Basalmost Paraves

There is a set of 4-winged, flying, primitive birds (eg. Anchiornis, Aurornis, Xiaotingia etc.) that at times have been classified as avialans and sometimes as dromaeosaurids. They actually belong to the basalmost Paraves. 
They are the Tetrapterygidae and the Scansoriopterygids.

I suggest that these flying primitive birds evolved from pterosaurs.
Later, some of these primitive birds settled on the ground and became secondarily flightless (eg. eudromaeosaurids, oviraptors etc).


https://en.wikipedia.org/wiki/Paraves

the work of Xu et al. (2003), (2005) and Hu et al. (2009) provide examples of basal and early paravians with four wings,[10][11][12] adapted to an arboreal lifestyle who would only lose their hindwings when some adapted to a life on the ground and when avialans evolved powered flight.[13] Newer research also indicates that gliding, flapping and parachuting was another ancestral trait of Paraves, while true powered flight only evolved once, in the lineage leading to modern birds.[14]

https://en.wikipedia.org/wiki/Tetrapterygidae

Tetrapterygidae (meaning "four-wings") is a group of four-winged dinosaurs proposed by Sankar Chatterjee in the second edition of his book The Rise of Birds: 225 Million Years of Evolution, where he included Microraptor, Xiaotingia, Aurornis, and Anchiornis.[1] The group was named after the characteristically long flight feathers on the legs of all included species, as well as the theory that the evolution of bird flight may have gone through a four-winged (or "tetrapteryx") stage, first proposed by naturalist William Beebe in 1915.[2] Chatterjee suggested that all dinosaurs with four wings formed a natural group exclusive of other paravians, and that this family was the sister taxon to the group Avialae, although most phylogenetic analyses have placed the animals of his Tetrapterygidae elsewhere in Paraves, such as Xiaotingia, Aurornis, and Anchiornis being placed in Avialae.[3]

Zhenyuanlong

Zhenyuanlong  is a secondarily flightless member of Paraves.

http://www.nature.com/srep/2015/150716/srep11775/full/srep11775.html (2015)

A large, short-armed, winged dromaeosaurid (Dinosauria: Theropoda) from the Early Cretaceous of China and its implications for feather evolution

Regardless of the precise phylogenetic relationships of dromaeosaurids, Zhenyuanlong provides the first glimpse of feather morphologies in a short-armed dromaeosaurid. Feathers are not preserved on the holotype of Tianyuraptor, and the shortness of the forearm in this taxon led to the suggestion that its arms lacked aerodynamic function15. Although the arms of Zhenyuanlong are short, they supported large and complex wings comprised of pennaceous coverts, primaries, and secondaries, some of which are asymmetric. Whether these wings served any type of aerodynamic function is a separate question that can only be answered with biomechanical analysis, but the wings of Zhenyuanlong are strikingly similar to those of Microraptor in general size, morphology, and composition, albeit they are supported by much smaller arms.

The integumentary similarities between Zhenyuanlong and Microraptor-type animals could suggest one of several explanations. First, the large short-armed dromaeosaurids may have had some volant abilities, unrecognized previously because Tianyuraptor was preserved without feathers and its small arms were assumed to be un-flightworthy15. Perhaps there was not a large functional and behavioural gap between animals like Microraptor and Zhenyuanlong. We find this unlikely, however, given the striking differences in body size between them, and the incredibly short arms of Zhenyuanlong which do not appear optimized for flight (although we reiterate that biomechanical modelling is needed to properly test this). Alternatively, the integumentary similarities between small and clearly volant dromaeosaurids7 and larger and presumably non-volant dromaeosaurids could suggest that the larger and short-armed Zhenyuanlong evolved from more volant ancestors and maintained a many aspects of the integument through the inertia of common descent or for other selective reasons, not because it needed them for flight. It may be that such large wings comprised of multiple layers of feathers were useful for display purposes40, and possibly even evolved for this reason and not for flight, and this is one reason why they may have been retained in paravians that did not fly.

https://www.washingtonpost.com/news/speaking-of-science/wp/2015/07/16/scientists-find-a-new-dinosaur-with-well-preserved-bird-like-wings-but-not-for-flight/

Zhenyuanlong suni didn't have wings well suited for flight -- but it did have the feathers one would need to get off the ground. Because of this, they suspect that suni came after a flying ancestor, losing the capability for muscle-powered flight but retaining the related plumage, perhaps to use its wings for mating displays.

