Shell Crushing Pterosaurs and Bad Posture

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.

Postures of pterosaurs in VERY schematic fashion, with red/orange/yellow representing forelimbs, and blue/aqua representing hindlimbs.

Postures of pterosaurs in VERY schematic fashion, with red/orange/yellow representing forelimbs, and blue/aqua representing hindlimbs. Wellnhofer’s model is left, Witton’s model is middle, and a hybrid is right.

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.

Image from Witton’s Pterosauria, from the inside cover, showing two distinct ornithocheiroids and their tucked-in elbows. Used with permission and copyright Witton (2013). Do not use without his permission.

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.

Dsungaripterus Skeleton Elbow Flex & Extend & Side - smThe 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.

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5 Responses to Shell Crushing Pterosaurs and Bad Posture

  1. Duane Nash says:

    But nothing to preclude the use of all three postures you go over? I am a little confused by the wording in the last paragraph “pterosaurs were an ungainly or graceful bunch on the ground”. Is this an either/or thing or are you suggesting a spectrum from cumbersome to graceful? Or a typo or missed qualifier in there?

    • I’m leaving that line as an exercise for the reader, but it is an either/or proposition with ambiguous wording. It’s not meant to be conclusive: This isn’t a conclusive post.

      I would say, though, that the track evidence tells up the Wellnhoferian sprawl is unlikely, at best.

  2. Jaime, good post. Was not aware of that paper.

    Since nearly every ptero-clade had a distinct forelimb/torso/hindlimb ratio each major variation should be considered on its own. Anurognathids had right angle femoral heads. Anhanguerids did not. Basalmost pterosaurs, like MPUM 6009, could not reach the ground with their short arms, Pteranodon and Nyctosaurus had wing metacarpals so long they did not permit finger/ground contact. So every case deserves individual attention.

    To your point, not all pterosaurs walked quadrupedally all the time. The proof is in their tracks:

    With reference to your drawings, remember the radius and ulna could not suppinate or pronate (as in birds and bats) so when you rotate the elbows out, you also must rotate the radius medially and ulna laterally, which rotates the laterally-to-posteriorly extended fingers (1-3) anteriorly to laterally, which is not reflected in any pterosaur tracks. The GIN 125/1010 specimen attributed to Noripterus definitely has the elbows back, seen here:

    I assembled a re-inflated Pteranodon skeleton, so I can tell you that the combination of torso, pectoral girdle, humerus, etc. results in a rear-flexed elbow orientation with several degrees of abduction (in dorsal view) in that taxon. Most pterosaurs are similar in standing dorsal view. Hyperextension of the humerus and hyperextension of the elbows are two common problems in ptero reconstructions.

    Also key to pterosaur terrestrial locomotion is the relationship of the femur to the prepubis, which anchors muscles of adduction (and could not be used in respiration). So at rest the femur was aligned with the prepubis. In motion the femur would have moved fore and aft from this position. I note your reconstructions don’t appear to align the femur to the prepubis.

    In lateral view I have provided a small Pterodactylus pterosaur matched to a Crayssac track animated here:

    Hope this proves helpful. Do we have any dsungaripterid tracks? I’d like to see those, if so. I noted earlier that some Pteranodon specimens (tall crested species) had flat feet while others (long crested species) did not here:

  3. Interesting post, Mr. Headden. What is your opinion on more derived azdarchid pterosaurs, say Quetzocoatlus, being depicted with upright elbows flexed to the side? Also, what is your opinion on Basal Pteronodontia (i.e Nyctosauridae) being depicted with hair like filaments?

    • First question: I’ll be getting around to what makes sense for pterosaurs generally and directionality of their arms when walking, but so far it doesn’t seem like there’s a difference among them. The ability to pull the elbow close to the body seems present in most pterosaurs, but so too to extend the arm outwards but flexing the elbow down. It depends.

      Second question: It looks like most, if not all, pterosaurs were fuzzy. This strongly depends on where Anurognathidae lies in the family tree: if it’s the most basal group, or near it, filaments are inferred for all non-pterodactyloids by extension, as Sordes pilosus is a rhamphorhynchoid/”pterorhynchid” near to Darwinopterus.

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