About those soaring giraffe-size pterosaurs

Pteranodon (c) Mark Witton. Pteranodon fossils present a compelling case for their flight abilities.

Paleontologist Mark Witton shares evidence that giant pterosaurs once flew, even continent-jumped, without much effort. His paper, co-authored by Michael Habib, is published in today’s PLoS One. In their view, which goes against convention, there is plenty of evidence that half-ton giant pterosaurs, as large as a giraffe, once soared through the skies and over the oceans at high speeds.

"Were giant pterosaurs old, flightless members of a species that could fly before they hit the flight-size limit and simply retained their flight anatomy as they grew under flightless conditions later? Possibly, but, again, the evidence doesn’t seem to suggest it."

Lots of particularly small things make me happy. I like the way that the blue light on my laptop charger doesn’t come on immediately when I plug it in. I like snoozing for five minutes after my alarm goes off in the morning. I like songs that have alternate single and double beats, and I really like those round, soft-plastic CD cases that you get with free with promotional CDs through the post every now and then. You know the ones? They fit CDs and DVDs so snugly: no excess packaging there, no sir. And the CDs look so happy inside them. They’re like socks for discs.

"Even a cursory glance at the Pteranodon skeleton shows it to be little more than a pair of wings with a head up front, a sure-fire indication of a flighted lifestyle if we ever saw one."

That’s not to say that big things aren’t great too, though. Big, honking car engines. Fireworks. King-size beds and massive, 3D SurroundoSoundavision IMAX screens: all big, all great. Average-sized things, though? Well, not so much. Fact is, it’s hard to be impressed with average-sized things. You can be content or satisfied with them, sure, but our constant drive to squeeze technology into packages no larger than postage-stamps or, conversely, construct the grandest structures possible shows on the whole, we’re most impressed by the biggest or smallest of all things, the desire for which depends on whatever object/form/bodily appendage we’re talking about.

"Ironically, the bigger species are often represented by such fragmentary remains that any guess to their size involves a hefty amount of extrapolation from smaller, more completely known critters."

Even scientists get dragged into our obsessions with big and small, doing crazy things like building the enormous microscopes to see the smallest things they can or smashing tiny particles together to make the biggest bangs (both cases satisfying our cravings for big and small things, I suppose). Palaeontologists are just as bad as the rest, frequently proclaiming to have found the biggest or smallest types of animals to have ever existed. However, while the tiny animals score points for cuteness, they don’t really capture the imagination as much as the giants. Ironically, the bigger species are often represented by such fragmentary remains that any guess to their size involves a hefty amount of extrapolation from smaller, more completely known critters. As such, some cleverclogs often follows the initial claims of monster size with a demonstration that Supersizeosaurus was more sensibly proportioned than originally proposed and, in actuality, it would not have generated earthquakes with every footstep nor eclipsed Belgium if it stood at full height. Still, the fact remains that the fossil record likely contains most of the largest animals to have ever lived, and the draw to find out not only how large they were, but also how they evolved their spectacular size is hard to ignore.

The largest animals to ever grace the skies, the pterosaurs, have not escaped this draw. A flying reptile spanning 7 m, the famously crested pterosaur Pteranodon, has been known for well over a century and is now very well represented from over 1200 specimens. 7 m is quite a wingspan, but even grander pterosaurs known as azhdarchids, estimated to span up to 13 m across the wings, were found in the 1970s, knocking Pteranodon into the runner up category in the size contest by some margin. Unlike Pteranodon, however, the largest azhdarchids are known from material so scrappy that you could fit the world stock of their remains on your lounge coffee table without much effort. They seem to represent the remains of flying animals, however, but working out exactly how these poorly known giants attained flight has been difficult. Where they particularly lightweight for their size compared to living animals? Were they super-specialists that required very specific environmental conditions to takeoff and fly? Was the very nature of the Earth itself different, either through reduced gravity or different atmospheric conditions?  Typically, pterosaurologists have favoured the former suggestion: giant pterosaurs, it’s been said, had to weigh virtually nothing for their size: 70 kg or so for a pterosaur around the size of a typical giraffe, for instance. More recently, this idea became unfashionable because several people have been pointing out how daft it is: you can’t take an animal as large as a giraffe and expect it to function with no more body mass than a large-ish pig. Even with a whole month of Sundays, that just ain’t going to happen. This has prompted controversial claims for another, not entirely silly idea: giant pterosaurs couldn’t fly at all.