Primitive birds flying

Primitive birds (basalmost paraves) flapped their wings and flew using the same set of muscles as their pterosaur ancestors.

http://press.princeton.edu/witton/sa...pterosaurs.pdf At one stage, it was thought that the flight muscles of pterosaurs were very [modern] birdlike, with the arm lifted by a muscle, m. supracoracoideus, anchoring on the sternum rather than the shoulders. In birds, this muscle arcs over the glenoid to attach on the dorsal surface of the humerus, elevating the wing with a pulley-like system (e.g., Kripp 1943; Padian 1983a; Wellnhofer 1991a). Detailed reconstruction of the proximal arm musculature of pterosaurs shows that this is not the case, however, and that the [pterosaur] 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.

https://repository.si.edu/bitstream/...=1&isAllowed=y

Furthermore, the supracoracoideus muscle, and hence an ossified sternum, is not necessary to effect the recovery stroke of the wing. Thus the main evidence for Archaeopteryx having been a terrestrial, cursorial predator is invalidated. There is nothing in the structure of the pectoral girdle of Archaeopteryx that would preclude its having been a powered flier.

Concerning the lack of asymmetric feathers in flying basalmost paraves, the following seems relevant: https://gwawinapterus.wordpress.com/...-introduction/ The basal deinonychosaur Anchiornis [a primitive bird] might offer a possible explanation. It had symmetrical feathers, but they were arranged in an unique away; in species with asymmetrical feathers, the most distally attached wing feathers are the longest ones. In Anchiornis, the longest are anchored near the wrist, making the center of the wing the broadest area. This is not an unusual profile among flightless maniraptors – oviraptors like Caudipteryx have this sort of arrangement as well. Anchiornis, however, differs in that the feathers at the front (as in, anchored more distally) of the longest feather decrease rapidly in size as they are closer to the end of the supporting digit; this results in a rounded, yet slightly pointy wing shape. It is possible that this arrangement could had been an early adaptation to the demands of powered flight, before true asymmetrical feathers evolved. If so, it is possible that Anchiornis did engage in powered flight, or even a method of escape akin to rudimentary WAIR. So far, no tests have been conducted to examine the aerodynamic capacities of it’s wings. In Anchiornis, the entire wing forms the airfoil. http://www.sciencedirect.com/science/article/pii/S0960982212011943 Primitive bird: In modern birds (Neornithes), the wing is composed of a layer of long, asymmetrical flight feathers overlain by short covert feathers [1-3]. It has generally been assumed that wing feathers in the Jurassic bird Archaeopteryx [4-9] and Cretaceous feathered dinosaurs [10, 11] had the same arrangement. Here, we redescribe the wings of the archaic bird Archaeopteryx lithographica [3-5] and the dinosaur Anchiornis huxleyi [12, 13] and show that their wings differ from those of Neornithes in being composed of multiple layers of feathers. In Archaeopteryx, primaries are overlapped by long dorsal and ventral coverts. Anchiornis has a similar configuration but is more primitive in having short, slender, symmetrical remiges. Archaeopteryx and Anchiornis therefore appear to represent early experiments in the evolution of the wing. This primitive configuration has important functional implications: although the slender feather shafts of Archaeopteryx [14] and Anchiornis [12] make individual feathers weak, layering of the wing feathers may have produced a strong airfoil. Furthermore, the layered arrangement may have prevented the feathers from forming a slotted tip or separating to reduce drag on the upstroke. The wings of early birds therefore may have lacked the range of functions seen in Neornithes, limiting their flight ability. Longrich NR, Vinther J, Meng Q, Li Q, Russell AP. https://en.wikipedia.org/wiki/Bird_flight Elliptical wings are short and rounded, having a low aspect ratio, allowing for tight maneuvering in confined spaces such as might be found in dense vegetation. As such they are common in forest raptors (such as Accipiter hawks), and many passerines, particularly non-migratory ones (migratory species have longer wings). They are also common in species that use a rapid take off to evade predators, such as pheasants and partridges.

