## How light could a giant azhdarchid be?

### October 19, 2015

I imagine that by now, everyone who reads this blog is familiar with Mark Witton’s painting of a giant azhdarchid pterosaur alongside a big giraffe. Here it is, for those who haven’t seen it:

(This is the fifth and most recent version that Mark has created, taken from 9 things you may not know about giant azhdarchid pterosaurs.)

It’s one of those images that really kicks you in the brain the first time you see it. The idea that an animal the size of a giraffe could fly under its own power seems ludicrous — yet that’s what the evidence tells us.

But wait — what do we mean by “an animal the size of a giraffe”? Yes, the pterosaur in this image is the same height as the giraffe, but how does its weight compare?

Mark says “The giraffe is a big bull Masai individual, standing a healthy 5.6 m tall, close to the maximum known Masai giraffe height.” He doesn’t give a mass, but Wikipedia, citing Owen-Smith (1988), says “Fully grown giraffes stand 5–6 m (16–20 ft) tall, with males taller than females. The average weight is 1,192 kg (2,628 lb) for an adult male and 828 kg (1,825 lb) for an adult female with maximum weights of 1,930 kg (4,250 lb) and 1,180 kg (2,600 lb) having been recorded for males and females, respectively.” So it seems reasonable to use a mass intermediate between those of an average and maximum-sized male, (1192+1930)/2 = 1561 kg.

So much for the giraffe. What does the azhdarchid weigh? The literature is studded with figures that vary wildly, from the 544 kg that Henderson (2010) found for Quetzalcoatlus, right down to the widely cited 70 kg that Chatterjee and Templin (2004) found for the same individual — and even the astonishing 50 kg that seems to be favoured by Unwin (2005:192). In the middle is the 259 kg of Witton (2008).

It occurred to me that I could visualise these mass estimates by shrinking the giraffe in Mark’s image down to the various proposed masses, and seeing how credible it looks to imagine these reduced-sized giraffes weighting the same as the azhdarchid. The maths is simple. For each proposed azhdarchid mass, we figure out what it is as a proportion of the giraffe’s 1561 kg; then the cube root of that mass proportion gives us the linear proportion.

• 544 kg = 0.389 giraffe masses = 0.704 giraffe lengths
• 259 kg = 0.166 giraffe masses = 0.549 giraffe lengths
• 70 kg =0.0448 giraffe masses = 0.355 giraffe lengths

Let’s see how that looks.

On the left, we have Mark’s artwork, with the giraffe massing 1561 kg. On the right, we have three smaller (isometrically scaled) giraffes of masses corresponding to giant azhdarchid mass estimates in the literature. If Don Henderson (2010) is right, then the pterosaur weighs the same as the 544 kg giraffe, which to me looks pretty feasible if it’s very pneumatic. If Witton (2008) is right, then it weighs the same as the 259 kg giraffe, which I find hard to swallow. And if Chatterjee and Templin (2004) are right, then the giant pterosaur weighs the same as the teeny tiny 70 kg giraffe, which I find frankly ludicrous. (For that matter, 70 kg is in the same size-class as Georgia, the human scale-bar: the idea that she and the pterosaur weigh the same is just silly.)

What is the value of such eyeball comparisons? I’m not sure, beyond a basic reality check. Running this exercise has certainly made me sceptical about even the 250 kg mass range which now seems to be fairly widely accepted among pterosaur workers. Remember, if that mass is correct then the pterosaur and the 259 kg giraffe in the picture above weight the same. Can you buy that?

Or can we find extant analogues? Are there birds and mammals with the same mass that are in the same size relation as these images show?

# References

• Chatterjee, Sankar, and R. J. Templin. 2004. Posture, locomotion, and paleoecology of pterosaurs. Geological Society of America, Special Paper 376. 68 pages.
• Henderson, Donald M. 2010. Pterosaur body mass estimates from three-dimensional mathematical slicing. Journal of Vertebrate Paleontology 30(3):768-785.
• Witton, Mark P. 2008. A new approach to determining pterosaur body mass and its implications for pterosaur flight. Zitteliana 28:143-159.

### 31 Responses to “How light could a giant azhdarchid be?”

1. Luigi Says:

The azhdarchid still looks a little more robust than the 544kg giraffe, though. Maybe skeletal pneumaticity could potentially make it proportionately lighter than the 3 meter tall Giraffe? Mark Witton did mention in an old blog post (“9 things you may not know about azhdarchid pterosaurs”) that, ignoring the long skulls, elongate cervicals, and wing membranes, the torso of an azhdarchid is just about the same size, if not only slightly larger than a human’s.

