Manual Digit 5 in Pterosaurs: It’s Still in There!

Figure 1. Manual digit 5 in pterosaurs. Click to access a webpage of several examples.

Time to Bust Some More Paradigms
There are three traditional hypotheses that go back over 200 years. The experts tell us all pterosaurs lack: (1) manual digit 5, (2) the wing ungual and (3) the pedal digit 5 ungual. We can see all three on predecessor taxa, such as Huehuecuetzpalli, Cosesaurus, Sharovipteryx and Longisquama, so it’s a wonder why at least some vestige isn’t preserved in Triassic pterosaurs, at the very least. Some snakes and whales retain vestiges of “lost” limbs. Some birds retain wing claws. So it seemed worthwhile to look more closely at pterosaurs to see if some vestiges of these three traits were retained.

Here we’ll take a look for them one at a time.

pterosaur wings

Figure 2. Click to enlarge. The origin of the pterosaur wing and whatever became of manual digit 5. A. Sphendon. B. Huehuecuetzpalli. C. Cosesaurus. D. Sharovipteryx. E. Longisquama. F-H. MPUM 6009, a basal pterosaur. Manual digit 5 is in green. Metacarpal 5 is a darker green. Carpal 5 is yellow. The reduction to a vestige began with Huehuecuetzpalli. Digit 5 was no longer than metacarpal 4 in Cosesaurus and Longisquama. It became half that size in basal pterosaurs and relatively much smaller in derived pterosaurs with an elongated metacarpus.

Manual Digit V
The pterosaur wing is a former hand that has been modified for flight (Figure 2), but in a different way than those of birds and bats. There was once some controversy over the identity and homology of the pterosaur fingers and pteroid. Goldfuss (1831) interpreted the pteroid as digit 1 and the wing finger as digit 5. However, Owen (1870) interpreted the wing as digit 4 and argued the pteroid was not a digit, an idea that is now universally accepted. That leaves us with a digit 5 that apparently disappeared sometime in the ancestry of pterosaurs (according to Bennett 2008) because no one had ever seen digit 5 in any pterosaur. After all, why look for something that isn’t supposed to be there? You’d have to be a heretic to test such a tradition.

Manual Digit 5 was Retained as a Vestige in Every Pterosaur
In the chaos of broken bone at the base of the metacarpal 4 in every pterosaur you’ll usually find the tiny remnants of digit 5, its metacarpal and carpal if you look hard enough. As in basal fenestrasaurs (Figure 2), digit 5 had three phalanges, including a sharp, curved ungual. The ungual helps to identify the rest of the finger. Typically the finger elements were curled one way or the other during taphonomy. Several samples can be seen here.

There was likely no use for this finger. Whether it was covered in skin or remained visible cannot be determined. It appears, as you might expect, on the former posterior margin of the metacarpal 4. In all pterosaurs metacarpal 4 has been rotated 90 degrees to facilitate wing folding in the plane of the wing. That’s why digit 5 now appears on the present dorsal surface of metacarpal 4 (contra Bennett 2008). The presence of digit 5 on the dorsal surface indicates that the entire metacarpal 4 rotated on its axis, rather than twisting only at its distal end.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References:
Bennett SC 2008. Morphological evolution of the forelimb of pterosaurs: myology and function. In: Buffetaut E, Hone DWE, editors. Zitteliana Series B, 28 (Special Volume: Flugsaurier: pterosaur papers in honour of Peter Wellnhofer). p. 127–141.
Goldfuss GA 1831. Beitränge zur Kenntnis verschiedener Reptilien der Vorwelt. Nova acta Academiae Caesareae Leopoldino−Carolinae Germanicae Naturae Curiosorum 15: 61–128.
Owen R 1870. A monograph on the fossil Reptilia of the Liassic Formations. III. Part II: Order Pterosauria. – Monograph of the Palaeontological Society, London: 41-81
Unwin DM, Frey E, Martill DM, Clarke JB, and Riess J. 1996.
On the nature of the pteroid in pterosaurs. Proceedings of the Royal Society of London B 263:45–52.

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