Tuesday, February 22, 2011

Too Big to Fly? Giant pterosaurs take wing in PLoS ONE

My post today is in the spirit of the old phrase "better late than never". As some of you may have already noted, Mark Witton and I produced a manuscript on giant pterosaurs back in November of 2010. David Hone was kind enough to link to the manuscript (which is freely available online in PLoS ONE) a while back, but I thought I'd shock everyone and actually discuss the manuscript a bit.

The basic idea of the manuscript was actually pretty straightforward - there have been a few papers published since 2004 that have suggested flightlessness for giant pterosaurs and/or suggested that they would be flightless at the body masses currently estimated for them. Mark and I were immediately skeptical of a number of the assumptions in these papers, and furthermore, we felt that it was about time that someone actually put forth a publication that made a quantitative argument for the ability of giant pterosaurs to fly. There has been a general consensus on that fact from the research community for a long time, but strangely enough, no one had previously sat down and actually laid out all the reasoning in a peer reviewed publication.

The arguments we make in the paper come in basically two types: 1) We demonstrated that pterosaurs cannot be simply modeled as birds and 2) the anatomy of pterosaurs independently suggests the ability to fly.

The first of these arguments was critical because prior assessments of pterosaur flight dynamics have often been derived from simple scaling of bird models. More to the point, all of the papers suggesting that giant pterosaurs were flightless did so through analogies with living birds. There were three such papers of interest. The first was Chatterjee and Templin (2004) that indicated a mass limited for a launching Quetzalcoatlus of 75 kg. Given the current mass estimates of 250 kg, that was quite a conundrum. However, as it turns out, Chatterjee and Templin assumed a bird-like launch strategy. More recent work by myself, Mark, and Jim Cunningham strongly indicates that this assumption is weak, and that pterosaurs were most likely quadrupedal launchers. Changing the launch model blows the lid off the 75 kg mass limit (and for those of you that are curious, 250 kg isn't the limit, either. More on that some other time, but in short, pterosaurs could have achieved larger sizes than Quetzalcoatlus).

The 'pterosaurs are not birds' problem also breaks down the argument of the second manuscript in question, Sato et al. (2009), which used accelerometer data from tube-nosed seabirds during takeoff and cruising flight as a benchmark for analyzing pterosaurs. The avian data in that manuscript are excellent, but alas they cannot be extrapolated to pterosaurs. The most recent manuscript to suggest flightlessness in pterosaurs is Henderson (2010), which applied an excellent new method of mass estimation to some sadly outdated models of pterosaur body shapes. The resulting estimate for body mass in Quetzalcoatlus was more than double that of even the highest previous estimates, but this estimate drops back close to that of Witton (2008) if an updated body volume is used. It is also worth noting here that we found no evidence for major size differences between the handful of giant azhdarchid pterosaurs - taking into account distortion, the Hatzegopteryx humerus is the same size as the Quetzalcoatlus humerus, more or less. As such, the largest known pterosaurs from multiple locations probably all come in around 10.5 meter wingspan mark.

So that's the bird analogy issue in a nutshell. But what about direct evidence of flight ability from the anatomy of the critters themselves? Mark and I examined the forelimb skeleton of giant pterosaurs, and showed (with both quantitative analysis and comparative anatomy) that the forelimb of big pterosaurs was considerably more robust than would be expected for simple quadrupedal walking. Furthermore, if one uses the most recent estimates of wing shape and body mass for the giants, the estimated climb out range and soaring distance available are both really quite impressive. So then, even if we assume that the largest pterosaurs could only flap for a short burst (which is likely), they would have had a couple of kilometers of range before they had to switch to soaring flight (thermals, etc) which shoots another hole in the "too big to fly" argument.

Add into all that our analysis of terrestrial competence in these animals (which was decent for some giants, like azhdarchids, but pretty limited for Pteranodon and kin), and some general issues about estimating body mass and you have our PLoS ONE paper in a nutshell. Good times. For those of you still searching in earnest for the original paper, you can turn your browsers here.