I stand up next to a mountain

Holy Christmas; what a busy week it’s been. It’s been manic, man. Nothing but rushing across countries and through cities at all hours to arrive for various deadlines, late finishes at work, long dinner dates with colleagues, frank, heavy discussions on technical issues, a chronic lack of sleep and an exceeding nervous moment of delivery on my own behalf. What a week. Sitting at home as I am now and reflecting on the whole affair, it has all the ingredients of something potentially very nasty indeed. But, dear readers, it’s actually been a considerably more enjoyable week than you might imagine. In fact, it's been one of the more enjoyable experiences of recent memory. Yes folks, I’ve finally made the launch into the surprisingly rock n’ roll world of palaeontological conferences, and it’s been great. That’s right: staying up until all hours with like-minded postgraduate students, being kicked by your supervisor to get you up in the morning, having beer spilt all over your clothes by world authorities in animal aerodynamics, discussing pterosaur systematics over bowls of muesli with globally recognised pterosaur experts and facing expensive repair bills as your transportation breaks down on the way to the gig… it’s a lifestyle that’s a little like being on tour with The Who should they have Albert Einstein on lead vocals.

Yes, the Peter Wellnhofer Flugsaurier Meeting has now been and gone, with pterosaur experts around the world now retiring back their lairs until their powers are needed again. I’m pleased to report that it was a huge success with the presentation of fascinating research, amazing specimens, a chance to meet people only previously known by name or E-mail address and, quite seriously, a genuinely positive feeling throughout. There are plenty of disagreements in pterosaur palaeontology but, despite some very strong opposing views, the whole affair was carried off without the bloodying of noses or throwing of beer bottles. Hats should definitely be taken off to the organisers of the meeting and particularly to Dr David Hone, the chap who has been at ground zero with the meeting from the start. Dave not only coordinated the arrangement of the speakers and their various presentations, but also managed to gather numerous world-famous specimens from various German museums to be examined in the same room for one memorable afternoon. Darren Naish, my fellow Flugsaurier traveller and at least partially responsible for making me stay up until very, very small hours of the morning twice in the last week, has more on this at Tetrapod Zoology.

In fact, I owe particular thanks to Dave Hone because he offered me the opportunity for my first presentation at an international conference. Initially, I only submitted poster abstracts for inclusion at the meeting but, when a scheduling gap was identified, Dave noted my apparent verbosity and offered me a chance to turn written words into spoken. Not wanting to regret turning down such a second chance, I offered a presentation on some work I’ve done on estimating pterosaur mass, or, in layman’s terms, working out how much the durned things weighed. Pterosaur workers are perhaps more interested in the masses of their extinct critters than other palaeontologists because, without knowing mass, we can only make very limited observations on their flight styles. Half-decent mass estimations can lead to all sorts of intricate aerodynamic work, from calculations of the forces working on various limb bones to reconstructing takeoff methods. But, before all that can be worked out, we need to ensure that we have good, solid mass estimations. Do we?

Well, in my view at least, maybe not. I’ve already hinted that my talk contained a whiff of controversy – in fact, it smells of controversy more than an entire hillside fertilised in the stuff – as my method of estimating pterosaur mass is a little different to most and has produced significantly different results. Y’see, while some folk have extrapolated pterosaur mass from modern animals like birds or bats, most pterosaur workers have estimated pterosaur mass by determining the volume of the pterosaur body and then multiplying it by a suitable density (thanks to differential tissue volumes, tissue densities and air spaces, animal bodies are very often more or less massive than water – crocs, for instance, are typically slightly denser than water while birds are typically less dense). Although this might initially sound quite daunting, it’s actually pretty simple: volume can be determined through simplifying the pterosaur body into a bunch of geometric shapes, and density can be extrapolated from modern animals, most often birds. No sweat, right? Maybe not: in actuality, this method is pretty problematic. You only have to look at the range of mass estimates given for the same pterosaurs to see this: some estimates of a 10 m span Quetzalcoatlus are five times higher than the lowest. Problem is, along with disagreement over pterosaur volume, no-one seems to agree on what density to use in their mass estimations. There are a number of so-called ‘average bird densities’, but it turns out that bird body density is really quite variable and directly related to ecology. Simply put, less dense birds tend to fly further and for longer periods than their denser colleagues, and this all ties in with foraging methods and lifestyle. This means that if we’re trying to understand pterosaur ecology we cannot model all taxa with the same density for fear of biasing the results. Should we be surprised that all pterosaurs come out as gull-like marine soarers if we model them all with gull-like densities, for instance?

