Pterosaurs,
as fenestrasaur tritosaur lepidosaurs matured isometrically. That’s a widely overlooked fact, even by pterosaur workers. Hatchlings had adult proportions with small eyes and long rostra — if their 8x larger parents had small eyes and long rostra. Hatchlings also had adult-proportioned wings. So presumably they were able to fly shortly after hatching (and drying out a bit) — if their parents were able to fly. But not all adult pterosaurs were able to fly…
Figure 1. GIF animation, 4 frames, showing three pterosaurs specimens in 3 sizes (see scale bars) with short, medium and long wings, drawn to the same torso length. The question is: did Quetzalcoatlus fly?
Flightless pterosaurs
Earlier we looked at two related pterosaurs, the no. 57 specimen (Sos 2482) and the no. 42 specimen in the Wellnhofer 1970 catalog (Fig. 1). Both are adults. Both are in the azhdarchid lineage that arose from a tiny pterodactyloid-grade dorygnathid, the no. 1 specimen (TM 10341) in the Wellnhofer 1970 catalog and ultimately gave rise to the giant pterosaur, Quetzalcoatlus (also in Fig. 1). A magnitude or more greater in size and with wings only half as long as the flying no. 42 specimen,
Quetzalcoatlus is widely considered a flying pterosaur.
Can that be verified? Other clades of large (larger than a pelican) pterosaurs all have elongate wings, ideal for soaring. Azhdarchids, apparently deep shoreline waders, did not. The distal two long phalanges (sans the ungual) were shorter in azhdarchids, but the wing was not otherwise reduced, as in the flightless pterosaur, no. 57 (Fig. 1). Witton and Naish 2008 provide a history of workers pondering this question. Unfortunately they provided a bat-wing membrane attached to the ankles or shins with anteriorly oriented pteroids, ignoring key references for pterosaur wing shape (Peters 2002, 2009 and references therein) while ignoring fossilized evidence of pterosaur wing tissue, as others have done.
As anything gets larger,
either ontogenetically or phylogenetically, they generally put on weight at the cube of their length. Air-filled pterosaurs were not as solid, so that ratio was undoubtedly lower. Even so longer, larger wings on larger pterosaurs makes sense, as in living large birds that fly and are also air-filled.
But that is countered by the isometric growth of individual pterosaurs as they mature to adulthood. Whatever works for hatchlings and tiny pterosaurs, is working just as well for giant adults. Could that mean that all ontogenetic stages of Quetzalcoatlus could fly? Or none of them? Or only half-sized juveniles at about ten percent of the adult weight? With flight, it’s always a balancing act: thrust, lift, drag, weight.
Wings can still provide great thrust
for terrestrial excursions even if they cannot get a big pterosaur off the ground (Fig. 2). So that’s a possibility under consideration, too. After all, why not use all the thrust available?
Figure 10. Quetzalcoatlus running like a lizard prior to takeoff.
To prevent an extant flying bird, like a cockatiel, from flying, or flying well,
it’s surprising how little of the tips of the feathers need to be clipped. Link here. Basically its the difference between no. 42 and Quetzalcoatlus above (Fig. 1). With this in mind, I cannot join those who say giant Quetzalcoatlus could fly or fly between continents, until supporting evidence comes alone. Rather, giant azhdarchids become hippo analogs in this respect: they were probably constant deep waders (Fig. 3) capable of charging or running from danger. Storks, which azhdarchids otherwise resemble, tend to fly away because they have long, not truncated wings and can do so.
Figure 3. In my opinion this saddle-bill stork wading in water appears to be the bird closest to azhdarchid morphology and, for that matter, niche. It can fly from danger on elongate wings. Not so sure that Q could do the same.
References
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing—with a twist. Historical Biology 15:277-301.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29: 1327-1330.
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.
Witton M and Naish D 2008. A Reappraisal of Azhdarchid Pterosaur Functional Morphology and Paleoecology. https://doi.org/10.1371/journal.pone.0002271. online here.
The Pterosaur Heresies 
When you talk about clipping wing tips that’s an artificial interference. That’s the same as saying because if you cut out four toes of a bear it wouldnt be able to walk properly a horse cant possibly gallop.
The flight mechanics of feathery wing are much more complex than raw wingspan, control of air stream and turbulence are a huge part if bird flight. Other than that, birds with different wing spans can have a number of other different adaptations, such as muscle mass and wing ratio. Take for instance a Wandering Albatross and a Harpy Eagle, they have a huge difference in wing span, but not so much in weight or body length.
Couldn’t it be that Azhdarchids had proportionaly more powerful flight muscles or a different, broader, wing profile than the default pterosaur?
Good question, Maricio. It’s one I cannot answer with certainty.
Let’s look at the anchor for the wing muscles in Q.
http://www.reptileevolution.com/quetzalcoatlus.htm
The pectoral girdle is smaller and narrower in Q than in pteranodontids and ornithocheirds.
To your point, I reconstructed a deeper wing chord in Q species.
Let’s also look at the phylogenetic trend in this clade. In smaller more primitive taxa like Jidapterus and Chaoyangopterus the wingtip, when folded/flexed, extends a short distance above the back. In Q the wingtip rises to the back of the arm pit.
Clipping a wingtip is indeed artificial interference. What happens in Q is natural, as in the ostrich, which got too big to fly, so it evolved shorter wings.
In summary, you have a shorter wing relative to torso length, a smaller sternal complex relative to torso length, and much greater weight to get off the ground. As a pilot I learned that airplanes cannot get off the runway if they are overweight and that airplanes with a shorter wingspan need greater takeoff speed, all other factors being equal. I wish it were otherwise, but the present factors appear to tip the scales against flight in giant azhdarchids. If you can find one with a broader sternal complex (for power) and longer wings (for lift) then the scales will tip toward flight.
Thanks for the answer, David. The requirement of higher speeds for takeoff with a narrower body ratio is indeed very strong! Also, thanks for the information about Q’s pectoral girdle.
I guess having grown up with Gurney’s Skybaxes (https://s-media-cache-ak0.pinimg.com/736x/94/0c/53/940c530695f7b1ba88fd77a9d41e5bd3–dinosaur-fantasy-art.jpg) really makes it harder to accept that maybe they couldn’t fly.