Quetzalcoatlus northropi (Fig. 1, Lawson 1975) is well known as the largest pterosaur of all time. It is known chiefly from most of the wing, which dwarves that of the more complete specimen of Q. sp. (Kellner and Langston 1996), which was found a mere 40km away from sediments of a similar age (Latest Cretaceous). Other giant azhdarchid pterosaurs competing for “the largest pterosaur of all time” are known from less complete remains.
Figure 1. Click to enlarge. Quetzalcoatlus specimens to scale. Here Q. northropi is 2.5x taller than Q. sp, if nothing else changed other than size.
Some workers (Henderson 2010) have questioned the flying abilities of Q. northropi. Others (Witton and Habib 2010) have given it tremendous flying abilities, able to soar between continents. Both have relied on scaling the small specimen up to the size of the giant.
I was curious
to compare the large and small specimens. Several years ago I took photos of the large specimen wing at the Langston lab in Austin, Texas. The tracings of the large specimen were scaled down to the size of the small specimen (Fig. 2). They are — almost – identical.
Figure 2. Quetzalcoatlus sp. compared to the large specimen wing, here reduced to match that of the smaller specimen. I lengthened the unknown metacarpus to match the Q. sp. and other azhdarchid metacarpi. Note m4.2 is narrower and shorter on the larger specimen, which doesn’t make sense if Q. northropi was volant. It might have been shorter still if Option 1 is valid. At this point, either is possible.
Scaling pterosaurs helps one understand some of the “big” questions. Everyone knows that to double the height of the animals is to cube its weight. The same holds true for pterosaurs. So then we might ask, if the larger specimen had higher wing loading, why wasn’t the wing spar more robust? As you can see, the wing elements were not more robust in the giant — AND — m4.2 was more gracile (Fig. 2).
The answer to that question is not so obvious, as we learned before. The proportions of giant azhdarchids were quite similar to those of the tiniest proto-azhdarchids, as you can see below (Fig. 3).
We also see distal wing phalanx reduction in the evolution of the flightless pterosaur, Sos 2428 from tiny ancestors, n42, and n44 (from the Wellnhofer 1970 catalog, Fig. 3) with longer wings.
Figure 3. The flightless pterosaur, Sos 2428 to scale along with two ancestral taxa, both fully volant. Note the reduction of the wing AND the expansion of the torso. We don’t know the torso of Q. northropi. It could be small or it could be very large.
Here in the flightless pterosaur (Fig. 3), perhaps more importantly, the torso expanded greatly in every direction during the evolution of flightlessness. The pelvis was also much larger in Sos 2428.
We don’t have enough torso material from the Quetzalcoatlus northropi specimen to understand its volume. While it is possible that the torso remained small, as in Q. sp. (Fig. 1), it is equally possible that it could have expanded to become voluminous, as in Sos 2428.
Until we know, we can only guess, but the relative reduction of the distal wing elements, beyond what we see in the smaller specimen, adds weight to the argument that flight was more difficult for the giant.
More data would help settle this issue.
We take our clues wherever we can. Don’t overlook the little stuff.
Henderson DM (2010). Pterosaur body mass estimates from three-dimensional mathematical slicing. Journal of Vertebrate Paleontology 30(3):768-785.
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. PlosOne 5(11): e13982. doi:10.1371/journal.pone.0013982