Why we think giant pterosaurs could fly

Every so often the idea of flightless giant pterosaurs circulates in the press or on social media. It doesn't take much to ignite these discussions: a new giant pterosaur fossil, a PR event from a museum, or simply artwork emphasising the size of giant flying reptiles will see someone, somewhere, questioning their flightworthiness. These suggestions are often made with strong conviction, to the extent of dismissing or even arguing with scientists who study pterosaur anatomy and biomechanics. After all, how can any sensible individual think that animals with 10 m wingspans and body masses hovering around 250 kg were capable of flight? At most they were gliders, or flighted as juveniles and flightless as adults, right?

Confession time: as someone actively involved in research and outreach on these animals, I often find these discussions frustrating, for two reasons. The first is that, among those who actually study pterosaur functional morphology – that is, those who make detailed observations and measurements of pterosaur fossils, compile biomechanical data and use computer modelling to objectively test their flight capacity – there is no controversy about the volant nature of these animals, even at their maximum size. Peer-reviewed claims that pterosaurs were flightless are genuinely rare (perhaps limited to Sato et al. 2009; Henderson 2010 and Prentice et al. 2011 in the last decade) and have a consistent record of being flawed on some critical anatomical or functional detail (Witton and Habib 2010). There is no debate about giant pterosaur flight among those of us who study their fossils: the press and social media fuss about the topic is a genuine palaeontological nontroversy.

The second source of frustration is that, away from technical literature, discussions of giant pterosaur flight frequently suffer from major cases of Dunning-Kruger effect, especially when parties have knowledge of planes. I've experienced this a lot in my career, and not just in the wilds of social media: many of my TV and film consultancy jobs have required defending basic tenets of pterosaur anatomy - even their basic, there-for-all-to-see proportions preserved in articulated fossils - to folks who just can't or won't believe what the fossils show. Having a casual understanding of engine-driven man-made flying machines does not equate to knowing all there is about everything that has ever flown, but you would not know this from some conversations.

Our controversial giant azhdarchid friends Arambourgiania philadelphiae (middle) and Hatzegopteryx thambema (right), compared to a record-breaking giraffe and the Disacknowledgement.

Whether through naivety of palaeontological theory or unwillingness to accept good data, the lack of accessible overviews of current thinking on giant pterosaur flight probably fuels this ongoing nontroversy. What's needed, it seems, is a synthesis of modern ideas on giant pterosaur flight and justification for why pterosaur experts don't challenge this idea. I've attempted this below, including sufficient methodological detail and references so that anyone wanting to understand these ideas will have a useful jumping-off point, and to establish what needs to be overturned to challenge the null hypothesis of giant pterosaur locomotion. Our focus will be on the largest of all pterosaurs, giant members of the clade Azhdarchidae, as they are the main focus of most flightless claims (above). They are also among the most familiar giant pterosaurs, their number including species such as Quetzalcoatlusand Hatzegopteryx. The general points made below pertain to all large pterosaurs, however.

Giant azhdarchids are invariably known from scant remains, sometimes a handful of fragments representing bones from across the skeleton or, in the case of Quetzalcoatlus northropi, an incomplete left wing (e.g. Lawson 1975; Frey and Martill 1996; Buffetaut et al. 2002; Vremir 2010; Martill and Moser 2018). Ordinarily, fragmentary remains are a barrier to interpreting the locomotory strategies of extinct organisms but flighted lifestyles adapt animal bodies to such an extreme degree that just a few bones can betray volant habits. It’s evident that even the largest pterosaurs bore wing anatomy comparable to their smaller, incontrovertibly flightworthy relatives. Although no complete giant wings are known, our fragments indicate similar linear forelimb bone proportions to smaller azhdarchids. Their wing joints – including details of their elbows, wrists and wing finger knuckle – are well understood, and indicate typical properties of pterosaur wing motion and function. We can make a number of predictions concerning muscle extent for giant taxa, the most important being related to the presence of a huge deltopectoral crest on their humeri. This broad flange of bone, situated at the proximal end of the humerus, anchored many muscles running from the shoulder to the wing and powered flapping motions in flight (Bennett 2003), so is a clear correlate for powered flight in giant species. The seemingly-small deltopectoral crest on the Hatzegopteryx humerus is sometimes raised as evidence of reduced flight ability: it’s actually just badly preserved with lots of bone missing on all margins (Buffetaut et al. 2002).