2. Mike Taylor Says:

Yes, pneumaticity certainly did lighten pterosaurs extensively. But how extensively is it possible to pneumatise an animal and have it still function? Obviously (to me, anyway), not to the extent of the 70 kg version of the azhdarchid that is as heavy as the tiny giraffe.

3. Mark Witton Says:

Interesting post, Mike! There’s some irony here in that the original version of the giraffe/pterosaur image was done to say ‘<100 kg estimates are too low: 250 kg makes more sense'. The inclusion of a person was to make the same point you do above: '70 kg is the same as that human, which is ridiculous for a giraffe-sized animal'. Now the same lineage of art is being used to argue that even those 'heavy' pterosaur estimates are too low: my own weapons have been turned against me! It's like being in an obscure, very low-stakes science fiction movie. My thoughts…

Don's 2010 estimate was discussed by Mike Habib and I in a 2010 paper (http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0013982). In that, we noted that his model of a giant azhdarchid had a torso about 3 times longer than what we would predict for an azhdarchid pterosaur. As we all know, azhdarchoids (including azhdarchids) had very small torsos – about a third longer than the humerus (this looks a bit like this: https://twitter.com/MarkWitton/status/626484515310030849/photo/1). For a giant, we expect a torso about 70 cm long – Don's was something like 2 m! Ergo, the 544 kg estimate does not really reflect accurate modelling of azhdarchid anatomy. We recalculated Don's model with an rescaled torso, and the mass tumbled down to 240 kg, which is in line with other upper-limit values. What's especially interesting here is that Don's approach was volumetric, whereas my approach (and the 259 kg value) was based in part on a scaling equation – this is a case where the two approaches marry up quite well. I can't recall a study of azhdarchid mass which has made a compelling case for heavier masses than these.

Turning to your reality check, I do wonder how appropriate it is to compare the mass of a giraffe with a flying animal, as there are fundamental differences in their construction. As we know, the mammalian anatomy of a giraffe is pretty solid, but azhdarchids are quite the opposite. The entire wing (including a really massive forelimb skeleton – the humeral diaphysis is 80 mm across, and the distal end exceeds 200 cm across. The carpals are similarly proportioned) is pneumatised with c. 2 mm thick bone walls. As you know, the neck and skull are full of air too, as are the dorsals and sacrals (not sure about the caudals, but they're so tiny they don't really matter here). So those enormous bits of anatomy – the arms, head and neck – look a lot more massive than they actually are. Perhaps more than any other tetrapod, we're really looking at ballooned osteology surrounded by relatively little soft-tissue with these animals. We should expect very low tissue densities in some components of azhdarchid anatomy, and that's going to seriously lower their mass. We can't say that for anything in a giraffe body.

Differences in lifestyle and gait are significant here too. As running, unguligrade herbivores, giraffes have big, intestine-filled torsos and powerful, well-muscled hindlimbs. These features add a lot of mass, but giant pterosaurs had no need for either. Being plantigrade (we know this from giant pterosaur footprints and smaller pterosaur fossils) helps keep the leg muscle mass down (no need to perpetually hold that ankle in place) and we've already tackled torso size above. Moreover, as quadrupedally launching fliers, azhdarchids can concentrate muscle mass around their shoulders and proximal limbs. That means that most of what you're seeing south of the proximal forearm is just bones and ligaments – not much mass there at all. Azhdarchid forelimb muscles were sufficient to power all active locomotion (launching, flapping and running), so the hindlimb mass is pretty light too. I'd expect giraffe necks to be relatively heavily muscled as well, especially given how males physically compete with them. I guess what I'm attempting to describe here is that giant pterosaurs have one, tightly centralised core of muscle, giraffes are well-muscled all over, and we all know how heavy muscle is.

So… yeah, I'm not really convinced that quarter-tonne estimates of giant pterosaur mass are problematic. Remember that the game for most flying animals is maximising linear proportions and surface area without increasing mass – azhdarchids appear to have excelled at that. It might be interesting to further this reality check exercise by throwing in imagery of giraffes in lateral aspect. The anterior view shown in my painting makes them look quite svelte. I expect they wouldn't look so contrasting if we could see the length of their torsos, broad limb musculature and depth of their necks.

4. Mike Habib Says:

Neat exercise!

One catch, though, is that as wonderful as Mark’s latest version is, I don’t think it is meant to be a measured illustration in all details. The overall height of the azhdarchid matches predictions, but Mark and I both restore the largest azhdarchids with a shoulder to hip distance of about 0.65-0.70 meters. I think the torso in Mark’s illustration is, therefore, actually a bit larger than the dimensions used when doing the numerical mass estimates. Or another way of looking at it, your resized giraffe for the 259 kg estimate looks to have a torso around the right size. The mass would then be somewhat increased by bolting on a much larger head and forelimbs, but subsequently decreased by pneumaticity, which (being very rough about it) probably about offset one another.

