Pterosaur wing extensors and flexors illustrated

Earlier we looked at the mistakes made in the Witton 2013/Bennett 2008 hypothesis regarding the location of muscles and tendons in pterosaurs and the flip-flopping of extension and flexion. Today we’ll look at these tissues from another angle, essentially a cross-section angle.

Bennett (1991, 2001) illustrated several wing joint surfaces. Here  (Fig.1) I use these as valid data, starting at the elbow. The more medial (further from the viewer) joint surface is in blue in the middle column. The  more medial (closer to the viewer) is in black at left. Anterior is toward the left, as in the smaller example of the entire left wing in dorsal view at right. Extensors in pink. Flexors in green.

Figure 2. Click to enlarge. Pteranodon extensor and flexor tendons in cross-section. Bennett's extensor (former flexor) tendon process is that little pink strip beneath metacarpal 3. Look how much larger it can be when the drawbridge of metacarpals 1-3 is palmar side down, as in all other tetrapods. Note how the radius is high proximally and low distally. This marks the neutral position, the same as whenever your palms face each other. This is not a supinated forearm.

Figure 2. Pteranodon extensor and flexor tendons in cross-section. Bennett’s wing extensor tendon (his former flexor, lower right figure) is that little pink strip beneath metacarpal 3. Look how much larger it can be when metacarpals 1-3 are palmar side down, as in all other tetrapods. Note how the radius is high proximally and low distally. This marks the neutral position, a slight crossing of the radius from the lateral condyle of the elbow to the thumb side of the wrist. the same as whenever your palms face each other. This is not a supinated forearm in which, by definition and configuration the radius and ulna are parallel.

Bennett (2008) sought a large extensor muscle and tendon to extend the wing finger. He also wanted to loop the tendon, like a violin string over its bridge, over the preaxial carpal sesamoid on the anterior rim. Strangely, Bennett found that large extensor on the flexor side. The flexor is the only muscle that starts on the humerus and ends at the fingers. So he invented a way for pterosaurs to flex the wingfinger the opposite way. That’s the only way to make that big flexor into a big extensor.

Such an invention based on pure imagination is not necessary. By Bennett’s (2008) own account, there is no fossil record for his hypothesis.

You get a bigger extensor by following fossil evidence.
And it certainly does not involve any awkward twisting, an unsubstantiated claim made by Bennett (2008) against Peters (2002) that was accepted by Witton (2013).

You can still get a big extensor muscle, even if it doesn’t start on the humerus and loop over the preaxial carpal sesamoid. The big extensor starts where other extensors on lizards do, on the wrist. The extensor gets phylogenetically bigger as metacarpal 4 gets thicker and hollows out anteriorly (Fig. 1). The extensor also gets bigger because the preaxial carpal expands the cross-section of the wrist where the extensor is anchored. The preaxial carpal is anchored by a tendon to the pteroid. The pteroid is anchored to the propatagium, which spreads stress along the entire anterior humerus with emphasis on the deltopectoral crest.

Now it must be remembered in neither Sphenodon nor Varanus do finger four extensors originate on the centralia (which migrate to become the pteroid and preaxial carpal).  In fact those two bones are typically devoid of muscle origins or insertions. However, as we learned yesterday, certain origins and insertions shift between Sphenodon and Varanus. Furthermore, in Huehuecuetzpalli the wrist became poorly ossified. Perhaps this stage permitted the migration of the two central wrist bones to the medial rim. We can see the ‘before’ and the ‘after,’ but the transition is cloaked in soft tissue mystery.

So, rather than a single muscle/tendon complex, we find in pterosaurs a chain of muscles, tendons and bones that served as wing finger extensors (Fig. 2). There is no need to turn flexors into extensors as Bennett (2008) has convinced other pterosaur workers. And there’s no untenable twists in the flexors and extensors in this model. All that is necessary is for metacarpal 4 to rotate on its axis and for two central wrist bones to migrate.

Figure 6. Pterosaur wing flexed and extended, compared to Cosesaurus and demonstrating the hypothetical movement of the loosely articulated pteroid and preaxial carpal.

Figure 3. Pterosaur wing flexed and extended, compared to Cosesaurus and demonstrating the hypothetical movement of the loosely articulated pteroid and preaxial carpal.

This heretical model is unpopular, because its author is unpopular, but it is simpler and follows fossil evidence and living examples. Hopefully this simpler hypothesis based on fossil and living taxa will pass the Occam’s razor test for readers.

References 
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.
Haines RW 1939. A revision of the extensor muscles of the forearm in tetrapods. Journal of Anatomy 73:211-233.
Haines RW 1946.
 A revision of the movements of the forearm in tetrapods. Journal of Anatomy 80: 1-11.
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods
Ichnos, 7: 11-41.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. – Historical Biology 15: 277–301.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29:1327-1330.
Wild R 1978. Die Flugsaurier (Reptilia, Pterosauria) aus der Oberen Trias von Cene bei Bergamo, Italien. Bolletino della Societa Paleontologica Italiana 17(2): 176–256.
Witton M. 2013. Pterosaurs. Princeton University Press. 291 pages.

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