Pterosaur wing folding problem solved!

Traditional paleontologists (Witton 2013, Bennett 2008) locate the pterosaur wing extensors and flexors on the proximal processes of manual 4.1, the big wing finger (Fig. 2). Unfortunately this brings up several problems.

Extensor problems
The extensor tendon process is unfused in many taxa. This is a phylogenetic trait, not an ontogenetic one. Even if ontogenetic, pterosaurs were flying within moments of hatching so such a connection still requires a pull on this process opposing the drag of the rest of the wing. Engineering-wise, this would tend to pull that joint apart. Its much better mechanically and matches living lizard morphology to attach the extensor tendons more distally on both sides of the m4.1 (proximal wing phalanx). The extensor tendon process actually acts more like a stop, preventing the wing from overextending, like the olecranon process does on various tetrapod arms. Likely nothing critical was connected to it.

Figure 1. Pterosaur (Santanadactylus) wing folding. Note when the wing is perpendicular to the metacarpus the flexor must be off axis in order to complete the wing folding process. The insertion  must be distal to the joint because the flexor process of m4.1 extends dorsally over the metacarpus during wing folding. Otherwise the ventral (palmar) flexor would be cut off from the swinging dorsal process.

Figure 1. Pterosaur (Santanadactylus) wing folding in ventral view (while flying) or medial view (while walking). Note when the wing finger is perpendicular to the metacarpus the flexor must be off axis in order to complete the wing folding process. The insertion also must be distal to the joint because the flexor process of m4.1 extends dorsally over the metacarpus during wing folding. Otherwise the ventral (palmar) flexor would be cut off from the swinging dorsal process. This also gives  a pretty good view as to what is happening with the metacarpal alignment, palmar side down for digits 1-3.

Flexor problems
When the wing finger is folded at right angles to the metacarpus the flexor process of the wing finger is essentially in line with the metacarpal axis, removing all leverage for further closure. In other words, no matter how much you pull, the flexor process does not get closer. In fact, it starts to rotate away when the wing finishes closing. To remedy this engineering problem the flexor insertion point must be located further distally, away from the rotation axis (Fig. 1). This provides leverage until the wing finger is fully closed (flexed) against the metacarpus. In addition, the thin sheet of dermal muscles between each aktinofibril may also serve to reduce the wing and fold it further by pulling the individual aktinofibrils closer together (Fig. 1). Both methods probably worked in concert.

Finger 3
This illustration (Fig. 3) is the first to show details on the unique metacarpophalangeal joint of digit 3. There were two planes of articulation at the metacarpophalangeal joint and each would pop into place as necessary for flight or terrestrial locomotion. This is why digit 3 impresses posteriorly while the others most often impress laterally.

Unlike archosaurs and mammals, lizards do not have finger extensors that anchor on the distal humerus. In lizards an extensor originates on the humerus and inserts at the proximal bases of the metacarpals. All finger extensors originate on the dorsal intermedium or dorsal ulnare, as they do here in pterosaurs  (Fig. 1). But see below for the alternate view.

On the other hand (pun intended) in lizards certain flexors do originate on the humerus. In some lizards (Iguana) after passing through the flat palmar aponeurosis the flexor splits on the palm toward the fingers. In other lizards (Varanus) there is no palmar aponeurosis. In Sphenodon the palmar aponeurosis is very well developed, so phylogenetic bracketing gives that condition to tritosaurs including pterosaurs. Other flexors (Flexores breves superficiales) orginate at the wrist. The main flexor of the digits is the flexor digitorum longus/ It has five distinct heads in Varanus. Three arise from the medial condyle of the humerus. Two arise from the ulna and ulnare. The ulna head forms a broad, thick flexor plate which receives the three humeral heads on the superficial surface, just above the wrist. Five strong tendons arise from this plate distally and extend to the unguals. These are the tendons that curl anurognathid wing fingers.

In Bennett (2008) and Witton (2013) it was the flexor digitorum longus, with origin on the humerus, that flipped service to act as an extensor to open the wing (Fig. 2). This occurred after the finger stopped flexing at all and started hyper-hyper extending, according to Bennett (2008) and Witton (2013), without fossil evidence.

Short flexors
In Varanus each digit is provided with deep flexors which arise in pairs from the carpal bones, and are inserted on each side of the proximal phalanges. There is less room to do so in fenestrasaurs, which have appressed metacarpals, as we’ll see in future posts.

Pteranodon myology (Witton 2013) based on Bennett (2008).

Figure 2. Pteranodon myology (Witton 2013) based on Bennett (2008). Here the flexors are the extensors and vice-versa. And both attach to the proximal processes of the first phalanx of the wing (m4.1), which is suboptimal according to what we just learned in figure 1 and doesn’t match living lizards. Note that no tendons cross the palmar and dorsal surfaces of the wrist, which is different than all other tetrapods.

Witton (2013) and Bennett (2008) see things differently
Witton (2013) follows Bennett (2008) in supinating the forelimb, stopping flexion and turning extension into hyper extension until the dorsal surfaces of both the wing metacarpal and the wing finger are in contact during folding. (Ouch!). That’s why, in Witton’s and Bennett’s view pterosaur flexors extend the wing finger and their extensors flex (fold) it. Moreover, both workers attach their flexors and extensors to the proximal processes, not foreseeing the problems that arise when that is done (Fig. 1) and not following lizard morphology.

The Peters (2002) axial rotation of metacarpal 4 solution is supported by fossil evidence and involves a minimum of evolutionary weirdness compared to the Witton/Bennett model. We’ll see more of that in coming posts.

These data were part of a 2009 manuscript that was rejected by traditional pterosaur referees who supported the Bennett (2008) model. That’s why it has always been an uphill battle, even with good fossil evidence. That’s why I’m pushing to have traditional paleontologists have a good look at the fenestrasaurs, a clade they have studiously avoided (and yes, I’m also talking about Hone and Benton 2007-2009 here) over the last dozen years.

There will be several posts on pterosaur myology (muscles) over the next few days. Hopefully these will help us understand pterosaur anatomy better. Today’s post is only part of the story.

Bennett SC 2008. Morphological evolution of the forelimb of pterosaurs: myology and function. Pp. 127–141 in E Buffetaut and DWE Hone eds., Flugsaurier: pterosaur papers in honour of Peter Wellnhofer. Zitteliana, B28.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. – Historical Biology 15: 277–301.
Witton M. 2013. Pterosaurs. Princeton University Press. 291 pages.

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