Friday, November 21, 2014


Comparison of basal pterosaur, basal paraves and basal coelurosaur. This is a work in progress.

Basal Pterosaur: Rhamphorhynchidae
Basal Paraves: Scansoriopterygidae
Basal Coelurosaur: Proceratosauridae
(or Compsognathoidea)


Back1Notarium: absent (0) present (1)0?0
Breathing2Respiratory air sacs: absent (0) present (1)11x0
3Breathing pump: absent (0) present (1)11x0
4Rib lever processes: absent (0) present (1)11x0
Chest5Keeled breastbone: absent (0) present (1) 11x0
6Furcula (wishbone): absent (0) present (1)00?
Feather7Stage 2 feathers: absent (0) present (1)11x0
8Pennaceous feathers: absent (0) present (1)0x1x0
Foot9Hyperextended second toe: absent (0) present (1)0x1x0
10Hinge-like ankle joint: absent (0) present (1)111
Leg11Thigh bone held: horizontal (0) not horizontal (1)111
Pelvis12Pubic bone: pointing to back (0) to front (1) downward (2)11
13Pubic bones: not fused (0) fused (1)00x1
14Acetabulum: not perforated (0) partially perforated (1) completely perforated (2)?1x2
15Pelvic bones: not fused (0) fused (1)11x0
Skull16Beak like jaw: absent (0) present (1)1?0
17Teeth: absent (0) present (1)111
18Crest: absent (0) present (1) 111
19Neck attaches to skull; from rear (0) from below (1)000
20Serrated teeth: absent (0) present (1)00x1
21Semicircular canals:  expanded (0) not expanded (1)???
Tail22Caudal vertebrae: less than 15 (0) greater than 15 (1)111
23Caudal rods: absent (0) present (1)11x0
24Muscle mass of M. caudofemoralis longus: small (0) large (1)00x1
Wing25Strap-like scapula: absent (0) present (1)11x0
26Scapula orientation to backbone: subparallel  (0) parallel (1)11x0
27Glenoid fossa: elevated (0) not elevated (1)00x1
28Propatagium: absent (0) present (1)11x0
29Patagium: absent (0) present (1) 11x0
30Wing membrane: absent (0) present (1)1x00
31Elongated 4th finger: absent (0) present (1)1
32Number of fingers: 2 fingers (2) 3 fingers (3) 4  fingers (4)4x32/3
33Pteroid bone: absent (0) present (1)1x00
34Capable of flapping flight: absent (0) present (1)11x0
Wrist35Semilunate carpal: absent (0) present (1)0x1x0
36Proximal carpals: not fused (0) fused (1)1??
37Distal carpals: not fused (0) fused (1)1??
38Carpometacarpus: absent (0) present(1)0x1x0
39Angle of abduction:  less than 25% (0) greater than 25% (1)??0
General40Warm blooded: absent (0) present (1)11?
41Neural flight control system: absent (0) present (1)1?0
42Pneumatic bones: absent (0) present (1)11x0

x = Difference

Sunday, November 9, 2014

Shoulder Joint


From the article on page 267 (by Frey et al.):
As in birds, the glenoid fossa in most pterosaurs is elevated by a dorsolaterally directed elongation of the coracoid and lies almost level with the vertebral column
Among living tetrapods, birds are unique in having completely separated the locomotor functions of fore and hindlimbs. The propulsive excursions of the forelimbs, which primarily involve elevation and depression in a transverse plane, differ fundamentally from those of most other tetrapods (pterosaurs and bats excepted) in which the forelimbs protract and retract in anteroposterior planes.
Pterosaurs and birds present a number of striking parallelisms in the structure of their flight apparatus and the glenoid is yet another example of their independent derivation of similar features.
In both rhamphorhynchoid and pterodactyloid pterosaurs the glenoid is distinctly saddle shaped with laterally as well as dorsally facing regions of the articular surface.
The origin of the pterosaurian glenoid must have involved the same evolutionary migration of position and orientation that has been outlined here for the avian lineage.
In contrast to the bulbous humeral head of birds, however, the humerus of pterosaurs bears a saddle-shaped facet, thus constraining the wingbeat excursion. This difference is likely a reflection of the relative structural versatility of the two wing types: an outstretched, sail-like membrane supported principally by a single digit versus a flexible airfoil composed of individual feathers, each with its own structural and functional integrity.
The [pterodactyl pterosaur] coracoid is about 75 per cent of the length of the scapula. It is expanded at its contact with the scapula, but has a more gentle decrease in width over its length. A small, blunt coracoid process is present, but it is not possible to tell if a groove separates it from the glenoid fossa. The sternal articulation is concave, faces posteroventrally, and lacks a posterior expansion. A large glenoid fossa faces anterodorsally with a dorsoventrally concave and anteroposteriorly convex saddle shape.
Wing skeleton. Both [pterodactyl pterosaur] wings are present in NGMC 99-07-1 (Text-figs 2, 4; Table 2). The humeri are complete though the right deltopectoral crest has become detached and rotated from its anatomical position (Text-fig. 2). The humeral head has an anteroposteriorly concave and dorsoventrally convex, saddle-shaped articulation so that it mirrors the shape of the glenoid.
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] 

