Pterosaurs really do seem to have become the theme here of late, and I’m hardly one to miss a chance to talk about them. This time it’s the last bone of the wing finger, less well known as phalanx IV-4 since it is the 4th bone of the 4th digit. It might seem like there is little to say, but there are some features of this bone that are worth noting.
For a start, it tapers to a point which is not seen much even in animals where there is no ungual (claw) bone. It does happen, but not often to this degree. This is actually a little odd since, while strain on the wing will be less at the tip than the centre, the tapering extends over the whole length of the bone rather than only really being concentrated at the very end as you might expect. Certainly it’s not like the whole wing finger gets progressively thinner down it’s length as a result of the ever decreasing forces, so why taper the 4th phalanx alone. I’m not aware of any obvious mechanical reason (though my knowledge of aeronautics is not that great, I know enough to find it odd) and I can’t recall any explanation having turned up in the literature.
This bone is also generally the shortest of those in the flight finger, but not always. It is always the shortest in the pterodactyloids, but a couple of basal forms have a IV-4 that is bigger than 3 or 1. Incidentally, it’s also proportionally longer in the rhamphorhynchoids than the pterodactyloids (i.e. it makes up more of the wing).
Finally it often has a slight pathology (as seen here) of the tip curving to point slightly (or occasionally, very) posteriorly. This is a pathology and not an evolutionary adaptation since it is rare and never seen consistently on taxa or (to my knowledge) individual specimens. In other words, while some wings and odd bones show it, you don’t see all members of a single species with it and I don’t think I have ever seen it on both wings of a given specimen. However, what causes it is another matter – it could be a developmental problem where the bones are deformed during growth or the effect of stresses and strains of flying causing the bones to bend and twist. There’s no direct evidence either way, but given that you don’t see the other bones of the wing distorted in this way then I suspect the latter rather than the former as otherwise all the bones would be affected and not just the tips.