Of course, this all assumes that the smaller azhdarchids (5 meter span) can be effectively scaled up to give the dimensions of the giants.

5. Markus Says:

Very interesting topic. It seems really hard to get real good weight data for giraffes at all. That´s quite often the case for big animals, as you have usually only a certain ranges of lengths (or in this case heights) and a certain rannge of weights. In many cases this ranges can give a misleading idea about the actually sizes weights and the proportions. Even keeping the interindividual variation in proportions in mind, the data from single individuals are quite often the best thing to get a basic figure to work with. Especially in big animals we have also the problem that weights are quite often simply estimated, but not actually weighed at all. This can easily cause overestimations. In some cases even widely cited figures for weight are actually nothing but estimations, which are sometimes highly dubious. Even after some search I could not find any really measured size and weight data for a single adult giraffe, only for new-born juveniles. Perhaps the best thing would be to get data from veterinarians from zoos of perhaps documented data from hunted trophy specimens. I found weights of 1,2 metric tons and 1,4 metric tons for the gigantic giraffe bull Schorse which lived until two years ago at Hannover Zoo. He was one of the very largest giraffes in captivity and was 5,8 m tall (take a look at photos of Schorse, he was truely enigmatic, especially he showed the phenotype of old males close to the extreme). But again, this are apparantly still nothing but estimates, and perhaps it weighed more when it was younger.
What I found at least are weights for giraffe necks which are said to usually weigh 500-600 pounds, what would be still around as much as the whole Quetzalcoatlus in Mark´s estimate. However, that is probably really not the very best comparison, despite the apparantly similar shape, as their body proportions and their anatomy (giraffes have really quite massive bones with a thick corticalis) are still way to different from those of an azhdarchid pterosaur.
I tried to get a better idea by using birds with comparable proportions for a comparison, for example marabous, storks and herons. They have all quite long necks and thin long legs, pneumatic bones and big to large beaks. In this case we still face the problem of unconnected weight and size data, as the given weights are still only ranges of weights and heights and wing-spans of different individuals, what makes it again hard get the proportions right for a particular size. Of course the comparison is still problematic, as the proportions of animals like Quetzalcoatlus and herons and storks and marabous still differ, especially due to the different walking style. So I tried different methods for comparison, but the problem of unconnected length-weight-data makes it really problematic (this can´t be emphasized enough). I would have prefered to have used some low data for marabous, das they seems to be the best analoge, but the ranges of parametres are really hard to get into line. I just used the lowest figures of 1,2 m TL, 2,25 wingspan and 4 kg weight. The TL is already problematical, as it is of course different from the standing height as the length of the beak is included. I tried to make a simple break by just reducing the TL for 20% to get the standing height for Leptoptilos crumenifer, what would be 96 cm. However, this already seems quite strange, and probably way too heavy, if compared with other animals like herons. Scaled up to 5,6 m it would weigh 793 kg. I have a hard time to accept this, and blame the problem of unconnected weight and size data for a big part of this.
I tried the biggest size and weight data too, with 1,52 m height, 3,19 wingspan and 9 kg weight. Scaled up I came to 450 kg based on height and 277 kg based on wingspan.
As another comparison I took the wingspan to see how it would work. Scaled up from 2,25 m it would weigh 351 kg at 10 m wingspan.
The data about marabous seems for various reason problematical, for this reason I used data for grey herons as well. This birds are really quite tall looking, but if you look at their skeletons alone, they aren bigger than those of a typical broiler in the 1,0-1,3 kg range.
I took the low sizw and weight data for the grey heron with 0,9 m height, 1,75 m wingspan and 1 kg. Scaled up from the height I come to 240 kg. Using the 1,75 m wingspan I come to a meager 187 kg.
Of course this is still also problematical as even the proprortions of marabous and herons doesn´t really fit those of Quetzalcoatlus with its super-long neck. But I wanted to add this quick and simple mathematical comparison, also to show the big problems we face with varying proportions and problematical origins of weigh and size data.

6. Mike Taylor Says:

Thanks, Markus, those are some really interesting data points.

I guess we should also ask question like: what would a condor weigh if we scaled it up to an 11 m wingspan? (I do know that birds often have shockingly low masses.)

7. Darius Says:

This was pretty much what I always thought, but was too lazy to check myself, so very helpful.
To me the 259kg estimate actually appears plausible based on this.