It is not an easy task to get all the needed information about the shoulder joint but this is how it appears:
Rhamphoryncidae had a saddle joint. Both the glenoid fossa and the humerus head were saddle-shaped.
Basal paraves had a half-saddle joint. The glenoid fossa was still saddle shaped but the humerus head was bulbous.

General form of a full saddle joint:

PTEROSAUR scapula, coracoid and glenoid
Some pterosaur bones are quite unusual. This scapulo-coracoid is photographed from both sides. The glenoid cavity of the shoulder joint can be seen, where the humerus articulates the wing to the body.
 A large [pterodactyl] glenoid fossa faces anterodorsally with a dorsoventrally concave and anteroposteriorly convex saddle shape.
The lower (right?) coracoid doesn't seem to be articulated with the scapula either, with its narrow proximal neck lower than and partially overlapping the base of the scapula. I would argue the coracoid would be bent so that the neck is in a different plane from the oval distal end, and this makes the C-shape we see in the Epidendrosaurus and Scansoriopteryx holotypes. The glenoid may be oriented ventrally or laterally- I cannot discern it.
On the other hand:
It is important to mention that scansoriopterygids retained a caudoventrally oriented glenoid,a subrectangular coracoid with reduced biceps tubercle, and a distally fan-shaped scapular blade, all representing plesiomorphic character states in respect to paravians.

DINOSAUR scapula, coracoid, glenoid and tiny arm.
Scapula orientation in theropod dinosaurs is quite interesting and it is worth looking, to begin with, at what orientation is displayed in primitive reptiles. The scapula is generally held at an angle of 90 degrees to the horizontal line held by the backbone – in other words it was held in a perpendicular fashion. At the other extreme, extant birds rotated the scapula so that it lies parallel to backbone – a position also evolved by the pterosaurs.

Theropods, and non-avian dinosaurs in general (but not bird-like theropods), evolved a condition that can be described as something in between – an intermediate position if you will. The scapula is held in an oblique position laterally to the ribcage but actually determining the exact position is somewhat problematic. There are not that many fully articulated specimens that can be referred to and there is always the spectre of both taxanomic and taphonomic variation to throw yet another spanner into the works.

Here is a very interesting video:
Agnolín and Novas. 2013. Avian ancestors
In this way, the scapulae of unenlagiids lie close to the vertebral column, dorsal to the ribcage, with the flat costal surface of the scapular blade facing ventrally, a condition seen in microraptorans (i.e. Microraptor), basal avialans (e.g. Archaeopteryx, Rahonavis), and ornithothoracine birds (Senter 2006), in which the shoulder socket sits high on the back, and the margins of the glenoid are smooth, thus this surface becomes shalower and consequently more continuous with the rest of the lateral surface of scapula
(Burnham 2008). In sum, the lateral orientation of the scapular glenoid in unenlagiids
(and probably also in other basal averaptorans), together with the absence
of acute ridges delimitating the glenoid cavity, suggest that the humerus in these
taxa was able to be elevated close to the vertical plane, 
taxa was able to be elevated close to the vertical plane, as proposed by Novas and Puerta (1997) (Figs. 5.1, 5.2).
It is important to mention that scansoriopterygids retained a caudoventrally oriented glenoid, a subrectangular coracoid with reduced biceps tubercle, and a distally fan-shaped scapular blade, all representing plesiomorphic character states in respect to paravians.

