As readers know, I’ve been fighting the experts on each of these four key traits of the pterosaur hand (Fig. 1). Here’s the evidence for:
1. Metacarpals 1-3 were oriented palmar side down. Here in dorsal view (Pterodactylus scolopaciceps BSP 1937 I 18 (Broili 1938, P. kochi No. 21 of Wellnhofer 1970, 1991), only metacarpal 4 axially rotates posteriorly to enable wing folding in the plane of the forelimb (not flexing against the palm as our fingers do. In lateral view that leaves room above metacarpals 1-3 for the big wing extensor tendon that wraps around the big wing finger knuckle to unfold the wings. And it leaves room for the 3 small extensors to extend digits 1-3.
2. Manual digit 5. With the axial rotation of metacarpal 4, the vestige that is manual digit 5 rotates with it to a dorsal exposure. Yes, it’s tiny because it’s useless. But if you look close enough, virtually all pterosaurs retain this vestigial digit right where you see it. If it is disarticulated you’ll have to look harder for it.
3. Since mc 1-3 were palmar side down, so were the fingers. Here, however, fingers 1-3 are all disarticulated so that their former palmar surfaces are now anteriorly oriented. This happens because the unguals (claws) are tall and thin, so they typically fall down flat during crushing. In life they would have been palmar side down, which is ideal for tree grappling and when implanted while quadrupedal, they would typically splay laterally to posteriorly, matching known tracks.
4. Pteroid articulation and orientation. The pteroid is articulated to the proximal carpal (radiale, Peters 2009), not the preaxial carpal, which is articulated to the first distal carpal. These are former centralia that have migrated similar, but convergent with the situation in mammals described here.
What you see is what you get.
Please don’t fall for hypotheses that postulate other origins and orientations for the pterosaur fingers and pteroid based entirely on imagination, earlier reviewed here. More on pterosaur hands here.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29: 1327-1330.
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133. Wellnhoffer P 1991. The Illustrated Encyclopedia of Pterosaurs. London: Salamander. 192 pp.