A collection of giant azhdarchid bones: A, cervical vertebra of Arambourgiania philadelphiae; B, humerus of Quetzalcoatlus northropi, C-D, the rather broken proximal humerus of Hatzegopteryx thambema. Other than their huge size (scale bars represent 100 mm) and some details of robustness, these bones are identical to those of smaller, incontrovertibly flying pterosaurs.

The only significant difference between the wings of giant and smaller azhdarchids concerns bone robustness, especially that of their joints. We would predict expanded wing bone diameters in large fliers as they enhance resistance to bending, and as giant animals they are going to experience proportionally greater bending stresses. If they flew, giant pterosaurs should have very large wing bones indeed, and - as is evident from the adjacent images - this is exactly what we find. Witton and Habib (2010) noted that the humeral shafts of giant azhdarchids are comparable in diameter to those of giant mammals, like hippos, despite the pterosaurs being a fraction of their weight (Witton 2008; Henderson 2010). Giant azhdarchid wing bones perform exceptionally well in bending strength tests, being able to resist multiple bodyweights before failing (Witton and Habib 2010), and their expanded diameters maintaining relative failure levels comparable to those of small or mid-sized pterosaurs, despite their size (Witton et al., in prep).

Pterosaur humeral scaling: as pterosaurs got bigger, their wing bones and joints expanded disproportionately to accommodate greater stresses incurred in flight and launch. Image from Witton (2013).

But for all their expansion, giant azhdarchid wings retain exceptionally thin bone walls. Even in Hatzegopteryx, the most robust of the group, they’re only 4-7 mm thick. This is a high value for a pterosaur, but still (in relation to bone diameter) at the low end of the cortical thickness spectrum. Other giant species have cortices of just 2 mm or so, values only just slightly larger than those of mid-sized pterosaurs. Expanded but bone-lite wings are another feature of flying creatures, being common to most flying birds and all large pterosaurs, and offering their owners a lightweight but bending-resistant flight skeleton. However, what optimises these skeletons for flight compromises resistance to buckling forces, which is why most non-volant animals tend to have much thicker cortices. The correlation between thin bone walls and flight is not watertight (Hutchinson 2001) but it's a feature we would predict for any seriously large flying animal, and is thus consistent with volant habits in giant pterosaur species.

It’s often asked how animals as large as the biggest azhdarchids could attain and sustain flight. It’s important to stress that no-one imagines giant azhdarchids as breezy fliers flitting around Cretaceous plains like busy songbirds. As animals operating close to the size limits of flight for the azhdarchoid bauplan (Marden 1994; Habib and Cunningham pers. comm. in Witton 2010; Habib 2013) we should assume a flight frequency comparable to our largest modern fliers – creatures like bustards, geese, swans, albatrosses and so forth. Though different in flight mechanics these birds are united in their relatively low launch frequencies, taking to the air when they must (such as to evade danger) or when they have long distances to travel. Launch is very energy-demanding because of their great body masses, and in some cases specific environmental conditions are needed (such as space for taxiing in albatross), limiting their options for frequent takeoff. We should assume the same was true for large azhdarchids: their functional morphology and trackways show strong terrestrial abilities (Hwang et al. 2002; Witton and Naish 2008, 2013) and they probably spent a lot of time grounded, only flying when harassed, or wanting to move far and fast.