Though most pterosaurologists would scoff at this idea, there’s actually been very little biomechanical work indicating that the largest pterosaurs could fly. In print, at least, pterosaurologists have just assumed that the giants could fly from the shapes of their bones. By contrast, in the last six years, three sets of authors have presented evidence suggesting otherwise. There’ve been indications that the largest pterosaurs would hit over half-a-tonne on the scales, making them far too heavy to fly. Two analyses of their takeoff mechanics have found that realistically weighted pterosaurs just can’t run or flap hard enough to reach the skies, and all three cases have applied a commendable approach to studying extinct animals: trying to understand their flight mechanics through modern representatives. Palaeontologists have been doing this for centuries, and it seems to stand up to scrutiny. Animals demonstrably converge on the same solutions to evolutionary problems, so the use of analogues is probably all right, at least to a point. In so, we can model the way birds take off and fly and then apply it to pterosaurs: their bauplans aren’t that different, right? What goes for one probably goes for the other, and plenty of large birds struggle a little to takeoff or sustain flight. Have you seen an albatross takeoff? Not much dignity there, that’s for sure. Accordingly, bird-based models have been constructed where the cap on pterosaur flight is the 5 -- 6 m wingspan mark, or between 40 –- 70 kg. Any pterosaurs bigger than this will be permanently grounded. Sounds reasonable, right?

Well, maybe not

"These studies suggesting giant pterosaurs were flightless seemed a little blinkered, not really factoring in the wealth of data that one might want to consider before declaring any pterosaur in their upper size range flightless."

My colleague at Chatham University, Michael Habib and I decided that this didn’t quite sit right with us. For one thing, these studies suggesting giant pterosaurs were flightless seemed a little blinkered, not really factoring in the wealth of data that one might want to consider before declaring any pterosaur in their upper size range flightless. Biomechanical models are good, but they’re only one form of evidence. Hence, we set out to investigate this idea from as many angles as possible, and the results of our studies have been published today in PLoS ONE. We investigated many components of giant pterosaur palaeobiology, trying to determine exactly how large the biggest pterosaurs were, what their anatomy and preservational environments indicated of their habits, compared their anatomy with that of birds and constructed our own model of their possible flight dynamics. The results of each investigative avenue suggested to us that the pterosaurs may have been grounded somewhat prematurely and, indeed, they were actually spectacular fliers, amongst the most fantastic the planet has ever seen. But don’t just takes these words for it; allow me to explain.

What giant pterosaur fossils tell us about their flight abilities

The first place to start, it seemed, was ensuring that we had a good idea of how ‘giant’ these giants actually were. After all, without knowing this, we can’t really assume that we have any handle on their mass and subsequent flight ability. Pteranodon is very comfortably known to measure around 7 m from wingtip to wingtip, but size estimates for the largest azhdarchids range between 10 and 13 m, a difference that equates into the latter being twice as heavy as the former. This massive inaccuracy stems from our incomplete knowledge of giant azhdarchid anatomy – we know of something like five cent of a giant azhdarchid skeleton, and even this meagre estimate would involve pooling their remains from around the world into one individual. This doesn’t put us in the best position to estimate their size but, doing the best we can with smaller pterosaurs as our guides, Mike and I did our damndest to constrain their size to a useable figure.

Illustration (c) Mark Witton.

We quickly found that we were quite happy with the 10 m span estimates, these being supported by scaling up different datasets from smaller, closely-related pterosaurs. This gave us a minimal value, but examination of the material alleged to represent larger animals suggests it’s been vastly over-egged. One fossil, alleged to represent a giant of 12 m, has been crushed and twisted to appear larger than it actually was; once this is corrected for, a 10 m span looks likely. Another, a single, incomplete neck bone nearly 80 cm long, was scaled to an animal with a 13 m wingspan without factoring in observations that most animal necks grow disproportionately quicker than the rest of their bodies: in short, a particularly long-necked azhdarchid does not equate to a particularly big wingspan. We cannot precisely span this second individual because the relationships between neck vertebra lengths and wingspans simply aren’t known, leaving us with our other bones that suggest 10 m (well, 10.4 to be precise) is the most reliable size estimate of the giant pterosaurs to date.

With this in mind, we turned our attention to trying to get a handle on the half-tonne estimates of azhdarchid mass. It turned out that these figures not only overestimated the wingspans of the largest-known azhdarchids by a metre or so, but also were proportionally incorrect in many other regards. The half-tonne azhdarchid, it was alleged, had a body almost 2 m long but, based on numerous other, smaller pterosaurs, Mike and I predict a body length of only 65 cm for even the biggest pterosaurs. Scaling the body back to this size brought the half-tonne mass tumbling down to more broadly accepted figures of 250 kg or so and, crucially, back into flight-possible realms (see below).