It has to be considered that the Giraffe is seen in an oblique anterior view. It’s torso would look bigger (in length) if seen from the side, so their bodies may well end up being similar in terms of volume.
Then we have to consider the giant skull and longer neck and limbs, but also the extreme degree of pneumaticity. I’m not sure whether there are current estimates on this, but is it reasonable to say that it may only be 70% as dense as the giraffe overall?
I know something along those lines has been proposed for some sauropods on this blog, so it may be legit to say that Quetzalcoatlus is only 70% as massive as it looks.

In short, the azhdarchid may be more voluminous, and certainly more impressive in terms of dimensions, than the downscaled giraffe, but the latter appears smaller than it is due to the angle it’s drawn at, while the pterosaur is smaller than it looks because it’s a baloon.

8. Mike Taylor Says:

Well, Darius remember that if a giraffe is reduced to 70% of its weight, that will still leave it at a height that is the cube root of 0.7 of what it was before: that is, 0.8879 of its previous height. To put it another way you’d only be able to take 11% off if initial height. So I think the pterosaur has to have much lower density that 0.7 for the 250 kg mass to make sense.

9. palaeosam Says:

Ok, this is a little too much fun. I should be preparing for an office move but instead I am scaling giraffe silhouettes. I thought I’d try scaling down your average giraffe to the size of an adult male ostrich (still pneumatic but less so as I understand it than a flying bird would be). The adult male ostrich being 156 kg and 2.75 metres tall, Following your calculations, 156 kg = 0.1 of the 1561 kg giraffe’s mass and 0.464 of its 5.5 m height. The resultant 156 kg giraffe stands an underwhelming 2.552 m. Here is an image of the scaled down giraffe, note the sizeable height advantage of the ostrich over the giraffe on the right. https://palaeosam.files.wordpress.com/2015/10/giraffe-ostrich-compared.png – these from Phylopic with the Ostrich attributed to Lukasiniho and the Giraffe to Kathy (link to licence: http://creativecommons.org/licenses/by-nc-sa/3.0/)

I don’t think the height and width in 2 dimensions are doing us any favours: the bobcat (Lynx rufus) is, nose to rump, the same length as the Eurasian griffon vulture (Gyps fulvus) beak to tail and their upper masses are close but the vulture is clearly much wider of the two. In Mark Witton’s diagram, we see the narrowest angle of the giraffe and the widest angle of the Azhdarchid (barring spread wingspan of course) so reality checks at a glance it would really be fair to see both front-on and side-on views of both.

10. Mike Taylor Says:

I agree that comparable views of the two animals would be good, and I fondly hope that Mark is working on that right now :-)

Interesting work with the ostrich. My gut reaction is that I’m surprised the ostrich comes out as big as it does compared with the same-weight giraffe. On that basis, you’d expect Mark’s giraffe-height azhdarchid to weigh not much less than its giraffe buddy, which is more than even I’ve suggested.

11. Ictonyx Says:

As an example of a flying bird / terrestrial mammal comparison, how about grey heron vs. European rabbit? Both in the 1 – 2 kg ballpark but I think a grey heron is around 95 cm in total length (and about as tall when upright) and a European rabbit around 40 cm long.

To my eyes, they don’t look remotely similar in size, but the weight tells a different story. Also, I feel the ostrich may be misleading – I thought their bone density was much higher than that of flying birds? Don’t know though.

I’m with Mark Witton; 250 kg giant azhdarchids look reasonable from where I’m standing, especially given the ridiculously short torso, which isn’t really displayed in the painting used here due to the anterior view – I bet a lateral view of the azhdarchid and giraffe would make the azhdarchid look way smaller. I think the giant skull just makes it ‘read’ huge.

12. Mike Taylor Says:

Well, the rabbit seems a much less good comparator for the stork than the giraffe is for the azhdarchid, given the rabbits are ball-shaped while giraffes are (for mammals) attenuated.

13. Markus Says:

In the case of condors at least there is good data on weights and wingspans. I found a mean average wingspan of 2,83 m and a mean average weight of 12,5 kg for male Andean condors. Scaled up to 11 m wingspan it would weigh a whopping 719 kg.
Another thing we have to keep in mind is plumage. In birds the feathers contribute a comparably big mass to the body weight. Pterosaurs had no feathers, but of course it is hard to say how much their wing membranes did weigh.

14. Mike Taylor Says:

Markus, thanks for the condor data. Let me just take a moment to say, more generally, how awesome it is to have such engaged and knowledgeable readers on this blog. It’s heart-warming to post something like this, based on ‘shopping an artwork, and then watch the actual data come flooding in.