Here is a good overview of the shoulder girdle of modern birds:

Friday, October 31, 2014

Dino to bird claims

What we see again and again is that there is no actual link between ground-based coelurosaur dinosaurs and arboreal paravians. They inhabit different niches (obviously) with no link between them. And even more importantly, they do not share characteristics. Almost all (if not all) the bird-like characteristics that are found in the paravians are not found in the ground-based coelurosaur dinosaurs. That is because they are not related.

So the question arises:
How in the world could there be so many claims for years and years that birds evolved from dinosaurs? 
In addition to the points noted above:

First, is the misleading convention of calling paraves "dinosaurs". So any bird-like character found in paraves is said to confirm the dino to bird theory. But paraves are not dinosaurs, they did not evolve from dinosaurs. People focus on the wrong place. The Achilles Heel of the dino to bird theory is that there is no connection between actual dinosaurs and paraves.

Next is to misinterpret the characters of actual dinosaurs as if they were bird-like or "proto" bird-like characters. Thus for example, we get the claim of "protofeathers" on ground-based coelurosaur dinosaurs, which does not stand up. 

And also we get secondarily flightless paravians being called "non-paraves maniraptors". As if they were transitional between actual dinosaurs and arboreal paravians. That does not stand up. They are secondarily flightless members of paraves.

Also the cladistic analyses that have been done, generally include only dinosaurs and use an inappropriate outgroup. The very significant exception to this is the James and Pourtless study, which not co-incidentally found other explanations as credible as the dino to bird theory. 

Thursday, October 30, 2014


In the dino to bird theory, there is a good deal of claimed exaptation.
Feathers, enlarged brain and abducted wrists are claimed to have evolved before they were used for flight. These are simply stories. These stories are made up in response to evidence that contradicts the dino to bird theory.
Exaptation and the related term co-option describe a shift in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another.

Carpal asymmetry [abducted wrists] would have permitted avian-like folding of the forelimb in order to protect the plumage, an early advantage of the flexible, asymmetric wrist inherited by birds.
However, it is likely that mobility of the wrist was initially associated with other functions, such as predation (Padian 2001).
It had originally been proposed that this flexibility could be attributed to hunting, but the same changes are seen in maniraptorans that were herbivores and omnivores so it is unlikely that hunting provides the answer. Instead, the authors of the new study propose, the ability to fold the hands backwards would have protected the feathers of the arms. This would have prevented the feathers from getting damaged or from being in the way as the dinosaurs moved about, although the authors recognize that this hypothesis requires further evidence.
Perhaps more significant, however, is how this wing-folding mechanism may have allowed birds to take to the air. Birds do flex their wrists while flapping their wings to fly, and so it appears that the wrist flexibility that first evolved in dinosaurs was later co-opted for flight in birds. This is what is known as "exaptation," or when a previous adaptation takes on a new function. Indeed, as more is discovered about the evolution of birds, the more traits paleontologists find that evolved for one function but have been co-opted for another at a later point (feathers themselves being the most prominent
example). There is relatively little separating birds from their feathered dinosaur ancestors.

As Darwin elaborated in the last edition of The Origin of Species,[14] many complex traits evolved from earlier traits that had served different functions. By trapping air, primitive wings would have enabled birds to efficiently regulate their temperature, in part, by lifting up their feathers when too warm. Individual animals with more of this functionality would more successfully survive and reproduce, resulting in the proliferation and intensification of the trait.
Eventually, feathers became sufficiently large to enable some individuals to glide. These individuals would in turn more successfully survive and reproduce, resulting in the spread of this trait because it served a second and still more beneficial function: that of locomotion. Hence, the evolution of bird wings can be explained by a shifting in function from the regulation of temperature to flight.
Exaptation is a term used in evolutionary biology to describe a trait that has been co-opted for a use other than the one for which natural selection has built it.
It is a relatively new term,proposed by Stephen Jay Gould and Elisabeth Vrba in 1982 to make the point that a trait’s current use does not necessarily explain its historical origin. They proposed exaptation as a counterpart to the concept of adaptation.
For example, the earliest feathers belonged to dinosaurs not capable of flight. So, they must have first evolved for something else. Researchers have speculated early feathers may have been used for attracting mates or keeping warm. But later on, feathers became essential for modern birds’ flight.