With their size more neatly constrained, we then asked ourselves whether we should be expecting the giants to fly in the first place. I mean, do the inferred muscle attachment sites and calculated bone strengths of their bones indicate that they could fly to begin with? Well, the arm bones of all giant pterosaurs bear the same anatomical hallmarks of flight as their smaller brethren, chiefly the enormous expansion of the areas of the arm bones responsible for anchoring the flight musculature and the elongation of their distal arm elements into wings. What’s more, the bone widths of giant pterosaur arm bones are enormous by any standards: the humeri (upper arm bones) of the biggest azhdarchids are as wide as those of modern hippos of giraffes. Thing is, even the grandest azhdarchids were featherweights compared to these guys, suggesting their bones were far bigger than a purely terrestrial animal of their mass would need. Granted, azhdarchid arm bones would be a little weaker through being hollow (like many birds, pterosaurs inflated their skeletons with air), but, even so, the robustness of their bones far exceeds expectations of terrestrial animals of their size. The plot thickens further when we find that pterosaur arm bones and muscle attachment sites become much broader as they get bigger, a trend that isn’t reflect in birds: avian bones become slightly more slender as they grow bigger. It seems most parsimonious, then, to assume that the giant pterosaur skeleton was evolved to cope with some enormous stresses and strain, and the most likely cause of these forces, given the rest of their anatomy, is flight.

"Pteranodon fossils present their own compelling case for flighted abilities. Thousands of Pteranodon have been found 200 km out into an enormous seaway and, with apparently limited swimming capabilities and no indication that so many specimens drifted out to sea from local shorelines, it seems likely Pteranodon flew there."

But is there another interpretation that can be applied here? Were giant pterosaurs old, flightless members of a species that could fly before they hit the flight-size limit and simply retained their flight anatomy as they grew under flightless conditions later? Possibly, but, again, the evidence doesn’t seem to suggest it. Previous work on the terrestrial capabilities of azhdarchid pterosaurs seems to suggest they were quite savvy on the ground: they were amongst the most proficient walkers and runners of all pterosaurs, in fact, so they could have lived on the ground without too much worry. Indeed, their fossils are frequently found in sediments deposited in continental environments, too, so they were clearly frequenting terrestrial habitats anyway. However, these traits don’t necessarily preclude flying. Obviously, plenty of birds and bats are quite happy to live in terrestrial environments and can walk and run very well, so the fact azhdarchids appear similarly adapted doesn’t really tell us anything. Moreover, it’s worth noting how much bigger the largest azhdarchids are compared to the alleged size-limits for flight: they would have retained and developed all of their flight anatomy and under a mechanically appropriate scaling regime whilst growing to twice the wingspan and five-times the mass of our size-limits on flight. All that bone and muscle is physiologically expensive to develop and maintain, and it seems a little odd that it would remain so prominent in animals that hadn’t flown since they were half their adult size.

The same is true, though to a lesser extent, for Pteranodon, though it’s only surpassed the alleged flight limits by a metre or two. However, Pteranodon fossils present their own compelling case for flighted abilities. Thousands of Pteranodon have been found 200 km out into an enormous seaway and, with apparently limited swimming capabilities and no indication that so many specimens drifted out to sea from local shorelines, it seems likely Pteranodon flew there. Plus, the arms of Pteranodon are much longer than its legs, so much so, in fact, that they almost certainly hindered walking and running. We have good evidence that grounded pterosaurs moved on all fours, even the oddly proportioned Pteranodon, suggesting Pteranodon was ill-suited to a life on the ground. Conversely, even a cursory glance at the Pteranodon skeleton shows it to be little more than a pair of wings with a head up front, a sure-fire indication of a flighted lifestyle if we ever saw one.

Birds and pterosaurs: uncomfortable mechanical bedfellows

Airquetz illustraton (c) Mark Witton

OK, so everything about giant pterosaur fossils indicates that giant pterosaurs could fly, but those flight studies can’t get them to take off? We know such studies can accurately model the flight of modern flying animals, so what gives? It seems to us that we’ve been stretching the bird-pterosaur analogue much too far: sure, pterosaurs and birds are successfully comparable to a point, perhaps as much as birds can be compared to bats, but the analogy is far from perfect. For one thing, bird and pterosaur anatomy is fundamentally distinct: they have different skeletal proportions, different soft-tissues making up their flight surfaces (membranes vs. feathers), different numbers of joints in their wings and different flight muscle structure: packaged together, there will be enough contrasts between these packages to make a real difference to their flight mechanics. And most importantly, it seems almost certain that these differences dictated that birds and pterosaurs attained flight in drastically different ways.

