Finger 5 in Triassic pterosaurs

Earlier
we looked at examples of manual digit 5 in pterosaurs here, here and here. Traditional paleontologists don’t recognize it. In fenestrasaur pterosaur precursors, like Cosesaurus, Sharovipteryx and Longisquama, manual digit 5 is small, but easier to see because it is relatively larger. You can see the evolution of the pterosaur hand here.

In pterosaurs manual digit 5 is extremely tiny and hard to find (Fig. 1). It can be lost during taphonomy. Fossilization tends to scatter the elements over metacarpal 4, which is often cracked and covered with sinewy soft tissue, factors that make identification all the more difficult. More to the point, based on the current paradigm, no one looks for it during preparation. These are NOT the best examples.

Figure 1. While difficult to discern at this scale, here is where I think the elements of manual digit 5 reside on these four Triassic pterosaurs.

Figure 1. These are not the best examples. While difficult to discern at this scale, here is where I think the elements of manual digit 5 reside on these four Triassic pterosaurs. Images from Dalla Vecchia and Cau 2014. Metacarpal 5 is located, as in other tetrapods, lateral to metacarpal 4, but in pterosaurs metacarpal 4 is axially rotated 90 degrees and metacarpal 5 ends up dorsal to metacarpal 4.

Complete loss of a body part is rare.
Often enough, as in basal whales and snakes, a vestige, like manual digit 5 in pterosaurs, remains. Granted it’s not easy to see. On the other hand, we’re not looking for it. Start with the proximodorsal surface of metacarpal 4. While there look for c5, mc5 and digit 5 composed of three elements including the ungual.

While we’re here looking at the metacarpus,
note that metacarpal 1 is connected only to metacarpal 2 and metacarpal 2 is connected only to metacarpal 3 and metacarpal 3 is connected only to metacarpal 4 — as in all tetrapods. There is no draw bridge-like rotation of mc1-3 to attach dorsal-to-dorsal to metacarpal 4, as promoted by pterosaurs workers who think fingers 1-3 were palmar side anteriorly during flight. That large space dorsal of metacarpals 1-3 is where all the finger extensors remain free to operate, with the wing finger tendon largest by far.

References
Dalla Vecchia FM and Cau Andrea 1014. Re-examination of the purported pterosaur wing metacarpals from the Upper Triassic of England. Historical Biology. To link to this article: http://dx.doi.org/10.1080/08912963.2014.933826

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