliding is involved in the evolution of flapping flight.” He questioned why the team's model would focus on gliding parameters when the forelimb shape was consistent with flapping, not gliding, and the hindlimb would have generated so much drag.http://www.economist.com/blogs/babbage/2011/11/evolution-flight
Rather, it was a challenge to the discipline's long-held belief that flapping wings, with their complicated nerve, muscle and bone structure, must, surely, have evolved from a simpler variety that allowed its owners to glide.
At first, Dr Padian and Dr Dial were expecting to find fossilised bats that were gliders, not flappers. To their surprise, they discovered nothing of the sort. They then expanded their search to all flying and gliding animals. What they found was, if anything, even more shocking: gliding and flapping fauna appear to have no direct common ancestor. This suggests that the prevailing [trees down] theory is wrong. It also, however, leaves open the question of where powered flight comes from.http://gwawinapterus.wordpress.com/2014/02/21/on-the-origins-of-flight-no-gliders/
An alternative option is what is reflected by an examination of gliding animal groups: gliding simply does not translate into flight, simply never leads to powered volancy, and that the flying vertebrates we know and love, as well as insects, have simply evolved under extremely anomalous sets of behaviour completly unrelated to gliding.
Based on birds and bats, as well as modern gliders, gliding appears to be effectively independent from flight, gliding animals seemingly unable to produce powered flight and animals that have evolved powered flight having developed it in unusual circumstances. Exceptions might exist, including possibly pterosaurs, but as it stands vertebrate flight has only been able to evolve from behaviours that promote forelimb movement, with aerodynamic speciation not leading at all to volancy.http://www.nature.com/news/ancient-bats-got-in-a-flap-over-food-1.9304
“It is a simple but amazing observation that there are no flying lineages of vertebrates with gliders as a sister group,” says evolutionary biologist Nancy Simmons at the American Museum of Natural History in New York.More:
A FANCIFUL RATIONALIZATION:
The passage below offers up a very weak rationalization for thinking that even if the trees down theory is correct the arboreal creatures could still be dinosaurs. As the writers admit, "It would require stages in the origin of flight not preserved in our current sample of extinct nonavian dinosaurs."
AMER. ZOOL., 40:486–503 (2000)
Phylogenetic Context for the Origin of Feathers1
STUART S. SUMIDA2,* AND CHRISTOPHER A. BROCHU†
The cursorial hypothesis, in its modern incarnation, is the simplest model for the origin of flight in the context of the preferred phylogenetic hypothesis. But is it a necessary corollary, as implied by some authors? We can infer the mechanism for the origin of flight from a phylogenetic tree, but this will always be a secondary inference. A strict reading of current cladograms does not necessarily reject a trees-down model of flight origins—the fossil record is incomplete, and one can always posit an unpreserved arboreal dinosaurian relative of birds. Additionally, rejection of a ground-up model for flight origins need not imply rejection of the dinosaurian hypothesis (Gauthier and Padian, 1985; Padian and Chiappe, 1998a), and some authors who derive birds from theropods explicitly prefer a trees-down model for flight origins (e.g., Chatterjee, 1999). The origin of a group, the origin of a structure, and the origin of a behavior or function are fundamentally different questions, and a cladogram primarily addresses the first. Some authors find legitimate reason to prefer a less-parsimonious scenario for a give cladogram on the basis of some sort of external evidence (e.g., Zaher and Rieppel, 1999).
One could argue that nonavian maniraptorans could climb, or that small arboreal theropods are yet to be discovered. Such a model would be much less parsimonious than the cursorial model, at least from the perspective of present phylogenetic understanding. It would require stages in the origin of flight not preserved in our current sample of extinct nonavian dinosaurs. The arboreal model is thus less parsimonious, but is not strictly falsified by the cladogram in Figure 1; an arboreal nonavian theropod may await discovery somewhere. The statement by Geist and Feduccia (2000) that cladistic analyses posit that “avian flight necessarily developed within a terrestrial context” (our emphasis) reflects a misunderstanding of how cladograms are constructed and what they can actually falsify.