Several ancient dinosaurs evolved the brainpower needed for flight long before they could take to the skies, scientists say.
Bird brains tend to be more enlarged compared to their body size than reptiles, vital for providing the vision and coordination needed for flight.
Scientists using high-resolution CT scans have now found that these "hyper-inflated" brains were present in many ancient dinosaurs, and had the neurological hardwiring needed to take to the skies. This included several bird-like oviraptorosaurs and the troodontids Zanabazar junior, which had larger brains relative to body size than that of Archaeopteryx.

See page 261 of "Riddle of the Feathered Dragons"

Did preadaptations for flight precede the origin of birds (Aves)? The origin of flight in birds is one of the great evolutionary transitions and has received considerable attention in recent years (Padian and Chiappe 1998; Clarke and Middleton 2008; Dececchi and Larrson 2009; Benson and Choiniere 2013; Dececchi and Larrson 2013). The evolution of birds is often considered coincident with the origins of flight, but many traits associated with flight evolved before the origin of Aves (Padian and Chiappe 1998).

Wednesday, October 29, 2014

Not required to become terrestrial

Here is a possible objection to the pterosaur to bird theory:
To derive birds from pterosaurs would require some pterosaur to give rise to terrestrial maniraptors, because non-paraves maniraptorans are terrestrial, and the bird lineage evolved within the maniraptorans
It must first be recognized that the so-called "non-paraves maniraptorans" are secondarily flightless and come after basal paraves*.
In the dinosaur to bird theory, "non-paraves maniraptors"  are incorrectly considered to come before (ancestral to) basal paraves.
However when we work with the idea that they are secondarily flightless (descended from basal paraves) then the objection above would not be relevant.
Pterosaurs did not give rise to terrestrial maniraptors. 
The pterosaur to bird theory proposes that flying pterosaurs gave rise to flying basal paraves.

* they are actually members of paraves

Tuesday, October 28, 2014

Artist's Renderings

Here are artist's renderings of the taxa we are interested in. They show that the pterosaur is much closer to basal paraves than the dinosaur is.




Saturday, October 25, 2014


Notice that the charts for basal Paraves and Rhamphorhynchus are very similar.
This is what you would expect if basal Paraves descended from Rhamphorhynchidae. 

Figure 3. Representative aerodynamic measurements for pitching stability and control effectiveness. Long-tailed taxa (a) have a stable equilibrium point at 10-25 (yellow line) and the tail is effective in generating pitching moments at low angles of attack (pale yellow box indicates measurable moments for given tail deflections). In short-tailed taxa (b), including extant Larus, the equilibrium point at 0-5 is unstable (red line) and the tail control effectiveness is reduced (no measurable moments for the given tail deflections). One example (Rhamphorhynchus) drawn from pterosaurs illustrates similar possibilities in a phylogenetically distant [actually quite close] taxon.

This is not convergence because ground-based dinosaurs and pterosaurs did not live in similar ways and/or similar environment, and so did not face the same environmental factors.
In morphology, analogous traits will often arise where different species live in similar ways and/or similar environment, and so face the same environmental factors. When occupying similar ecological niches (that is, a distinctive way of life) similar problems lead to similar solutions.[4]

Friday, October 24, 2014


Pterosaur and basal paraves wrists are very similar,

The pterosaur wrist consists of two inner (proximal) and four outer (distal) carpals (wrist bones), excluding the pteroid bone, which may itself be a modified distal carpal. The proximal carpals are fused together into a "syncarpal" in mature specimens, while three of the distal carpals fuse to form a distal syncarpal.

Now let's compare to Yixianosaurus.
The describers considered the exact placement of Yixianosaurus within Maniraptora to be uncertain, but because the hand length resembled that of another feathered dinosaurEpidendrosaurus (now Scansoriopteryx), they suggested it was a close relative of the Scansoriopterygidae.
Comparing the Yixianosaurus wrist to the pterosaur wrist is a very good comparison because I have been proposing that Rhamphorhynchidae developed into a taxon like Scansoriopterygidae.

