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.
Notice figure (b) in the chart below:Now let's compare to Yixianosaurus (Pennaraptora):
The describers considered the exact placement of Yixianosaurus within Maniraptora to be uncertain, but because the hand length resembled that of another feathered dinosaur, Epidendrosaurus (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 continued to be fused. One distal carpal was lost (along with the corresponding digit). The other two distal carpals continued to be fused.
Sullivan et al
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 grooveFhttp://en.wikipedia.org/wiki/Scansoriopterygidae
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.http://en.wikipedia.org/wiki/Trochlea
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 proximal carpal is seen articulating with the Ulna and Radius. This carpal in turn articulates with the Distal Carpal in a saddle like gliding joint.
A story to rationalize a purported dino to bird transition:
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.
FOR REFERENCE (MODERN BIRDS)
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.
"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
Metacarpal to humerus ratio:
Table 2 shows that Rhamphorhynchinae varies between 0.39 to 0.68.
Primitively, pterosaurs have comparatively small metacarpals, with the humerus at least 2.5 times longer.
Dorygnathus in general has the build of a basal, i.e. non-pterodactyloid pterosaur: a short neck, a long tail and short metacarpals — although for a basal pterosaur the neck and metacarpals of Dorygnathus are again relatively long.