I think the scaled-condor mass of 719 kg is fascinating, because it’s asking an awful lot for pterosaurs to be be so much better engineered than birds that they were able to attain that wingspan at only one third of the mass.

We all know that birds, like pterosaurs, are highly pneumatic. Feather mass is fascinating. I remember being astonished, when I first saw Brodkorb (1955), that the contour feathers of his bald eagle, at 586 g, were much more than twice as heavy as its skeleton, at 272 g. So the addition of feathers to a bird certainly has an effect on its mass. On the other hand, 586 g was still only 14.36% of the total body mass of 4082 g, so that a bird without contour feathers would still weigh 85% as much. For example, your isometrically scaled 11-m-wing-span condor would weigh 611 kg, which is still more than the highest published mass for a big azhdarchid, and about two and a half times Mark Witton’s favoured 250 kg.

Also: pterosaur wings were much bigger objects than plucked bird wings. Those membranes were not insignificant. Even if they were so awesomely engineered that they weighed only half as much as equivalent flight feathers — which I doubt — that still means the weight saving of substituting pterosaur flight membranes for feathers would only come to 7% of body mass. So our giant condor, if it could halve its flight-feather mass, would still come in at 670 kg.

In short: I think the evidence of the condor corroborates my feeling that we are underestimating the mass of big pterosaurs.

15. Ictonyx Says:

It’s interesting that the isometrically scaled condor would still weigh 670 kg with half the contour feather mass, but think how differently proportioned it is from the azhdarchid – most importantly, it has a normal sized torso instead of a tiny one.

The scaled-up condor would have a torso of, at a guess, 1.75 m long, assuming a 0.45 m torso for the life-size condor. Compare that 1.75 m to the 65 – 70 cm torso length of the azhdarchid and it’s no wonder the scaled-condor mass is around 3 times that posited by Witton for the pterosaur.

16. Mark Witton Says:

I’m very wary of attaching significance to mass estimates derived from scaling up birds to the size of a giant pterosaur. It won’t surprise you to learn that people have done this before (using bats as well), and the response (from a number of pterosaur workers, but most vocally from Mike H and myself) is that this is very problematic. As noted several times in the comments here, pterosaurs are not birds, or mammals, and possess a suite of nuances to their anatomy which can’t be ignored when considering their mass.

For instance, in addition to bearing tiny (man-sized) torsos, pterosaurs lack the enormous hindlimb muscles of birds. Birds need a substantial set of leg muscles just for launch, and these add tremendous weight to their bodies: they don’t just add bulk, but require a bigger and more robust set of bones to anchor too. Of course, these leg muscles are a burden because they not only have no purpose other than launch – they’re just ballast once they’re airborne – and they also mean birds likely scale somewhat differently to pterosaurs. Their hindlimbs have to grow disproportionately big to provide enough power for launch at larger size (standard scaling issues with size, volume etc.), and this likely explains the seemingly low size limit of flying birds: at some point (seemingly about 5 m) bird muscle mass scaling can’t increase any more and launch is impossible. Pterosaurs, of course, avoid this problem in being quad launchers: they only have to worry about scaling effects on one dual purpose (launch and flapping) flight engine. That allows them to get a lot bigger before hitting launch constraints, and provides a pretty fundamental, scale-relevant distinction right there.

It’s also worth considering what the ‘big bits’ on an azhdarchid are. The hypertrophied anatomy in azhdarchids is exclusively comprised of extensively pneumatised components: head, neck, forelimb. No other animals have expanded this set of elements as much as azhdarchids. Above, Mike T says pterosaurs would have to be more efficiently constructed than birds to weigh less: well, in this respect, and their muscle scaling, they are! As mentioned in my comment above, azhdarchids are really good at getting the most out of their mass – they’ve effectively taken every shortcut to achieve their enormous proportions. We can even note an effect of their short torso and disproportionate limb lengths too – their standing height is made all the taller by that. All this is not only going to have an obvious impact on scaling comparability, but also affect how we perceive their size against other animals. As much as images like those above are useful reality checks, they don’t tell the whole story.

My point here is not a novel one, simply that we can’t just ignore what makes pterosaurs unique and expect to gain insight into their mass through scaling up entirely different bauplans. To use an extreme example, you wouldn’t expect to gain insight into the mass of a sauropod using a salvator monitor. Sure, both are terrestrial, quadrupedal reptiles with long necks and tails, but they are constructed so differently that there’s little value attempting to scale one to the other. The same is true of birds and pterosaurs, or giraffes and pterosaurs: those differences in fundamental anatomy can’t just be ignored. Indeed, it’s likely that those differences are why one group can achieve their combination of size and mass while the other can’t. Mike H and I wrote a whole paper on this very issue, in fact: birds are probably our best analogue for pterosaurs, but they two groups should not be treated interchangeably – especially when it comes to scaling.