In the transition from pterosaur to basal paraves, the two proximal carpals were not fused. Two of the distal carpals continued to be fused. The outermost distal carpal was not fused.
This allowed the wrist to bend a great deal.
Extant volant birds possess a highly specialized wrist joint, in which two proximal carpals articulate with a fused carpometacarpus. The proximal part of the carpometacarpus forms an articular trochlea, comprising two convex ridges separated by a transverse groove, and is largely homologous to the structurally similar semilunate carpal (SLC) of derived non-avian theropods (Ostrom 1976).
Like other maniraptorans, scansoriopterygids had a semilunate carpal (half-moon shaped wrist bone) that allowed for bird-like folding motion in the hand.
The pterosaur proximal carpal articulated in a groove in the distal carpal and that is how it worked in basal paraves as well.
Figure 1: Diagram showing the position and general morphology of the transversely trochlear proximal articular facet of the carpometacarpus in selected theropod hands with the phalanges omitted (upper: proximal view; lower: dorsal view; medial side of hand to left).

(a) The basal coelurosaurian condition (based on Guanlong). (b) The basal paravian condition (based on Sinovenator). (c) The neornithine condition (based on Crossoptilon). Yellow indicates the ‘semilunate’ carpal; grey-yellow indicates the transverse groove; green indicates the metacarpals.
Trochlea (Latin for pulley) is a term in anatomy. It refers to a grooved structure reminiscent of a pulley's wheel.

This is also a very helpful link:
The homology of the ‘semilunate’ carpal, an important structure linking non-avian and avian dinosaurs, has been controversial. Here we describe the morphology of some theropod wrists, demonstrating that the ‘semilunate’ carpal is not formed by the same carpal elements in all theropods possessing this feature and that the involvement of the lateralmost distal carpal in forming the ‘semilunate’ carpal of birds is an inheritance from their non-avian theropod ancestors. Optimization of relevant morphological features indicates that these features evolved in an incremental way and the ‘semilunate’ structure underwent a lateral shift in position during theropod evolution, possibly as a result of selection for foldable wings in birds and their close theropod relatives. We propose that homeotic transformation was involved in the evolution of the ‘semilunate’ carpal. In combination with developmental data on avian wing digits, this suggests that homeosis played a significant role in theropod hand evolution in general.


The following seems to be inconsistent with the other references:
The wing of a modern bird, for example, has only two remaining carpals; the radiale (the scaphoid of mammals) and a bone formed from the fusion of four of the distal carpals.[14]
Bird limbs
"Fore-limb and hind-limb compared. H., Humerus; R., radius; U., ulna; r., radiale; u., ulnare; C., distal carpals united to carpo-metacarpus; CC., the whole carpal region; MC.I., metacarpal of the thumb; I., phalanx of the thumb; MC.II., second metacarpus; II., second digit; MC.III., third metacarpus; III., third digit. F., femur; T.T., tibio-tarsus; Fi., fibula; Pt., proximal tarsals united to lower end of tibia; dt., distal tarsals nited to upper end of tarso-metatarsus (T.MT.); T., entire tarsal region; MT.I., first metatarsal, free; I.-IV., toes." -Thomson, 1916

Friday, October 3, 2014

Arboreal theory and the pterosaur to bird theory

The arboreal theory of bird evolution ("trees down") is held by a number of researchers and academics. The core of the theory is that the lineage leading to birds began in the trees (not on the ground).
The pterosaur to bird theory is a refinement of this idea. The refinement is that pterosaurs were already flying in the trees, by flapping their skin membrane, and they later developed feathers.
The ancestor of birds was not a dinosaur, nor was it a thecodont. It was a pterosaur.

The trees down theory begins with a gliding arboreal creature. This is closer to being correct except that the gliding phase took place earlier, in the evolution of the pterosaur. This is a key point.

Wednesday, October 1, 2014

Why did they change?