With their arms entirely tied into being wings, birds have to takeoff using their legs alone: their arms simply cannot be used in any great way to shove them off the ground. Hence, a powerful leap or perhaps a run is required to get birds airborne, and their hindlimb bones reflect this. As flighted birds get larger, their leg bones become much more massive than the rest of their skeletons to accommodate the greater musculature necessary for these endeavours. Previous work by Mike has shown that our leathery-winged friends didn’t follow this pattern, however, their legs are hardly wimpy, but their strength and scaling rates are entirely eclipsed by the massive size and scaling regimes of pterosaur arms. Mike interpreted this as an indication that pterosaurs and birds were emphasising different limb sets for takeoff: birds prioritise their hindlimbs for takeoff, and pterosaurs favour their forelimbs (wondering what this may be like? (Check out this for more information). This crazy-sounding idea isn’t actually as nuts as it seems. Several bats, including vampires, use the same forelimb-assisted takeoff and, crucially, it’s a more powerful launch mechanism than the legs-alone approach. Because the main thrust is derived from the flight muscles (the largest, most developed muscles on the body of an actively flying animal), this ‘quadrupedal launch’ has much greater potential for launching heavier animals into the air. With this in mind, there suddenly seems clarity as to why previous pterosaur workers have been unable to launch their giant pterosaurs -- they’ve probably been approaching the launch mechanism from the wrong end.

The bit that gives me goosebumps

Being capable of becoming airborne is only part of the story, however. What if the giants were like turkeys, able to launch but, shortly after, tiring and falling back to earth? Hence, Mike and I wanted to know what the largest giants were capable of in the air. Because of their size, flapping would be a lot of work for a giant pterosaur and, like many modern large birds and bats, they almost certainly combined flapping with periods of soaring, along with exploiting updrafts of any kind where they could. However, to find that source of lift, our giants would have to fly under their own steam. Our flight model of a 10 m span azhdarchid, assuming 50 kg of anaerobically-enhanced flight muscle, an ultra-efficient through-flow lung system, a hide covered in fuzz to keep their engines warm and around 10 m2 of flight membrane predicted the largest azhdarchids could pound along at a staggering 120 kph (75 mph) for around two minutes before having to resort to gliding, after which they would cruise at a more sedate 90 kph (56 mph). Given how high they could potentially climb with their efficient lungs and ability to keep themselves warm, we imagine giant azhdarchids were capable of continent hopping, animals that could wake up a thousand miles away from the place they would spend the following evening. They could cross vast tracts of the globe in a few days should they choose to. With those abilities, oceans, mountain ranges and deserts would mean nothing to these guys. As such, far from being flightless, giant pterosaurs would be some of the most spectacular flying animals ever -- high-octane, high-speed aeronauts that pole-vaulted into the air before screaming off at speeds warranting a speeding ticket. There should be pictures of these things next to the word ‘awesome’ in the dictionary. Similar things could probably be said of other giant pterosaurs like Pteranodon, though Mike and I noted that Pteranodon probably flew in a different way to the giant azhdarchids because of its differing proportions. Relatively speaking, the narrow wings of Pteranodon may have made it the faster flier and been more suited for oceanic soaring, for instance, though we didn’t model the flight of this pterosaur specifically in our study.

"There are no convincing arguments to say that giant pterosaurs, nor any pterosaurs at all, for that matter, were flightless."

But here’s the best bit

So, in our view, there are no convincing arguments to say that giant pterosaurs, nor any pterosaurs at all, for that matter, were flightless. That’s not to say that some pterosaurs could not have abandoned flight. They appear to be perfectly happy when moving around on the ground and, theoretically, there’s no reason why they shouldn’t have settled into a purely terrestrial existence. If they did so this, though, there’s no evidence for it yet.

For the time being, though, we probably need to abandon the big idea of flightlessness in pterosaurs. What we can do, however, is acknowledge how great it is that we’re actually talking about these sorts of things in the first place: pterosaur research spent much of the 20^th century in the doldrums and, until the 1980s, we really didn’t develop our understandings of these animals very far at all. The fact that we’re now chucking ideas around like flightlessness, quadrupedal launching and similar jazz suggests that we’re much happier to think about pterosaurs progressively than we have been previously and, with a bit of luck, this open-mindedness will only serve to help understand them better. And on that note, it’s probably time to close this rather giant essay and, being Saturday night, a trip to the pub is probably in order. They may even be playing music with alternating single and double beats. That would make my night, that would.

Full reference for the PLoS ONE article:

Witton, M. P. and Habib, M. B. 2010. On the Size and Flight Diversity of Giant Pterosaurs, the Use of Birds as Pterosaur Analogues and Comments on Pterosaur Flightlessness. PLoS ONE, 5, e13982.

Mark Witton, PhD, is a paleontologist and paleo-artist with the Palaeobiology Research Group, University of Portsmouth, England. He contributes to Pterosaur.Net and Pterosaur.Net blog. More of his pterosaur artwork can be found at www.flickr.com/photos/markwitton. 

Other Flesh & Stone articles on Witton's work:

• Diminutive version of T. rex found

• Knowing a dinosaur's true colors doesn’t take fun away for paleoartists

• Raising the curtain on a new pterosaur

• Finding Lacusovagus by Mark Witton