Despite this, most pterosaur workers have been happy to estimate the masses of pterosaur bodies with single densities and, thanks to thoughts that pterosaurs had next-to-no soft tissue, pterosaur mass estimates are typically low. Like, really low. 16 kg for a 7 m span Pteranodon, sub-100 kg for a giant 10 m span Quetzalcoatlus, that kind of thing. Are these figures right? Well, to test this, I’ve tried to put together a method of estimating mass without going anywhere near soft tissues. No density required here, folks: just skeletal anatomy. Y’see, the dry mass of a skeleton is directly proportional to body mass in modern birds and mammals, and, intriguingly, the scaling relationships is nigh-on identical in both animal groups in spite of obvious differences in ecology and phylogeny. Armed with an array of 18 different pterosaur skeletal masses determined using geometric modelling and a regression analysis of bone wall thickness – bone shaft diameter to estimate skeletal pneumaticity (pterosaurs have hollow bones, folks), I plugged my modelled pterosaur skeletal masses into the skeletal-mass to body-mass regression and voila, total pterosaur masses estimated with no consideration of soft-tissue density at all. Just so you know, my masses of a giant 10 m span Quetzalcoatlus were around the 250 kg mark, while a tiny 30 cm span Anurognathus masses in at 39 g or so. This is potentially pretty neat but, erm, there’s one big problem: my masses are approximately three times greater than anything that’s gone before. Oh.

This, of course, creates the question of who’s barking up the wrong tree. Are my pterosaurs ridiculously heavy, or are other pterosaurs unfeasibly underweight? Judging this is difficult: without having a pterosaur land on a set of scales we’ll never know how great their masses really were, but four points are worthy of consideration on this issue.

Number one: My estimates correlate with bird masses of the same wingspan very, very well. Of course, we don’t have any birds with wingspans greater than 3 m today, but it’s reassuring that the pterosaurs I modelled beneath 3 m are not unreasonably heavy compared to their avian counterparts. Granted, we need to be careful comparing pterosaurs to birds in this manner, but at least modern birds demonstrate that animals of these masses and wingspans can fly.

Numéro deux: It pays to consider the amount of soft-tissue pneumaticity needed to generate the low masses previously cited for pterosaurs. In the case of a giant 10 m span azhdarchid, I’ve estimated a body volume of 500 litres through clay modelling. This means that something like 75 – 90 % of the body has to be full of air to generate the low 50 – 100 kg figures favoured by many workers. Unless pterosaurs were Mesozoic analogues of zepplins, these are ridiculously high percentages.

Nummer drei: Using data from exceptionally preserved fossils to reconstruct the wings of pterosaurs (yes yes, yes, I know – this is another whole can of worms. Thing is, we really don’t have the time to go into this now – check out this article for more, however), I’ve investigated the effect heavier masses have on wing loading and its relationship to aspect ratio. Plotting this data into a Principal Component Analysis that removes the effect of body size on wing loading, we can approximate the flying styles of my modelled pterosaurs. This folks, is where it gets really interesting. Lightweight estimates cast pretty much all pterosaurs as frigate bird-like marine soarers but, give these critters some weight and suddenly, almost magically, they start to plot in disparate parts of the graph, thereby increasing their flight style diversity dramatically. What’s more, they don’t just plot randomly. Azhdarchids are down with the thermal soarers, Pteranodon and its long winged friends are up with the marine soarers. The large footed ctenochasmatoids cluster with waders. Anurognathids plot with insect catchers. Both sedimentary and anatomical evidence support the placement of these critters in these positions: it’s where we might predict them to plot, basically. Of course, this placement does assume that my wing models are also correct but, seeing as these are as objectively derived from fossils as possible, to see my heavy mass data corroborating so well with functional morphological and sedimentary data is really, really intriguing. Of course, this is really just one theoretical model supporting another like a house of cards – collapse one and the whole thing will all come tumbling down. Still, for the moment at least, the results do support each other well.