When imagining giant pterosaurs flying, we need to have birds like the kori bustard in mind: large, powerful animals which are strong fliers, but unable to flit about the sky like small songbirds or bats. When these guys take off, they mean it. Photo by Arnstein Rønning, from Wikimedia, CC BY 3.0.

Indeed, in all likelihood giant pterosaurs couldn’t launch every few moments. Flying animals tend to allocate about 20-25% of their body mass to flight musculature, which gives our large azhdarchids 50 kg or so of flight muscle to use in launch and flight (Paul 2002; Marden 1994). Even so, models of muscle energy availability show that giant pterosaurs could not launch aerobically (that is, using muscle contractions supplied with oxygen) and they had to rely on stronger, but less endurable, anaerobic muscle contractions. Anaerobic muscle power is essential to launch in the largest birds and almost certainly played a role in extinct giant insect flight, too (Marden 1994, see graph below), so its inferred use in giant pterosaurs is quite plausible. This reliance on anaerobic muscle power would necessitate resting periods between launches (hence the inability to launch continuously like a small flyer) as well as after vigorous bouts of flapping. Witton and Habib (2010) predicted that the hard flapping window for a giant azhdarchid was about 90 seconds, after which a rest was needed. So, does that limit our giants to turkey-like burst flights?

Launch for giant azhdarchids - like Quetzalcoatlus northropi - would be no more challenging than it is for large birds. The dotted line on this graph represents the minimum muscle energy output needed for flight. Using the same mechanism of anaerobic muscle power as large living fliers, giant azhdarchids are on the right side of that line. From Marden (1994).

Probably not. One of the world's leading experts on animal flight, Mike Habib, found that Colin Pennycuick’s freeware Flightprogramme – software designed to model bird flight - can be easily modified to predict pterosaur gliding and soaring capabilities, even accounting for the differences between feathered and membranous wings (see Witton and Habib 2010 for this methodology). Using this software, Mike and I predicted that giant azhdarchids were supreme soarers, easily able to sustain long-distance gliding even at body masses of 180-250 kg (Witton and Habib 2010). Predicted giant flight velocities exceeded 90 kph and, in that 90 second flapping burst, giant azhdarchids would cover several kilometres - plenty of distance to seek areas of uplift such as deflected winds or thermals. Having located these, azhdarchids could easily adopt energy-saving soaring to recover their flight muscles, their glide ratios being consistent with those of large soaring birds such as storks, Procellariiformes and raptors. Mike has presented calculations that these giants would have sufficient on-board energy resources to travel the planet, their speed and flight range being sufficient to ignore most geographical barriers. Note that these models assume modern day parameters of atmospheric density and gravity: we do not need to modify these to keep giant azhdarchids airborne. Sure, if you did change these parameters you might make the job easier, but the giants already have very strong flight performance without it. If you don't buy this, remember that you can play around with Flight yourself: download the program, get the method and pterosaur parameters from our open access paper and go at it. None of the science behind these animals is mystical - the methods are entirely conventional and repeatable.

For all this talk of modelling pterosaur flight at quarter-tonne masses, two sets of authors have proposed that giant pterosaurs were simply too heavy to attain flight. Sato et al. (2009) based this on their understanding of procellariiform takeoff, modelling a maximum possible volant mass of 40 kg for these birds and assuming the same limit must apply to pterosaurs. The next year, Don Henderson (2010) compiled a series of volumetric estimates of pterosaur mass including a 450 kg Quetzalcoatlus. Don – probably correctly – assumed that such an animal would be too heavy to fly.

