Small post here. This one is going to seem incomplete, the title a tease. It’s a premise for more things. But you’ll see where I’m going with this. This concerns the issues of how we look at pterosaurs when they’re walking around on all fours. Normally, we might not care. However they moved is … however they moved, right? Well, not quite. Trackway evidence tells us that pterosaurs walked both quadrupedally and with their hands just a gauge wider than their feet. This posture tells us that the when you pull the hands in to the position of the feet, some pterosaurs would have some interesting postures.
It concerns the elbows. We assume (that’s everyone who’s looked at pterosaur wing anatomy in any detail) that the elbow is generally flexed. In flight, on the ground, etc. But how flexed is flexed? The angle changed in flight, as the wing collapsed for the upstroke, extended for the downstroke, and various situations where the inverse might be true. On the ground, though, it’s about how high you stand. Back in the 1980s, Peter Wellnhofer, with the benefit of an extensive collection of pterosaur fossils collected from the Chapado do Araripe in Brazil, site of the famous Santana exposures (Romualdo and Crato Formations; some refer to these as members of the Santana Formation), determined that the body posture of Anhanguera santanae was sprawled outward, the legs parasagittal but the arms decidedly not. The humerus stuck out sideways, the elbow flexed downward, and the forearm was mostly vertical. This made a low profile posture for the long-armed ornithocheiroids, most of whom shared the Anhanguera body plan.
The limb posture of pterosaurs has been occassionally approached. The most rigorous means typically involve the rigging of muscles to the bones to determine lines of action and effect. The latest of these was an attempt to determine if the muscle moments could infer elbow flexure and thus posture in a host of vertebrates, including pterosaurs, and discriminate them (amongst other taxa, including ceratopsians) from what are “typical” straight-elbow folks. That paper, by Shin-ichi Fujiwara and John Hutchinson, indicated the elbow adductor moments and elbow morphology (briefly touched on) indicated a likely upright, parasagittal posture for Anhanguera, thus contradicting Wellnhofer. Mark Witton has for some time now been illustrating pterosaurs with semi-vertical humeri, and this work compares well with Fujiwara and Hutchinson’s predictive model.
Some pterosaurs have humeri with distal ends that show the elbow was probably only slightly flexed: the end is broad, mostly flattened, indicated a low amount of mobility (if the intervening elbow cartilage wasn’t completely different, which is a possibility; limb fibrocartilage can have distinctly different shapes from the underlying bone, as anyone who’s prepared their dinner chicken or turkey knees might agree with). So the elbow stuck straight out? This meant the elbow was straigther than assumed, and perhaps may not have gone through such a wide arc when shifting between terrestrial and aerial locomotion. Mark Witton’s pterosaur art shows this prominently by folding the humerus close in, and rotated so that the elbow flexes to the rear as it does in, say, rodents. This pulls the elbow in near the ribcage (see above).
Before I continue and get to the meat of this with pterosaur shoulder anatomy (but I won’t in this post, though don’t worry, I will eventually) here’s a drawing showing a pterosaur in these various postures.
The the thing you will notice is that the side and rear-flexed upright postures (B, C) are so much taller than the Wellnhoferian posture. The feet are closer to the hands in all dimensions. (Indeed, some tracks suggest the hand and foot would strike at the same points on the ground across their strides.) The major difference above is that with C, the forearm would have to be almost as wide-spaced as in B unless, as in the illustration at the time, it were slightly flexed or bowed inward, or angled. There is, at this moment, no data to suggest this was the case, nor that it wasn’t.
Another analysis, this time in 2005 by Michael Fastnacht, concerned the pelvis of a dsungaripterid-like pterosaur from Germany which suggested that the muscle attachments of the hip and leg prevented the Wellnhoferian sprawl; instead, the body was tilted strongly upward. The only way this posture can be achieved is were the elbow less flexed during quadrupedal stance. Another point that is certain to be of interest is that in the Wittonian model, the center of mass, located somewhat at the shoulder joint, is a bit in front of the hands: This posture would work primarily only during a stride when the other forelimb could prevent a fall. Conversely, the odder posture keeps the CoM above or behind the hands, and thus the posture is stable regardless of movement. The upward tilt isn’t affected. However, there’s more to posture than moment arms: we must consider the relationship of muscle and other soft-tissue to the limbs, but also the joint structures themselves.
Whatever the “right” posture, it’s useful to keep in mind that pterosaurs were an ungainly or graceful bunch on the ground, and not the horrible sprawling demons of the 1800s.
Fujiwara, S.-i. & Hutchinson, J. R. 2012. Elbow joint adductor moment arm as an indicator of forelimb posture in extinct quadrupedal tetrapods. Proceedings of the Royal Society of London, B: Bioloogical Sciences 279: 2561-2570.
Fastnacht, M. 2005. The first dsungaripterid pterosaur from the Kimmeridgain of Germany and the biomechanics of pterosaur long bones. Acta Palaeontologica Polonica 50 (2): 273-288.