17. Matt Wedel Says:

I think this visual comparison (by Mike, not Mark!) goes off the rails in two ways. First, Ictonyx is absolutely right to focus on torso size. We’ve seen before that torso volume is a major driver of overall mass, but the relative volumes of the torsos are not easy to compare in the painting (which is okay, since it was painted for entirely other purposes). Mark mentioned up above that a big azhdarchid like this would have a torso perhaps 70 cm long. For the giraffe the torso is probably more like 1.5 meters (quick measurement from Don Henderson’s digital model in Henderson and Naish 2010). And although giraffe torsos are small-ish overall, they’re pretty rotund in cross-section. Even isometry would predict that a 1.5-meter giraffe torso would have only 1/8 the volume of a 75-cm smaller giraffe torso. If you additionally figure that azhdarhid torsos might have been less rotund, and filled with air sacs, the difference in mass is even larger.

Second, most of the visual impact of both animals is made by their extremities, which are very light for pterosaurs and fairly heavy for giraffes. van Schalkwyk et al (2004) found that the limb bones of giraffes are quite dense, especially the very long cannon bones. So although pairing a giraffe with an azhdarchid may be a great aesthetic choice for showing the height (1 dimension) of big pterosaurs, it’s probably the worst possible comparison for trying to use a picture (2 dimensions) to guess the mass (3 dimensions) – which is a pretty dodgy thing to do under any circumstances.

So I guess my question is, what’s the point here? That a ~250 kg mass for Quetz is surprising (which I agree with), or that it’s in error (not as far as I can see)?

UPDATE: my comment passed in the ether with Mark’s most recent one, which makes the same points, only better. In particular, I had never thought about the fact that the hindlimbs of birds are dead weight once they’re off the ground. How very interesting.

18. dale Says:

Blowing up birds to different size catagories would be intriquing. But they probably compare better to theropods. Try bats.

19. biologyinmotion Says:

What a great discussion! Thanks to Mike Taylor for kicking it off.

Regarding using living flying vertebrates as mass/scaling analogs for pterosaurs:

One reason this runs into so many problems is that the standard for comparing flying animal size is wingspan. Using span has some noticeable issues when comparing living birds or bats, but it makes particularly bad predictions when trying to make mass comparisons with pterosaurs. The bottom line is that pterosaurs (especially derived Cretaceous forms) were basically low span-loading specialists. That is, they had a wide array of morphological features that increased the ratio of span to weight (or, looked at the other way, decreased the ratio of weight to span). This does mean they were more efficient than birds and bats in some ways, though it isn’t so much that they were generally more optimized as much as particularly good at one thing.

This issue is especially noticeable when attempting to use birds like condors as examples, because living avian thermal soaring specialists all “cheat” the span loading trends by using slotted wingtips. In essence, a condor has functional span that is larger than its anatomical span. Pterosaurs, on the other hand, had a functional span that was equivalent to their anatomical span (or at least quite close – the lunate wing tips on pterosaurs, see Hone et al. 2015, would have somewhat increased their functional span to weight ratio).

Since selection is presumably acting on the functional span, comparing a bird with slotted wing tips to a pterosaur, by scaling from span, becomes immediately problematic. The problems compound when we consider (as noted above) that pterodactyloids, at least, probably had even more extreme ratios of functional span to weight than living soaring birds.

The problem persists even if we go to marine birds with long, un-slotted wings because the classic examples of living long-winged birds are not low span load specialists (they are, in fact, high wing load/high aspect ratio specialists). So, take a wandering albatross as a (highly popular) example. A big individual (from which I have a direct mass measure on hand) is a bit over 9 kg, and has a span of just under 3 meters (2.9m). Scaling it up to the span of Anhanguera santanae (4.09m) isometrically predicts a mass of over 24 kg – a conservative estimate given that some elements (like bones) would need to scale allometrically to maintain performance.

The mass estimates for Anhanguera from slicing models and bone scaling range from about 7kg to 12 kg, respectively. Unlike the giant azhdarchids, we have multiple near complete Anhanguera, which helps keep the estimates a bit more consistent (though obviously still quite variable). Getting the 4m individuals up to 24 kg actually requires making Anhanguera unusually dense (given its volume) which would be quite strange for a highly pneumatic animal.

