Azhdarchid pterosaur flight issues

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…

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?

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.

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.