QUETZALCOATLUS AND THE SIZE LIMITS OF VERTEBRATE FLYERS

The discovery of the giant pterosaur Quetzalcoatlus northropi raised numerous questions regarding the performance limits of flying animals. The discovery of additional giant pterosaur material in the intervening 38 years has further fueled debate, and the size limits for vertebrate flight continue to be a contentious topic. Azhdarchid pterosaurs, such as Quetzalcoatlus, are particularly informative regarding the morphological features required for flying animals to reach large sizes.

Takeoff is a likely limiting factor for large flyers. We investigated the potential maximum size of an azhdarchid pterosaur, assuming that no major morphological changes occurred beyond expected allometry. Prior literature demonstrates that animal takeoff is typically ballistic, powered by leaping. We find that bipedal leaping would be unable to successfully power the launch of giant azhdarchids, even with unrealistically low mass estimates and liberal reconstruction of hindlimb musculature. Under a quadrupedal launch model, which has been previously supported by comparative long bone analysis, the maximum instantaneous power output is greatly elevated. However, a “quad launch” requires an extra upstroke phase that could be limiting. Using a wingbeat frequency scaling model that predicts wingbeat frequency in both birds and bats, we estimated the timing of an initial post-launch upstroke with a folded wing for Quetzalcoatlus. Our results indicate that Quetzalcoatlus would reach the apex of its ballistic phase in approximately 0.75 seconds, but it would only require about 0.20 seconds for the initial upstroke following push-off. As such, wingbeat timing does not seem to have been limiting for giant azhdarchids. Instead, we predict that power output would eventually be limiting even with quadrupedal launch, and that this limit would likely occur at around 280-290 kg for an azhdarchid body plan.

We find no support for the suggestion that there is a singular, global mass limit for flying animals. Instead, the limits of animal flight are morphology dependent. Our work on Quetzalcoatlus highlights several common misconceptions regarding animal takeoff, such as the persistent myth that heavy flying animals must utilize long runs to launch.