20. Mark Witton Says:

One point I’d missed from earlier comments concerning torso size in my Arambourgiania painting: I think it’s basically OK for a 70 cm glenoacetabular distance – my assumptions and speculations on soft-tissue throw a curve ball into proceedings. Specifically, I think it looks bigger because of the lateroventral angle (probably one of the broadest faces of a pterosaur body), shoulder mass and fluffiness. I made efforts to grade the neck into the torso to account for neck musculature anchoring to the pectoral girdle and sternum (I’ve assumed this is more substantial and longer than in birds, and more like that of crocs of lizards) and air sacs. This has the effect of making the anterior torso look longer than it is. I quite like it, figuring that the resultant gradation between neck and torso might provide useful fairing for flight. The hindlimbs also have bonus posterior fluffiness on them too: they add some bulk to the back end. The result is a stretched looking torso, but checking the illustrated glenoacetabular length against Georgia the Human Scale Bar(tm)’s legs suggests the torso would not rise much above her knees if placed on the ground, which seems about right (if needed, I can confirm distal hindlimb length on the actual GtHSB later). It might not be perfect – I’m not a precision artist at the best of times – but it should be in the right ballpark.

Of course, there is another cautionary tale about comparing animal sizes directly here: soft-tissues (restored or otherwise) can be really misleading!

21. Mike Taylor Says:

It must be comforting for your wife to know that, were an azhdarchid torso to be placed on the ground next to her, it would not rise much above her knees. I suggest you use this confection as the inscription on your anniversary card.

“Soft-tissues (restored or otherwise) can be really misleading!” — so very, very true, and should really be our official motto.

22. Mark Witton Says:

I anticipate great interest in Georgia’s knees at the next SVPCA. “Oh, so _that’s_ how big an azhdarchid torso is.”

23. Mike Taylor Says:

The only way to really make the point will be for you to make a life-sized model, fit it will little wheels, and have Georgia pull it around on a string everywhere she goes.

24. Markus Says:

It´s really obvious that we face a huge amount of problems and issues at this topic. The reason why I started the comparison was mainly to get off the giraffe. As Matt already said, yes, it looks really good in visual terms, but it´s really not good for a visual comparison for the weight. The special anatomical traits of pterosaurs (like the lesser heavy hind limbs) made them really obviously lesser heavy at the same size. And this little word at the end of the sentence is already a problem, because it is really very problematic to compare the sizes of pterosaurs with those of birds. Even stork and herons which seems to comes comparably close to Quetzalcoatlus in overall shape, are stil different in proportions, especially due to the super-long neck and head in those pterosaurs. Wingspan is also problematical, as the torso-to-wingspan-ratio is apparantly different as well.
I tried to get a somewhat better idea how far this differences go and made a quick and dirty scaling comparison in PAINT. I used a Quetzalcoatlus silhouette (from here: https://upload.wikimedia.org/wikipedia/commons/e/e5/Quetzscale1.png ) and a photo of a marabou standing erect with expanded wings and stretched neck (from here, not perfect but the best one I cold find: http://4.bp.blogspot.com/-2RbbrnUooGs/VHRQdgRGCtI/AAAAAAAAKo0/8ZgSH6lRWaA/s1600/Marabou%2Bstork%2Bwith%2Bwings%2Boutstretched.jpg )
I tried to scale the marabou to the same torso size as the Qutzalcoatl. I had some problems, as the old photo makes it not that easy to get a perfect comparison. When I found that I had at least a roughly similar torso size, I measured the proportions again.
The wingspan of the marabou was at around 7,5 m, but as its wing were somewhat hold up, it would be probably closer to 8 m. The total length of the marabou with beak and neck in straight line would have been around 5 m.
For comparison, the Quetzalcoatlus in the illustration is depicted with a wingspan of around 10,2 m and a full length (in this case from tail tip to toes) of 8,5 m. Interestingly, at the same torso size the legs of Quetzalcoatlus and marabou are very similar in length. Head and neck are both around two times as long as those of the marabou.
I used this other comparison to see how much a marabou of 2,9 m wingspan and 8 kg (as mentioned earlier, this are sadly no dates from one indivudual, but only the upper sizes and weights of the average range) I came to a weight of 167 kg. Even keeping all the light-weight anatomy in mind, the much longer neck and head and wings and a probably wider torso could have well added some more weight too. I think at the end it wouldn´t be far away from Mark´s weight figure.
That was of course far away from being, well, super-precise, but perhaps at least an interesting step into another direction, i.e. that pterosaurs like Quetzalcoatlus had perhaps a lower weight at the same (calculated) wingspan as birds, as their torsi were proportionally smaller, and that it could better work to try to compare torso size instead of wingspan.
I think one very important thing however has become clear, that the super-low weight figures of well lesser than a hundred kg for Quetzalcoatlus and similar-sized pterosaurs are next to sure erroneous.

