There are those who think
the giant azhdarchid pterosaur, Quetzalcoatlus (Fig. 1), was flightless. Almost all others think Quetzalcoatlus was the largest flying animal of all time. The question is: were the wings of Quetzalcoatlus large enough to initiate and sustain flight?
Sometimes it just helps to compare
azhdarchids to azhdarchids to azhdarchids. In this case we’ll compare Quetzalcoatlus in dorsal view to two azhdarchids so small that traditional paleontologists don’t even consider them to be azhdarchids. BSPG 1911 I 31, (Figs. 2, 3) is a traditional, small volant pterosaur with a long neck and a standard pterosaur wingspan. JME-Sos 2428 (Fig. 2) is an odd sort of flightless pterosaur with a very much reduced wingspan. Neither of these taxa seems to ever make it to the cladograms of other workers.
When you compare azhdarchids to azhdarchids to azhdarchids
you get the overwhelming impression that IF Quetzalcoatlus was volant, it would not have reduced the distal wing phalanges so much. And yet it did, just like other flightless pterosaurs did. Since weight increases by the cube as size in dorsal view increases by the square, the wings of the giant should actually be larger than those of the smaller azhdarchid to handle the relatively larger mass.
So what did Quetzalcoatlus use its flightless wings for?
Thrust (Fig. 2).
Contra tradition, the azhdarchid bauplan
was initiated with Late Jurassic small pterosaurs like BSPG 1911 I 31, so misbegotten that traditional paleontologists have forgotten to give it its own generic and specific name distinct from the wastebasket taxon Pterodactylus, with which it is not related, as we learned earlier here.
Kellner AWA and Langston W 1996. Cranial remains of Quetzalcoatlus (Pterosauria, Azhdarchidae) from late Cretaceous sediments of Big Bend National Park, Texas. – Journal of Vertebrate Paleontology 16: 222–231.
Lawson DA 1975. Pterosaur from the latest Cretaceous of West Texas: discovery of the largest flying creature. Science 187: 947-948.
Witton MP and Habib MB 2010. On the size and flight diversity of giant pterosaurs, the use of birds as pterosaur analogues and comments on pterosaur flightlessness. PloS one, 5(11), e13982.
More data here: why-we-think-giant-pterosaurs-could-fly-not/