Searching for the Backstory of Airborne Giants

What was as tall as a giraffe, weighed a scant 550 pounds, and could fly? This is not a trick question. Around 68 million years ago, such creatures tromped and flew over what is now Texas. Paleontologists know this animal as Quetzalcoatlus northropi – a fantastically huge pterosaur that belonged to a group called azhdarchids. The biggest members of this leathery-winged lineage were the largest creatures ever to fly over our planet, and the increasing amount of information paleontologists have gleaned from their bones has repeatedly underscored a persistent riddle. How did these unusual flyers get to be so enormous?

Superlative size sparks our imagination and demands explanation. We’re constantly drawn to mysteries such as why there aren’t any gargantuan bony fish filtering krill from the seas as there had been in the Jurassic, and, of course, why the most massive land mammal of all time wasn’t even close to the prodigious size of dinosaurian titans.

Researchers often approach such puzzles through a sort of evolutionary reverse-engineering. In their attempts to unravel the how and why of giants, scientists search for adaptations and possible pressures from natural selection that would have pushed the evolution of large body size and required certain alterations to accommodate those changes. Looking at Quetzalcoatlus, we might expect that a a pterosaur with a 34-foot wingspan would have gained a foraging advantage in being able to range widely in search of food, and, as a corollary to such size, the soaring archosaur had to walk as a quadruped on land. But this is only half the story.

No one yet knows the reason why azhdarchid pterosaurs became so impressively huge, or if there even is an adaptationist explanation. But, as paleontologist and ancient flight expert Michael Habib explains in a recent review about the limits of airborne giants, we know that the quadrupedal stance of Quetzalcoatlus was a feature inherited from earlier pterosaurs and not an adaptation to cope with large size. As Habib points out, all trackways known for pterosaurs – even small ones – show that they moved on all fours while on the ground. A quadrupedal stance was a common, early characteristic of the group, Habib notes, and rather than being a consequence of large size, may have in fact allowed pterosaurs of Quetzalcoatlus proportions to evolve.

We can’t understand evolution without an appreciation of history. To consider any organism, living or fossil, outside of the lens of Deep Time is to intentionally blind ourselves to the differences between traits that are true, novel adaptations and those that are remnants of history and phylogeny. Not only do we need to consider “the functional abilities of organisms in relation to what is allowed by physical parameters,” Habib argues, but we must pair such a perspective with an understanding of the constraints on life through their evolutionary backstories.

Lessons researchers have gleaned from bird and bat flight don’t necessarily apply to the extinct pterosaurs. While there are some basic aerodynamic principles that apply to any flier, Habib points out, the way pterosaurs pole-vaulted themselves into the air and flew were distinct from any flying animal alive today. If we are to understand huge pterosaurs, we must see them as the unique creatures they were and not birds or bats writ large.

Consider takeoff. There is a certain point where an organism is too massive to fly. But there is no universal cutoff that can be applied. Constraints on takeoff and flight depend on anatomy, which is in turn influenced by evolutionary history.

Bustards, albatrosses, and wild turkeys of similar size – roundabout 48 pounds – are among the largest flying birds, Habib notes. And all these birds have different modes of getting into the air. Bustards take short runs, albatrosses must get a longer running start, and turkeys are capable of taking off from a standstill. In the case of turkeys, specifically, the birds have massive pectoral muscles, short wings, and strong legs for an initial push that allow them to take off much more directly than bustards or albatrosses. Nor is this arrangement unique to turkeys – quail and grouse share this suite of anatomical features. Their anatomy is constrained by their evolutionary relationships, so, as Habib writes, “Flight performance (or any other form of mechanical performance) is therefore necessarily constrained by phylogeny.”

So take this back to pterosaurs. Since there is no standard weight limit for takeoff, the anatomy and history of pterosaurs must be understood on their own merits to figure out how the flew and what evolutionary constraints they may have encountered. If birds can vary widely in the way they take off, then great care must be taken in using them as an analogy for the same behaviors in pterosaurs. And perhaps they shouldn’t be at all. Aside from major differences in the anatomy of bird and pterosaur wings, Habib points out that pterosaurs might have had large amounts of high-power types of muscle that, in terms of muscle physiology, made them quite different from birds.

In considering almost any aspect of giant pterosaur takeoff and flight, though, Habib warns against the adaptationist perspective of thinking that particular animals or lineages required certain characteristics to fly and therefore evolved what was necessary. Instead, events in the past opened up possibilities for the evolution of large flyers. Regarding takeoff, for example, Habib notes that a wide variety of large and small fliers – from flies to birds and pterosaurs – push off a substrate in some way to clear their wings and gain quick acceleration. “Therefore,” Habib writes, if this requirement of takeoff is as widespread as it appears then “it provides insight into the pre-requisite, chance aspects of form that must be present prior to the origin of powered flight.”

We still don’t know how and why Quetzalcoatlus and other immense azhdarchids got to be so enormous. But in order to solve those conundrums, Habib concludes, we need to change the way we think about these exceptional animals. Not only must we stop grouping all “flying vertebrates” together as if they were a monolithic group to which the same biomechanical rules apply, but it’s imperative that researchers understand the aspects of evolutionary history that simultaneously constrain and open evolutionary possibilities. To unravel the puzzle of the pterosaurs, we must appreciate how truly peculiar these ancient fliers were.

Top image from: Witton, M. Naish, D. 2008. A reappraisal of Azhdarchid pterosaur functional morphology and paleoecology. PLoS ONE 3, 5: e2271. doi:10.1371/journal.pone.0002271

References:

Habib, M. 2013. Constraining the air giants: Limits on size in flying animals as an example of constraint-based biomechanical theories of form. Biological Theory. DOI: 10.1007/s13752-013-0118-y