25. Mike Taylor Says:

Interesting stuff, Markus. Yes, to be completely clear (if I’ve not been before now) I absolutely agree that extant birds are not perfect models for pterosaurs. But I do think they are probably the best we’ve got, and for an important functional reason which is that they were both optimising for the same mechanical function (powered flight, of course). As Mark has pointed out, the giraffe comparison makes awesome art but questionable science.

As for scaling the stork to the same torso-size as Quetz: granted that “size” is a slippery concept, I think this has to be even worse than using wingspan or standing height. Being sauropod jockeys by nature, Matt and I are inclined to fixate on torso volume as dominating total animal volume — it comes out just over 70% for Giraffatitan in Table 4 of my 2009 paper. But of course that is not true of pterosaurs, and neither is it obvious that it’s true for birds. Looking at your stork photo, I would guess off the top of my head that the torso there accounts for maybe 25-30% of the total body mass.

What does this mean? That we have to be cleverer about scaling birds to “the same size” as pterosaurs, or more general in scaling any animal to the same size as any other. I wish I could propose something more constructive or specific.

26. Matt Wedel Says:

Looking at your stork photo, I would guess off the top of my head that the torso there accounts for maybe 25-30% of the total body mass.

I really hope that Mark Witton or Mike Habib can set me straight if I’m off track here, but since you’re firing from the hip, I will too: this seems extremely unlikely to me. I’ve been fortunate to get to dissect birds at all sizes from pigeons to ostriches, flying and flightless, and my off-the-cuff guess is that the torso is usually at least half of the mass (if we don’t include the bulky thighs with the torso). I think you’re being fooled again by the apparent size of the extremities, which look huge but are all thin (hindlimbs past knees), nearly two-dimensional (wings), or highly pneumatic (head, neck, and wings again). It’s just like looking at a sauropod and guessing that the torso is only 25-30% of the mass, just because the torso looks small next to the sticky-out bits.

I don’t mean to be harsh, but since folks like Mark W. and Mike H. have actually run the numbers here, I’m struggling to see how resistance to the ~250 kg mass is more than an argument from incredulity. Visual comparisons to things like giraffes and storks might seem like a sort of informal, Mythbusters-level back-of-envelope check, but for lots of reasons (already explained up-thread) they are probably bad comparisons. If you want to seriously question that result, it’s on you to read the relevant papers, find the flaws (if any), and propose better methods. That’s what I’d ask of anyone who was questioning our results; only fair to hold ourselves to the same standard.

27. Mark Witton Says:

Looking at your stork photo, I would guess off the top of my head that the torso there accounts for maybe 25-30% of the total body mass.

Matt is probably on the money here: in powered fliers we normally assume the flight muscles alone are 20-25% of body mass, irrespective of everything else in the torso. This is probably true for pterosaurs too – that torso is going to be more than 25-30% of any pterosaur once organs, bones and non-flight musculature are accounted for.

For further details, check out Don Henderson’s 2010 paper on pterosaur mass. It breaks down the contribution of body parts to overall mass. I can’t get an exact figure for the torso at a quick scan (sorry, no time to look in detail right now), but it’s clear from a glance at figure 9 that the torso dominates mass even in small-bodied taxa.

28. William Miller Says:

“at some point (seemingly about 5 m) bird muscle mass scaling can’t increase any more and launch is impossible.:”

I thought Argentavis was more like 7 m.

29. Mark Witton Says:

Not any more – Ksepka 2014 suggested it might be more like 6 m, and Pelagornis sandersi is – according to recent estimates, anyway – the bird with the largest wingspan (6.4 m). So yeah, I was a bit tight on bird wingspan constraints above, but the point stands about their maximum known span being nearly half that of a giant pterosaur.

30. William Miller Says:

Ah, wow, didn’t know Argentavis had lost the title! Thanks.

31. Fascinating discussion!
The high-tech (so to speak) aspects of pterosaur anatomy are astonishing (humerus: wall 2mm bone thickness with 200mm diameter at distal end, really?!)
I wonder if there was some specialized soft-tissue wrap, such as very thin layers of tough connective tissue, with crossed fibre alignments, to add strength (but not too much extra weight) to the long bones to avoid failure in tension or in torsion).

(Matt – “Even isometry would predict that a 1.5-meter giraffe torso would have only 1/8 the volume of a 75-cm smaller giraffe torso.”
Erm, vice-versa, surely?)