Mike and I addressed both these proposals in a 2010 publication about giant pterosaur flight. On Sato et al. (2009), we found numerous problems with the overt biomechanical links drawn between bird and pterosaur flight. Avian and pterosaur anatomy is comparable enough to assume some broad analogies in wing shape and flight styles (e.g. Hazlehurst and Rayner 1992), but the detailed kinematics of flight – including launch – are too distinct to assume that the size limits of one group apply to the other. There are reasons to think pterosaurs launched in a very different way to birds (see below) and were subject to a different set of scaling regimes and size limits (Habib 2008, 2013; Witton and Habib 2010). Sato et al. (2009) may have predicted a flight mass limit for long-winged, dynamically soaring birds, but the application of this limit to flying reptiles is not supported by our understanding of pterosaur and avian biomechanics.

We found a much simpler issue with Don Henderson’s half-tonne Quetzalcoatlus model: its body was simply too large. Don based his work on a silhouette in Wellnhofer’s (1991) pterosaur encyclopaedia, a reasonable decision given the paucity of reconstructions of this animal at the time, but ultimately a problematic one for making accurate mass estimations. Azhdarchoids were relatively poorly known in the early 1990s and Wellnhofer’s silhouette reflects this, being a mostly imaginary pterosaur only accurate in wingspan. Crucially, its body is monstrously oversized at 1.5 m long. Complete azhdarchoids discovered since this time, including that of the azhdarchid Zhejiangopterus, have shoulder-hip lengths only 30-50 % longer than their humeri (above) and - in lieu of giant pterosaur torso fossils - we have to assume this was true for the giants, too. The 544 mm long humerus of Q. northropi translates to a predicted torso length of just c. 750 mm – a fraction of the size used in Don’s estimate. Mike and I adjusted Don's calculations to a more reasonable body proportion and, presto, the predicted mass was in the more familiar quarter-tonne range, a value flight models are happy to see launching and soaring without difficulty (Witton and Habib 2010).

A critical hypothesis for giant pterosaur flight concerns recent interpretations of their launch strategy. This idea is that pterosaurs – probably all of them – took off from a quadrupedal start, not a bird-like bipedal one. The quad-launch hypothesis has origins in technical literature dating back to 2008 (Habib 2008, 2013, Witton and Habib 2010) but has a longer history through Mike H’s and Jim Cunningham’s contributions to the Dinosaur Mailing List. In retrospect, quad launch can be seen as a unifying hypothesis in studies of giant pterosaur flight, the piece of the jigsaw that allowed us to see how data from comparative anatomy, body masses and relative bone strength fit together. Before quad-launch, pterosaur flight models struggled to transfer giant azhdarchids from the ground to the air and were forced to cap their body masses at unrealistically low values (e.g. 75 kg in Chatterjee and Templin 2004) in order to launch them like big birds. Other than the fact that masses of 75 kg are untenable for creatures the size of giraffes (they’d need to be something like 70-80% air; Witton 2008), bipedal launch models suffer from several biomechanical issues involving bone strength, limb bone scaling and muscle size, as well as inconsistencies concerning pterosaur gaits. It's these issues which Mike and Jim investigated in their studies, making them the first researchers to approach pterosaur launch with objectivity, rather than a priori assuming an avian launch model, and bending pterosaur palaeobiology until it fit.

Ratios of limb bone strength in birds and pterosaurs. Positive values trend towards strength in humeri vs. femora, while negative values skew towards stronger femora vs. humeri. I've left the caption on for greater explanation. From Habib (2008).

Let's unpack these points in a little more detail. Firstly, the main launch limbs of flying animals are - above body masses of 500 g - stronger than their non-launching counterparts, and scale with more pronounced positive allometry (Habib 2008). This reflects launch being the most demanding part of flight. Look closely at a launching animal (high speed video helps) and you'll see that flight does not begin with a flap, but a leap: something like 80-90 % of launch effort stems from a powerful jump initiated by the main launch limbs. This explains why birds have proportionally robust and strong hindlimb skeletons but relatively slender wing bones: as they increase in size, their legs must become proportionally stronger to initiate flight at greater masses (Habib 2008). Pterosaurs, in contrast, show the opposite condition: their forelimbs are larger and stronger than their legs, with this relationship increasingly pronounced in larger species. Mike's 2008 study quantified this distinction, showing that the section modulus - a value proportionate to the strength of a given cross section - is consistently larger in pterosaur humeri than femora, and vice versa in birds, and that these ratios are more extreme at larger body sizes (see diagram, above). We presented similar data highlighting distinction in limb bone scaling in our 2010 paper, as well as quantifying the relative weakness of azhdarchid femora - their neck vertebrae are actually stronger than this major limb bone. Their humeri, in contrast, were very strong even in giant taxa modelled at the upper limits of pterosaur mass estimates (below). This is already a strong sign of a forelimb-dominated launch strategy in giants, and there's more to consider yet.