Altogether we have a picture of a flying, feathered, 4 winged, arboreal, primitive bird with a long bony tail, that flew like a pterosaur. With elliptical wings and symmetric feathers

At times people have argued that a flying pterosaur would not devolve into a gliding primitive bird. But this is misguided, because the evidence indicates that flying pterosaurs evolved into flying primitive birds, not into gliding primitive birds.

Here are the aspects related to flight capability:

• muscles used 
• keeled or not keeled sternum 
• flight feathers (asymmetric or not) 
• feathered hindlimbs

See here for more details:
http://pterosaurnet.blogspot.ca/2010/05/keeled-breastbone.html
http://pterosaurnet.blogspot.ca/2015/07/primitive-birds-flying.html
http://pterosaurnet.blogspot.ca/2014/09/flight-without-supracoracoideus.html
http://pterosaurnet.blogspot.ca/2013/02/flight-stroke.html


http://www.nature.com/articles/ncomms14576 (2017)

A stiffer feathered postpatagium in Anchiornis may have compensated for its aerodynamically inferior arm feathers to some degree

Fingers - Quick Summary

 https://en.wikipedia.org/wiki/Origin_of_birds#Digit_homology

There is a debate between embryologists and paleontologists whether the hands of theropod dinosaurs and birds are essentially different, based on phalangeal counts, a count of the number of phalanges (fingers) in the hand. This is an important and fiercely debated area of research because its results may challenge the consensus that birds are descendants of dinosaurs.

Dinosaurs have hands with digits 2-3-4-x-x. (Roman numerals represent fingers, numbers represent phalanges).
But birds have hands with digits x-2-3-4-x.
This is a problem for the dino to bird theory.
To overcome this problem, the dino to bird folk propose changes that include the following:

• the loss of digit I 
• the reappearance of the lost digit IV
• digits II-III-IV adopting the phalangeal count and characteristics of the earlier digits I-II-III (via a frame shift) resulting in x-2-3-4-x with digit III the longest.  

The pterosaur transition (from 2-3-4-5-x) would be:

• the loss of digit I
• digits II-III-IV lose one phalange each, resulting in x-2-3-4-x with digit IV the longest (as in scansoriopterids). 

Note: The dino to bird folk do not consider scansoriopterids to be on the line leading to birds, but to have branched from that line.


Frame shift
http://www.researchgate.net/publication/259253962_Thumbs_down_a_molecular-morphogenetic_approach_to_avian_digit_homology

Thus the change of the phalangeal formula (as in the PRH) is actually caused by the change of the transcriptome (as in the FSH [frame shift hypothesis]), which in turn is directly caused by the loss of digit I (probably shh and hoxD mediated).

https://commons.wikimedia.org/wiki/File:Bird_and_dino_hand_difference_debate.JPG

Dinosaur to bird (notice the original loss of digit IV and its re-appearance)

In the diagram, Neotheropoda ( 1 ), basal tetanurae ( 2 ), a coelurosaurian ( 3 ), the bird (?)Archaeopteryx ( 4 ) and modern bird ( 5 ).

If scansoriopterygids are the basalmost members of paraves, then the very first paraves were (or were very similar to) the scansoriopterygids, which have x-2-3-4-x with digit IV being the longest.
Which is consistent with a pterosaur ancestry and contrary to a dino ancestry.

See here for more details:
http://pterosaurnet.blogspot.ca/2014/07/pterosaur-fingers.html

Bristles and feathers - Quick summary

Dinosaurs (eg. tyrannosaurs) had bristles. They did not have any form of feather.
On the other hand, pterosaurs had Stage I and Stage II feathers. Those early stage feathers kept the endothermic pterosaur warm, which was necessary in flying.
Primitive birds (basal paraves) had pennaceous feathers. Those pennaceous feathers developed in the transition from pterosaur to primitive bird.

In order to understand this topic, it is essential to understand the development stages a feather goes through.
Drawing B represents Stage I. Note that the follicle appears at Stage II (drawing C):

See here for more details:
http://pterosaurnet.blogspot.ca/2014/01/helpful-background.html