The bat wrist (carpus)
is interesting. And I’d like to know more about it than I do.
Figure 1. Bat carpus compared to that of the basal bat Onychonycteris, and two other mammals, Home and Ptilocercus. The pink bone is the pisiform. Some carpals appear to fuse in certain taxa. Note that in Homo and Ptilocercus the carpals are locked together, but rotate as a unit proximally. That is not the case with bats, which appear to have mobility within the carpals, not at the proximal articulation with the radius and ulna. On Onychonycteris digit 1 is preserved below the other digits and so is hidden in this graphic.
The wrist bones
of Homo and Ptilocercus are pretty well locked in as a unit, but a sliding joint permits movement (rotation in two axes) at the radius and ulna.
Bats appear to be different,
perhaps due to their ability of completely fold their hands (wings) against their antebrachia (radius + ulna or forearm), in the manner of birds and pterosaurs, in the plane of the wing. Above (Fig. 1) I have attempted to identify homologous bones. Please advise if any mistakes were made.
In the basal bat,
Onychonycteris, the carpal bones were not locked into place. This could be a taphonomic issue with elements drifting. Or perhaps it represents an early stage in bat carpus evolution. Or both.
In the only bat carpus illustration
I could find (genus unknown), the proximal carpals appear to be fixed to the ulna and radius. The distal carpal elements appear to be loose. The metacarpals appear to overlap like Japanese fan struts. I don’t see the same proximal metacarpal expansion in the basal bat Onychonycteris.
Since bat wing claws are still present
on all of the fingers of Onychonycteris and with wrist rotation in the plane of the wing, the claws would still have been oriented toward the palmar side and thus could be used. Or if not used, this orientation of the fingers and claws provides clues to those of ancestral bats with smaller hands not yet used for flying that might have been likewise rotated 180 degrees while oriented ventrally on tree trunks and other vertical substrates.
I have asked for,
but have not yet received, the pdf for Greene 2005 listed below. I’m sure there will be more to say on this subject when that pdf comes in.
References
Greene WE Jr. 2005. The development of the carpal bones in the bat. Journal of Morphology 89(3):409-422. Article first published online: 6 FEB 2005 DOI: 10.1002/jmor.1050890303
The Pterosaur Heresies 