Raw data on azhdarchid limb bone strength from Witton and Habib (2010). Note the 'avian expectation' column - pterosaur bones do not scale in the same manner as bird bones, indicating a different regime of biomechanical selection pressures, and thus different limits on parameters like size.

Secondly, the avian skeleton has two large girdles for limb muscles: an enlarged shoulder and chest region for flight muscles, and an enhanced pelvic region to anchor those powerful hindlimb launch muscles. Pterosaurs, in contrast, have only one large limb girdle - their shoulders, making this the de facto likely candidate for powering their launch cycles. Using volumetric modelling, Paul (2002) predicted that a giant azhdarchid would have space for 50 kg of muscle in their pectoral region, a value appropriate for initiating flight in 200-250 kg animals (Marden 1994). This strategy is a far more economical use of muscle mass because the same muscles that power flight can also initiate launch, thus allowing quad launchers to have smaller torsos - and thus lower masses - than bipedal launchers. For all their power, the moment birds have launched their legs are effectively useless - they're just dead weight to be hauled around until it's time to land.

Reconstructed skeletons of large and giant azhdarchids in quad-launch poses, from Naish and Witton (2017). Note how the nearly completely known Quetzalcoatlus sp. - D - E - lacks a large site for hindlimb muscles - that's typical of all pterosaurs, and an important argument in favour of quad launch.

This is an critical point for giant pterosaur flight as it allows us to make hypotheses about body size maximums related to launch strategy. Because quad launch is a mass-efficient route to flight we can hypothesise that quad launchers could attain much larger overall sizes and masses than bipedal launchers (Witton and Habib 2010; Habib 2013). As everyone knows, this is borne out in our fossil record of volant birds, which max out at 5-6 m wingspans and masses of 22-40 kg (Ksepka 2014), while giant azhdarchids attained wingspans of 10 m and 200-250 kg body masses. Mass-efficient launch mechanics is almost certainly a major factor in how azhdarchids became so big, especially combined with the exceptional azhdarchoid ability for skeletal pneumaticity (Claessens et al. 2009).
A further line of evidence for quad launch concerns pterosaur trackways. Habib (2008) also notes that launch in living tetrapod fliers correlates to terrestrial gait: the number of limbs used to locomote on the ground is the same as the number used to take-off. Birds walk and launch with two legs, while bats walk and launch using all four. An extensive record of pterosaur trackways shows that pterosaurs were quadrupedal animals like bats, and it stands to reason that they also launched from four limbs: they would contrast with our living fliers if they had to shift gaits to take off. Our pterosaur footprint record includes trackways of quadrupedal giant pterosaurs (Hwang et al. 2002), so we can comfortably extend this observation to them, too. Incidentally, the fact that several bats take off quadrupedally is often overlooked in discussions of pterosaur launch: bird-like bipedal launches dominate our consciousness only because we see them taking off every day, but they do not represent the only way tetrapods can become airborne.

Quad launch cycle in vampire bats Desmodus rotundus, traced from video footage: this is a real, proven launch mechanic folks, not something dreamt up by pterosaur workers desperate to prove giant azhdarchids could fly. Several other bats launch in this way, too. From Schutt et al. (1997).

