National Geographic

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.

A reconstruction of Quetzalcoatlus. Image by Piotrus distributed under Multi-license with GFDL and Creative Commons CC-BY-SA-2.5 and older versions (2.0 and 1.0).

A reconstruction of Quetzalcoatlus. Image by Piotrus distributed under Multi-license with GFDL and Creative Commons CC-BY-SA-2.5 and older versions (2.0 and 1.0).

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

There are 17 Comments. Add Yours.

  1. David Peters
    August 30, 2013

    Not all pterosaur tracks indicate quadrupedal activity.

    Kim JY, Lockley MG, Kim KS, Seo SJ and Lim JD 2012. Enigmatic Giant Pterosaur Tracks and Associated Ichnofauna from the Cretaceous of Korea: Implication for the Bipedal Locomotion of Pterosaurs. Ichnos 19 (1-2): 50-65.DOI:10.1080/10420940.2011.625779 online

    The above bipedal pterosaur track makers were likely taller than humans.

    Peters D 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification. Ichnos 18(2):114-141. http://dx.doi.org/10.1080/10420940.2011.573605

    Includes several smaller tracks without manus impressions matched to anurognathid pedes.

    Lee YN, Azuma Y, Lee H-J, Shibata M, Lu J 2009. The first pterosaur trackways from Japan. Cretaceous Research 31, 263–267.

    This trackway starts off bipedal then the hands are put down.

  2. khots
    September 1, 2013

    from what the reconstruction photo of the pterosaur is the few bones that made its long neck.
    cranes and storkes curl their necks when flying. the bones on this structure clearly wouldn’t do it, right? the neck doesn’t seem anything close to those of giraffes and apatosaurus too…
    how does a neck like this work? both on land, and during flight?

  3. Rooony
    September 3, 2013

    Interesting !

  4. Andy Bough
    September 5, 2013

    I understand that many flying insects in the carboniferous forests grew to enormous sizes due to an increased oxygen level in the atmosphere. Could this also explain the size of pterosaurs?

  5. Jonny O
    September 8, 2013

    DUH! What is so hard to understand??? That we in the modern world cannot comprehend that paleoworlds were vastly different than ours? Perhaps these big pterosaurs inhabited areas where updrafts were superstrong – in response to the hothouse climate of the Late Cretaceous? Even today, animals have evolved to thrive on their local landscape/conditions: the South American anteater is perfectly adapted to eat what is the most readily available source in its habitat – anthills and their associated inhabitants, ants. People have to understand the realities of fossil critters.

  6. Guillermo
    September 8, 2013

    I think that the supersaurus its just as fully grown Diplodocus. If it was so big then how come there arent other bones? Being bigger would mean that its bones would had a bigger chance of preservation no?

    It seems to me like as soon as they find a new dinosaur they just claim its a new species and give it a fancy name.

  7. David Bump
    September 8, 2013

    I’m not sure I see the value of considering deep time and evolutionary history in this. Seems more like fitting the known aspects to what the current evolutionary history seems to be — the history doesn’t reveal answers to us, it just dictates how we shape our answers. If all the tracks had been bipedal, then we might figure at some time in the line leading to the giants, a quadrupedal stance would probably have developed to support the weight. Lack of quadrupedal tracks wouldn’t have been a problem for the notoriously spotty fossil record. There are some interesting related mysteries: There’s a giant flying bird, Argentavis, that’s dated to just yesterday by comparison: http://avian101.wordpress.com/2011/03/25/argentavis-magnificens/
    It’s not too clear just how or when the larger, short-tailed pterosaurs evolved from the smaller, long-tailed earlier forms, and they appear in an amazing variety of diverse forms, including the truly bizarre filter-feeding Pterodaustro: http://www.dinosaurcentral.com/rr_pterodaustro.php
    Of course, the biggest mystery about pterosaurs is where they came from in the first place, as nothing looks anything like a reptile starting to glide with the aid of an enlarged digit. Curiously, in the Triassic rocks where we might expect to find them, we do find a number of gliding lizards using several different designs.
    Oh yes, and finally, the pterosaurs mysteriously died out with the dinosaurs, giant aquatic reptiles, ammonites, and many forms of Mesozoic birds and mammals. Several disasters have been fingered as the culprit, especially the Chicxulub impact, but why so many creatures, in such broad classifications, with such diverse characteristics, went extinct all over the world, while others survived, is still a big question.

