Pterosaur wing origins

It has been said that pterosaurs appeared suddenly in the fossil record with no apparent precursors. Unfortunately, this is the belief of those who still believe pterosaurs are archosaurs. No series of archosaurs has ever been put forth that demonstrates a gradual evolution of pterosaurian characters, hence the paradigm mentioned above.

Lepdiosaurian fenestrasaurs (Peters 2000, 2009) do demonstrate a gradual evolution of pterosaurian characters, no matter which characters one chooses. Here the forelimb is transformed into a wing using a series of lepidosaurs.

1.7 Illustrates the basal lepidosaur, Sphenodon. Digit 3 extends further than digit 4. Metacarpal 3 is longer than 4. All the carpals are present including two centralia. The medial centralia is elongated.

1.1 Illustrates the basal squamate Huehuecuetzpalli. Here the carpals are poorly ossified, even in the adult. As a result the distal radius and ulna touch because the intermedium is absent. Metacarpal 4 is as long as 3 and digit 4 extends further than digit 3. Digit 5 is somewhat shorter.

1.2 Illustrates the basal fenestrasaur, Cosesaurus. Here the radius and ulna have little to no space between them (compare to 1.7) and the carpals are well ossified. Thus supination and pronation were dramatically reduced. Note the two centralia are now on the medial side of the carpus where they take on new identities as the pteroid and preaxial carpal (Peters 2009). Only fenestrasaurs, including pterosaurs, have this arrangement. The digits are robust and digit 5 is further reduced. Extradermal tendrils trail the ulna and finger IV, probably for decoration. Cosesaurus was occasionally bipedal and likely flapped its arms due to the presence of a pterosaur-like pectoral girdle.

1.3 Illustrates Sharovipteryx. This fenestrasaur reduced its forelimbs and elongated its hind limbs so much that the forelimbs never touched the ground during locomotion. Here the stunted radius and ulna are shorter than the hand. The gracile fingers are much longer laterally with deep claws. Digit 5 is further reduced. The extended neck skin acted like strakes. The flapping forelimbs acted like canards and probably provided some propulsion for this hindlimb glider.

1.4 Illustrates Longisquama. This fenestrasaur has a pterosaur-like wing membrane trailing the forelimb, which was relatively much larger, but otherwise similar to Sharovipteryx. The axial rotation of metacarpal IV has begun because digit 4 no longer had continuous flex lines with digits 3, 2 and 1. This rotation enabled digit 4 to flex in the plane of the wing for wingfinger folding. Such a wing would not have provided enough lift for flight, but it could have extended glides by providing thrust.

1.5 and 1.6 Ilustrate the derived fenestrasaur and basal pterosaur, MPUM 6009. Here the hand is much smaller, but the wing finger is hyper-elongated. The fiber-embedded wing membrane (poorly preserved in the original fossil) is large enough to provide thrust and lift for flight. While Longisquama used its wings primarily to ward off rivals and entice mates, MPUM 6009 could have used its wings to fly from place to place in powered flight. Later pterosaurs modified the wing elements in various relatively minor ways. Only one pterosaur, SOS 2428, reduced its wings and became flightless. Note digit 5 is not gone. It is a vestige. More examples of manual digit 5 in pterosaurs are here. The wing ungual is also not gone. However it does become disarticulated on most pterosaur specimens. Note that a trochlear joint for the reception of a wing ungual always tips the fourth wing phalanx. Examples that appear pointed are the result of a partially buried tip that needs further preparation.

A recent paper on pterosaur wing shape (Elgin, Hone and Frey 2010) purported to show that all pterosaur wing membranes attached at the tibia or ankle (see below). Unfortunately, all of the examples presented demonstrated the opposite, that the wing was stretched between the wing finger and elbow with a small fuselage fillet inboard as shown above and here. Elgin, Hone and Frey (2010) discarded one of the best examples of the narrow chord model, the Zittel wing. They considered other narrow chord wings as examples of "membrane shrinkage," presumeably due to desiccation. That is an invention enforced my misunderstanding examples of wing membrane tearing and folding. Wing membranes don't shrink. They are embedded with fibers, not water.