These points - bone strength, concentrations of muscle bulk, limb bone scaling and trackway data - are the cornerstones of the pterosaur quad launch hypothesis, an idea which explains many independently observed features of pterosaur biomechanics, bone proportions and absolute size. Crucially, all of these points can be investigated for giant azhdarchids, and there are no red flags suggesting quad launch did not apply to these pterosaurs. Ergo, we can assume that giant azhdarchids used the most efficient launch mechanism conceivable for a tetrapod, negating any need for unreasonably low mass estimates or cliff jumping to become airborne. We can understand why pterosaur humeri are so strong and their femora so (relatively) weak, as well as the impact this has on overall size. Through birds, we have as inverse proof of this relationship. Furthermore, quad launch negates the need for special assumptions about giant pterosaur flight, allowing us to cast the biggest azhdarchids as 'extreme' versions of the pterosaur bauplaun, not evolutionary weirdos that take us back to the biomechanical drawing board.

Despite the sound scientific basis to quad-launch it is sometimes dismissed out of hand, I think because many folks just can't imagine it working - this excellent video by Mike and Julia Molnar does a good job of showing the kinematics. My experience is that counter-arguments are made without knowledge of its supporting data as they focus on less knowable components of the launch cycle, such as the speed of wing action or intuitive ideas about how high pterosaurs could leap. These are poor argument because they a) are largely speculative, not based on measurable/observable phenomena like bone strength or trackways; and b) ignore the fact that any launch mechanic requires rapid deployment of wings or an ability to obtain good ground clearance. We have to assume giant pterosaurs could achieve these feats no matter what our preferred launch strategy is. Moreover, somewhat ironically, the elevated flight speeds necessitated by giant pterosaur mass actually minimises some of these concerns. Flapping amplitude scales negatively with animal size and flight speed, making ground clearance less of an issue for large fliers than smaller ones (Habib 2008).

Flying the gauntlet

Let's put all this together - congratulations if you've waded through this long, often technical post. Giant azhdarchids...

• are poorly known, but have anatomy consistent with volant habits in every known aspect.
• do not seem to have struggled with take-off energetics more than any other large flyer.
• are often anatomically mischaracterised, being overly compared to extant birds or modelled in ways which distort their likely flight parameters.
• evolved from animals with a fundamentally more efficient launch strategy than that of birds, which lifts their body mass ceiling well above that predicted for avians. Every tested aspect of giant azhdarchid anatomy points to retention of this launch strategy even at their huge sizes.
• have flight parameters which, when modelled using conventional animal flight software in modern-grade atmosphere and gravity, equate to excellent flight performance, analogous to that of large soaring birds.

The take-home message is that interpretations of giant azhdarchids as flying animals are based on numerous corroborating lines of investigation and hypotheses which support and predict one another. Moreover, the methods used in these studies are entirely conventional techniques of palaeontological inquiry, and to disregard or ignore them requires dismissal of entire scientific fields of study. Don't buy the limb bone strength studies? Fine, then you also don't buy beam theory or structural engineering. Don't believe the flight analyses? OK, but you're also challenging software written by noted experts in animal flight, using data measured from real flying animals and a deep understanding of aerodynamics.

This is not to say that we know all there is to know about giant pterosaur flight - far from it. They remain poorly known animals and we can only guess at their variation in flight performance. Who knows, maybe a flightless species will turn up one day - this is not a ridiculous concept, we just don't have any evidence for it yet. But, for now, anyone seriously wanting to challenge this interpretation needs to discredit a robust theoretical foundation of pterosaur flight mechanics and provide a superior interpretation of the many strands of evidence we've discussed. This seems like a tall order to me, but it's the gauntlet that anyone who says giant pterosaurs were 'too big to fly' or 'they needed different gravity' has to run. Such comments reflect an ignorance or unwillingness to engage with a growing body of sound technical research on these animals, and - unlike giant pterosaurs - their arguments just don't fly.

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