Today we get the second guest post on pterosaurs. In addition to his extensive background in biomechanics, Mike has started a blog with Justin Hall on biomechanics - you should check it out. Now, one with the main programming...

Great Flying Skeletals: Perspectives from Pterosaurs

Scott has graciously invited me to do a little guest post from the perspective of a biomechanist. While Skeletal Drawing focuses primarily on dinosaurs, I am going to depart a bit on this occasion and use pterosaurs to highlight some of the biomechanics issues that can occur with skeletal poses.

The Case of Running Pterosaurs

Many of you have probably seen pterosaurs reconstructed in the popular standard pose (particularly preferred by Greg Paul) where they are in mid-sprint, like so:

From here.  Copyright Gregory S. Paul. — From here. Copyright Gregory S. Paul.

It's an impressive pose for a flying animal. A lot of people really like the look, and it lines up nicely with the same pose in birds, to which pterosaurs are often compared. There's just one (big) problem with it: pterosaurs probably never sprinted around on their hindlimbs like the reconstructions show.

In the case of pterosaurs, the "standard running pose" is typically presented as a launch pose. However, in the late 90's, Jim Cunningham made a strong case for quadrupedal launching in Quetzalcoatlus at a series of presentations for both engineers and biologists. In 2008, I published a manuscript on a sizable comparative study I ran on bone structural strength estimates in the forelimb and hindlimb, which demonstrated that most pterosaurs probably launched quadrupedally rather than bipedally.

Now, I know you're thinking "Oh c'mon, Mike, you just don't like those bipedal running pterosaurs because they conflict with your personal results. You're biased!" I may be biased in some sense, but actually, that's not the problem. I would not mind bipedal, sprinting pterosaurs if another study had used different data to support the idea. But the reality is that no analysis has ever produced support for bipedal launching in pterosaurs. In fact, so far as I am aware, my paper was the first attempt at testing between the two modes of launch. There have not been a great number of biomechanical analyses run on pterosaurs, but there were a handful back in the 1970's and again in the early 2000's. A few of these considered their performance during takeoff, and the authors all assumed a bipedal launch mechanism, as in birds. The key word there is assumed - those studies asked the question "if pterosaurs launched like birds, then how would it work out?", but they never actually tested if a bipedal run was likely. I think the first lesson here is this:

Don't reconstruct skeletal images in poses the animal was not known to reach, unless you are specifically trying to argue the plausibility in conjunction with the pose, with appropriate empirical data present.

Most viewers of a skeletal reconstruction will assume that the animal could (and did) the action shown by the skeletal pose. More discriminating viewers may consider the issue more thoroughly, but either way this gets in the way of the point of a typical reconstruction.

A typical skeletal is supposed to show off the anatomy. If the paper you are illustrating happens to be arguing for a specific dynamic action, then it makes sense to show the animal in that pose. If there is a dynamic pose that others have shown to be plausible, then that's fine, too - but not as a standard pose, because there will nearly always be some animal that you come across later that can't do it. Nearly all terrestrial vertebrates can manage a slow walk, but only some can sprint - so choosing sprinting as your standard is risky. Inevitably, something is going to end up sprinting in your illustrations that never did so in life.

We can make an animal do anything we want in an illustration. Scott made an Allosaurus do a handstand. We could make Quetzalcoatlus launch by vaulting on its beak. These extreme examples are obvious, but less extreme cases can be difficult to detect. The ability to render a good illustration is powerful, because it can make the action or anatomy suggested by the image seem plausible, even if it's completely false or fabricated. If you're an illustrator, use your powers wisely.

Where Does that Wing Go?

One really tricky issue with pterosaurs is the wings - we don't know for certain where the wings attached in most species, and even if we pick a particular attachment point, there are a range of potential resulting wing shapes (if you want to read more about this issue, check out the section on flight over at pterosaur.net).

(Image from David Hone's blog.)

Of course, the soft tissue extent need not affect a skeletal reconstruction, but the typical methodology for skeletal illustrations is to include a black body outline. Usually this can made relatively conservative and follow typical muscle contours for vertebrates, but there is still a certain amount of conjecture there. When there is a major soft tissue structure involved, like the wing of a pterosaur, this can get very tricky. Any wing profile you show will be taken much the same as a pose: the viewer will assume you are explicitly supporting that particular wing shape.

One way to get around this issue is to leave off the wing membrane. A simple black muscle contour can be drawn around the wing spar and then the wing itself can be left out. This is, from my perspective, probably the best option if one is trying to simply show pterosaur skeletal anatomy in a neutral manner. However, if your illustration is for wide audience, then be forewarned that you will probably need to make a note that the wing is left out somewhere in the caption, or else non-specialists will be very confused.

This same problem can arise with other taxa, of course, especially swimmers - flukes and fins tend to be largely soft tissue, and sometimes there is no exact match to a bony contour (look at the tails of cetaceans, for example). In those cases, leaving off the fin or fluke might be especially confusing, so some sort of estimate might be required - but again, there is a danger of extrapolation. I do not know what the best option in that case would be, though half-tone might be an option for showing speculative components.

Up and Away?

The last major challenge for pterosaur skeletal illustration is probably choosing whether to show them flying or walking. As already discussed, bipedal sprinting is not likely, but pterosaurs certainly walked around (quadrupedally, as it turns out, based on the trackways), so one could easily show them walking, which would tend to match other skeletal reconstructions and work well for the standardization aspect. However, it can be difficult to show off pterosaur anatomy well with a walking pose, because the folded wing gets in the way. Some illustrators may also prefer to remind the viewer that the reconstructed critter was a flying animal. As such, there are some good arguments for a flying pose. As a biomechanist, I tend to be happy seeing either or both options, but this does bring one final consideration for skeletal standardization, which is that any given illustrator will inevitably need multiple standards. You can't really show a mosasaur walking, for example. As such, it may be that a separate standard is needed for each major mode of locomotion - flyers, swimmers, and walkers, as it were.

There are additional concerns that could be raised for pterosaurs, and for biomechanics interests, but those three general issues (plausible posing, soft tissue suggestion, and multi-modal animals) are some of the factors that speak most quickly to biomechanists like myself. Happy reading! Thanks again to Scott for the guest slot.

Mike Habib