El número cuatro: Finally, I would like to draw your attention to why we’re here in the first place: the picture above. This is, of course, an update of an old image that has been in need of a makeover for some time now. In case you’ve not guessed, the image shows the frequently mentioned pterosaur Quetzalcoatlus, a giant animal with a wingspan of 10 – 11 m and a standing shoulder height of 2.5 m. It’s a very, very big animal and, according to me, it has a mass of 250 kg. Nowhere near as big is the strange creature standing next to it, a bespectacled human scraping 1.75 m in total height. Being of a rather slender build brought on by an upbringing of low-fat foods and a meagre PhD student budget, he’s a rather thin, if ruggedly handsome specimen that masses in at 65 kg. Now, imagine if you can, the mass of this chap stretched over the skeletal frame of Quetzalcoatlus: it’s a slightly disturbing, anorexic sight to behold, I’m sure you agree. Far more comparable with the pterosaur is the handsome giraffe, a beast holding its shoulders 3 m or so above the ground. Giraffes can mass anywhere between 800 – 2000 kg, values significantly higher than those of Quetzalcoatlus. My point? Quetzalcoatlus and even moderately sized pterosaurs are big animals. For their stature, the heavy estimates I’ve accredited them are comparatively lightweight: 250 kg for an animal that can stare down the tallest land mammal alive in the modern age ain’t bad. By contrast, lower mass estimates just don’t give enough material to construct the entire animal.

And that’s pretty much it, folks: light estimates might make for good arm-waving stories, but I just don’t think they stand up to scrutiny. Thankfully, I think the delivery of my talk on this went well enough that those who were sceptical about heavyweight pterosaurs before are now at least considering the possibility that the consensus lightweight values might be flawed. I certainly didn’t get the barrage of sharpened objects being hurled at me that I expected and, most surprisingly, an awful lot of people seemed to agree with me. This is very encouraging, actually: I’m a little paranoid that I’ve missed something totally obvious with this work, but to have so many people come up and pat me on the back is most reassuring. Hence, this isn’t the end of this story: a paper on these results, plus a far more detailed analysis of the relationship between pterosaur mass and wing shape (which is actually a very important part of the whole project, despite my glazing over it here) is firmly underway. If I get it finished in time, I might submit it for the special edition of Flugsaurier Proceedings Volume that will be published as a special edition of Zitteliana next year. In the mean time, if anyone who was at the Flugsaurier meeting is still reading (and, indeed, if they’ve even begun), thanks very much for the kind and constructive words about the project – your encouragement is greatly appreciated. Oh, and thanks to those who used my pictures in their talks or even just came up and said how much they liked them. Oh, and finally, a general thanks to all those people who made Flugsaurier not just an interesting academic meeting, but a damn good time in its own right. Right, that’s more than enough writing on this - stay tuned for more conference-related pictures. Now, I’m now going to lie down – thanks to the marathon Munich-to-Calais driving session given by my PhD supervisor Dave Martill over last night, not to mention several nights on the trot with a minimal amount of sleep, I’m beat. A comfy sofa and a cup of coffee awaits.

UPDATE (18/09/07): The ever thorough Dr Naish has just begun a series of blog post covering the highlights of the Wellnhofer Flugsaurier meeting. You can check out his first post here, with the follow ups over here and here. Oh, and to my delight, there's even a blog dedicated to the image above, tucked away down here.

UPDATE (4/05/08): Hey, look at that, folks - there're even bigger pterosaurs to look at. Where be they? They be here.