Earlier and below (Fig. 2) we looked at large and giant pterosaur wings comparing them to the largest flying birds, including one of the largest extant flying birds, the stork, Ciconia, and the extinct sheerwater, Pelagornis, the largest bird that ever flew.
FIgure 1. A basal pteranodotid, the most complete Pteranodon, the largest Pteranodon skull matched to the largest Pteranodon post-crania compared to the stork Ciconia and the most complete and the largest Quetzalcoatlus. Note the much reduced distal phalanges in the complete and giant Quetzalcoatlus, distinct from the Pteranodon species.
we’ll look at how the largest Pteranodon (Figs. 1, 4) compares to much larger pterosaurs, like Quetzalcoatlus northropi (Figs. 1, 2) that have vestigial wingtips similar to those of the much smaller flightless pre-azhdarchid, SOS 2428 (Fig. 3).
Note the tiny three distal phalanges
on the wing of the largest Quetzalcoatlus, distinct from the more typical elongate and robust distal phalangeal proportions on volant pterosaurs of all sizes. Much smaller definitely flightless pterosaurs, like SOS 2428, shrink those distal phalanges, too. That’s the pattern when pterosaurs lose the ability to fly.
Figure 2. A previously published GIF animation. Q. northropi and Q. sp. compared to Ciconia, the stork, and Pelagornis, the extinct gannet, to scale. That long neck and large skull of Quetzalcoatlus would appear to make it top heavy relative to the volant stork, despite the longer wingspan. Pteranodon and other flying pterosaurs do not have such a large skull at the end of such a long neck (Fig. 1). The longer wings of pelagornis show what is typical for a giant volant tetrapod, and Q. sp. comes up short in comparison.Today we’ll compare the wingspan of the largest Quetzalcoatlus to the largest and more typical Pteranodon species (Fig. 2).
pterosaur workers refuse to consider taxa known to be flightless, like SOS 2428 (Peters 2018). It’s easy to see why they would be flightless (Fig. 3). Scaled to similar snout/vent lengths with a fully volant pterosaur like n42 (BSPG 1911 I 31) the wing length and chord are both much smaller in the flightless form.
Figure 3. Lateral, ventral and dorsal views of the flightless SoS 2428 (Peters 2018) alongside No. 42, a volant sister taxon.
Comparing the largest ornithocheirid,
SMNK PAL 1136, to the largest Pteranodon (chimaera of largest skull with largest post-crania in Fig. 4) shows that large flyers have elongate distal phalanges, distinct from body and wing proportions documented in the largest azhdarchids, like Quetzalcoatlus.
Figure 4. Largest Pteranodon to scale with largest ornithocheirid, SMNS PAL 1136. Note the long distant wing phalanges on both of these giant flyers. This is what pterosaurs evolve to if they want to continue flying. And this is how big they can get and still fly. Giant azhdarchids exceed all the parameters without having elongate wings. Note: the one on the left has a longer wingspan whir the one on the right has a more massive torso and skull together with more massive proximal wing bones and pectoral girdle. On both the free fingers are tiny, parallel oriented laterally and slightly tucked beneath the big knuckle of the wing finger. The pteroid points directly at the deltopectoral crest.
As the largest Pteranodon and largest ornithocheirid (SMNS PAL 1136)
(Fig. 4) demonstrate, as flying pterosaurs get larger, they retain elongate distal wing phalanges. And big, robust phalanges they are.
By contrast in azhdarchids and pre-azhdarchids
there is a large size bump after n42 (BSPG 1911 I 31) the fourth wing phalanx either disappears (see Microtuban and Jidapterus) or shrinks to a vestige. Then there’s Zhejiangopterus (Fig. 5), with a big pelvis, gracile forelimbs and a giant skull on a very long neck. Just that neck alone creates such a long lever arm that the pterosaur is incapable of maintaining a center of balance over or near the shoulder joints.
Figure 5. There are several specimens of Zhejiangopterus. The two pictured in figure 2 are the two smallest above at left. Also shown is a hypothetical hatchling, 1/8 the size of the largest specimen.
As mentioned earlier, becoming flightless permitted, nay, freed azhdarchid pterosaurs to attain great size. They no longer had to maintain proportions that were flightworthy. Instead they used their shortened strut-like forelimbs to maintain a stable platform in deeper waters. And when they had to move in a hurry, their wings could still provide a tremendous amount of flurry and thrust (Fig. 6) for a speedy getaway.
Figure 6. Quetzalcoatlus running without taking off, using all four limbs for thrust. That long lever arm extending to the snout tip in front of the center of gravity is not balanced in back of what would be the center of lift over the wings
For the nitpickers out there…
some specimens of Nyctosaurus (UNSM 93000, Fig. 7) also have but three wing phalanges, but they are all robust. The distal one is likely the fourth one because it remains curved. Phalanges 2 and 3 appear to have merged, or one of those was lost. Compare that specimen to a more primitive Nyctosaurus FHSM VP 2148 with four robust wing phalanges.
Figure 5. Cast of the UNSM 93000 specimen of Nyctosaurus. Missing parts are modeled here.
Peters D 2018. First flightless pterosaur (not peer-reviewed). PDF online.