I have been asked this question a few times:
“why would a pterosaur give up a functional skin-and-bones wing structure to evolve a different, feather-based structure with the same function”?
Here are some things to consider:
In the dinosaur to bird theory, it is said that the first feathers were for display and for insulation. If those answers make sense in the dino to bird theory, then they could be equally applied to the pterosaur situation.
Another answer is that the pterosaur’s long wing finger makes it difficult/dangerous for a pterosaur to fly in the trees. It would damage the bone and tear the membrane. Feathers made it possible to inhabit the forests.
Another answer is that the ability of the feathers to open up when elevating the wing, reducing drag, is an advantage over the skin membrane.
And additionally, even though we may wonder what value the pterosaur got from feathers, the fact is that the birds were successful and the pterosaurs went extinct. The feathered version must have had additional value.

The question about WHY pterosaurs would develop feathers is often teamed up with the point that pterosaurs were doing exceptionally well for millions of years. Why change?
But exactly the same argument can be made about dinosaurs who were doing exceptionally well for millions of years. Why would they change? Same question.

Pelvic Girdle

Basal paraves had a femur/acetabulum articulation that allowed them to abduct (splay, sprawl) their legs. That was very different than dinosaurs.

In theropods, the femoral component is cylindrical without any distinctive head and neck. It projects medially at a right angle from the shaft and fits into a perforated [completely open] acetabulum of up to 1.5 times its diameter. As a result, the hip joint is stable and fully congruent during parasagittal motion, permitting a wide range of flexion and extension but very little abduction and adduction
One important dinosaurian synapomorphy is the perforate [completely open] acetabulum, simply a "hip bone" (actually three connected bones, together called the pelvis) with a hole in the center where the head of the femur ("thigh bone") sits. This construction of the hip joint makes an erect stance (hindlimbs located directly beneath the body) necessary — like most mammals, but unlike other reptiles which have a less erect and more sprawling posture. Dinosaurs are unique among all tetrapods in having this perforate [completely open] acetabulum.
A partially closed acetabulum is seen in basal archosaurs and is characteristic of the scansoriopterids and Jurassic feathered forms such as Anchiornis initially described as near Aves by Xu et al (2009)
The [Hesperonychus] 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).
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).
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)
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 requiresthe hindwing to have at least 20° of negative dihedral (anhedral),all configurations were quite stable gliders. Glide angles rangedfrom 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.
Scansoriopteryx also lacks a fully perforated acetabulum, the hole in the hip socket which is a key characteristic of Dinosauria and has traditionally been used to define the group.
Scansoriopteryx is clearly more primitive
than Archaeopteryx in many respects such as its
saurischian-style pelvis which has remarkably short
pubes; elongate and robust ischia; and
comparatively small pubic peduncles. These
primitive features further suggest that the nearly
closed acetabulum is not a reversal, but a true
plesiomorphic condition
This results in a somewhat sprawling position for the [Archaeopteryx] femur that is corrected at the knee joint, resulting in a functionally vertical leg.
The pelvis has an incompletely open acetabulum, and there is no characteristic dinosaurian supra-acetabular shelf.
The femoral head turns forwards rather than extending perpendicular to the shaft.
There is a reference to the splayed posture of Scansoriopterids here (page 154):

There is a reference to the splayed posture of Archaeopteryx here (page 399):
head of [Scansoriopteryx] femur lacks a distinctive neck and is instead more proximally oriented as in reptiles with sprawling limbs
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.

Basal paraves had a femur/acetabulum articulation that was different than dinosaurs and that allowed them to abduct (splay) their legs.

Also see the info about splayed hindlimbs here and here.

Note the articulation of the femur and the acetabulum.
Figure 2. Pterodactylus kochi (cast) A short-tailed Jurassic pterosaur,about the size of a robin Solnhofen Limestone, Germany


As far as I am aware, the new specimen does not have a name and has yet to be described in the literature, but this Archaeopteryx appears to be one of the more complete and well preserved of the lot. In fact, the preservation and position of the bones are reminiscent of the Thermopolis specimen I saw in Wyoming this past year, although this new Archaeopteryx is missing one forelimb and the skull. Don’t be fooled by the fact that, at first glance, the fossil looks a little jumbled up. If you start by following the tip of the tail (on the right), the articulated vertebral column leads to the hips and splayed legs before curving up and back in the classic dinosaur death pose. The arm is displaced below the hips but remains articulated.

The 11th skeleton of Archaeopteryx. Photo by Helmut Tischlinger.