|Next fall we will publish an amazing book by Mark Witton. It exclusively covers pterosaurs and is intended to be equal interest to researchers and interested readers. While wandering around the internet, I came across this terrific artwork and a sneak peak excerpt that Mark posted on his flickr site.|
The image above depicts a Pteranodon taking off, but obviously using its arms, not its legs, as the main launch propulsor. This may seem a bit odd, but read the excerpt below and all will be clear. Mark also suggests you check out this video from artist Julia Molnar and biomechanicist Mike Habib to further visualize the quadrupedal launch procedure.
The first stage of achieving flight, of course, is becoming airborne. As alluded to above, quite how pterosaurs did this has long been a problem and running starts, gravity, headwinds and/or different atmospheric conditions have been pulled in to help. Whilst there is little evidence for the latter, you can just about scrape an argument together that pterosaurs living in coastal or marine environments may have been able to find enough cliffs or wind to regularly employ them in takeoff, but the same certainly cannot be said for the increasing numbers of pterosaurs turning up in inland continental deposits. Inland settings are often very flat, windless and densely vegetated places that really limit the potential for finding suitable pterosaur runways. It’s particularly interesting that the real pterosaur giants, the azhdarchids, are most abundant in terrestrial settings too, as they presumably needed the longest, widest runways, strongest headwinds and sloppiest slopes to takeoff: it’s hard not to wonder just how many places these guys could conceivably take off from under such constraints. Adding further fuel to this quandary is the recent calculations by two teams of authors that suggest pterosaurs massing above 70 kg (Chatterjee and Templin 2004) or even 41 kg (Sato et al. 2010) were unable to launch at all, regardless of environmental assistance. These masses are strikingly low when the arguments on pterosaur mass outlined above are taken into account and, moreover, the widespread distribution, longevity and apparent volancy of all known pterosaurs seems to suggest their flight was not so environmentally limited. In short, then, our problem is thus: how did realistically massed pterosaurs become airborne without the luxury of a Mesozoic airport?
The answer, it seems, comes from the fact that we’ve been looking at only half of the picture. It has always been assumed that a pterosaurs launching from a flat surface will have taken off in a bird-like fashion, using a leap or a run to attain the velocity needed for flight. While this assumption may appear quite safe and, indeed, the strength of pterosaur hindlimbs have been noted (Bennett 1996; Padian 1985), two American pterosaur workers, Jim Cunningham and Mike Habib, have long pointed out that assumptions of bipedal launch contradict other evidence (e.g. Habib 2008). For one thing, it’s notable that walking and launching are achieved using the same gaits in birds and bats: birds walk and takeoff bipedally, whilst bats do the same tricks as quadrupeds. We may expect, ergo, that pterosaurs may have also stuck to their quadrupedal gait whilst taking off. Similarly, it turns out that launching with the hindlimbs alone seriously pumps up the leg skeleton of the launcher: bird legs scale much faster with mass than other parts of their bodies – even their wings – to meet the demands of shoving their weight into the air with only one set of limbs. Pterosaurs, by contrast, do not have particularly robust legs for their size but do bear strong, overscaled forelimb bones that are particularly well-developed around the shoulders (Habib 2008). Under mechanical analysis, these bones are more than strong enough to catapult twice the weight of a given pterosaur into the air, whereas their hindlimbs would fail at much lower stresses.
What this all points to, then, is the high probability that pterosaurs took off quadrupedally and, in all likelihood, from a standing start. Firstly, the pterosaur would crouch before shoving up and forwards with its hindlimbs to vault over its own arms. Almost simultaneously, the powerful forelimbs push upwards, changing the trajectory of the pterosaur body from one moving primarily forward to one moving forward and skyward. Whilst this is occurring, the wing finger is being extended out to open the wing and, as the animal becomes entirely clear of the ground, a partial upstroke is achieved as the arms are swept above the shoulders. The wing is fully open by the end of the upstroke and full flap cycles can begin, which the pterosaur would continue as it ascends to a suitable height. All this would take place in a very quick, highly synchronised fashion and even the largest pterosaurs would be clear of the ground in a second or so.
Such a sight would certainly be worth shedding out some cash to see and it’s worth taking a quick moment to consider the prospect of a giraffe-sized monster azhdarchid launching itself into the air in this way. What’s more, while this may sound like the latest in the long line of crazy ideas that have been proposed about pterosaur habits, bear in mind that a number of bats (most famously vampire bats) launch in a very similar fashion and, because they employ the most powerful muscles in their body –those developed for flight – their takeoff is particularly powerful and efficient (Schutt et al. 1997). In fact, little vampires almost spring into the air vertically, a trick, it must be said, that would not be seen in larger pterosaurs. With greater masses to heft into the air, climbout angles would decrease as size and mass increased, meaning the largest pterosaurs would require a relatively clutter-free area immediately in front of their launch site. Still, it would seem odd for large, gangly, volant animals to hang around in woodlands or forests anyway, so this probably wasn’t much of an issue. The evidence for quadrupedal launching appears pretty convincing, then and, if sceptics need further evidence of this behaviour, there are rumours of a trackway that may show a pterosaur taking off in just such a fashion (Habib, pers. comm.. 2010).