    • David Peters
      September 27, 2013

      for David Bump, the origin of pterosaurs can be found if you Google: Cosesaurus. Follow the links and you’ll see the several origins of pterosaurs with short tails, all by convergence.

      • David Bump
        September 29, 2013

        Thanks, David Peters, I already knew about Cosesaurus, which doesn’t appear to be close to gliding. I did discover that Longisquama has been given an extended finger with an associated membrane, but that bizarre creature with plume-like structures attached to its back was clearly following a different drummer. Likewise with Sharovipteryx, which also gets thrown into the line-up, but was using its extremely long hind legs to support its main gliding surface. All of these are middle to late Triassic, and in late Triassic strata, fully developed pterosaurs also appear. I’d have to say the story of how pterosaurs got here is not in the known fossil record.

        • David Peters
          September 29, 2013

          Indeed, Cosesaurus was flapping, not gliding, as indicated by the stem-like coracoid and sternal complex (clavicles + inter clavicle + sternum). Longisquama, Sharovipteryx and pterosaurs were essentially 3 different directions cosesaur descendants evolved, with pterosaurs adding the long legs of Sharovipteryx without reducing the forelimbs and also adding the long fourth finger of Longisquama without adding the long torso, plumes and long fingers 1-3 of Longisquama. More at pterosaurheresies.

  8. David Bump
    September 8, 2013

    PS You’re probably aware of this, but there are gliding lizards living today: http://lazy-lizard-tales.blogspot.com/2012/01/flight-of-dragons.html

  9. Carlos
    September 10, 2013

    @Johnny: There’s about three things wrong with your statement:

    – The terminal Cretaceous wasn’t a hothouse world

    – Giant azhdarchids inhabitted prairie environs, not notable for strong updrafts or weird take off conditions

    – Giant anteaters didn’t evolve in respond to termites, they are descended from forest anteaters like tamanduas.

  10. philip.parsons
    September 11, 2013

    Jonny O. I’ve not often met the word DOH in scientific papers, but lets leave that for a moment. Updrafts! If you’re going to use an updraft, you need to be ‘up’ first. How do you get into an updraft when you’re on the floor? Perhaps the latest ridiculous idea that pterosaurs could ‘leap’ using all four limbs to attain instant flying speed would do it?
    Who said the late Cretaceous was a ‘hothouse’?
    Finally, flying is not a minor adaptation such as diet. It is a major adaptation, strictly controlled by the laws of physics. Quetzelcoatlas could not fly in the modern sense of the word – unless someone comes up with a radical solution. A beast the size of a giraffe which apparently only weighs the same as a large dog? I don’t think so!

  11. Rob Veltman
    September 12, 2013

    Just a short note to khots.. Storks and cranes fly with their necks stretched out. Heron curl up their necks when flying ;-)

  12. p.parsons
    September 27, 2013

    Quite right Rob Veltman. But why the difference? Look at the weight/moment of the legs against the weight/moment of the beak/head. Its all about keeping the Centre of Gravity (cg) under the centre of lift for balance. The old adage for model aircraft builders is:
    “If it’s nose heavy – it won’t fly very well; if it’s tail heavy – it won’t fly”
    Now look at some of the diagrams of Qetzalcoatlas and tell me if you think that is balanced!

  13. Carlos
    October 22, 2013

    I’m not going to make assumptions, but suffice to say that “arguments” akin to yours have defuted countless times in Tetrapod Zoology:

    – How is quadrupedal launching a ridiculous idea? It’s mechanics have been throughly explained, to the point that the only contestors have admited to not read Habib’s paper.

    – I guess you’ve never heard of cranes, flamingoes and countless other birds with long necks, then.

    – Quetzalcoatlus bears hallmarks of extensive flight musculature. To say it was just a glider is utterly ridiculous

  14. Carlos
    October 22, 2013

    I’m not going to make assumptions, but suffice to say that “arguments” akin to yours have defuted countless times in Tetrapod Zoology:

    – How is quadrupedal launching a ridiculous idea? It’s mechanics have been throughly explained, to the point that the only contestors have admited to not read Habib’s paper.

    – I guess you’ve never heard of cranes, flamingoes and countless other birds with long necks, then.

    – Quetzalcoatlus bears hallmarks of extensive flight musculature. To say it was just a glider is utterly ridiculous

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