No Wing Tip Claw?
It has long been a paradigm of pterosaur studies that the wing (manual digit 4) lacked an ungual or claw (Bennett 2008). In most pterosaurs, even well articulated ones, there is no obvious wing claw. There is only a trochlear joint a the tip of the wing. Such a joint typically marks the “knuckle” between phalanges, in this case at the base of the missing fifth phalanx, the ungual. The presence of a trochlear joint at the wing tip, even on Late Cretaceous pterosaurs like Pteranodon (Figure 1), indicates the likely presence of another phalanx, the ungual, now missing (displaced) or invisible due to poor ossification.
When Did the Wing Ungual Disappear?
Pterosaur predecessors, including Cosesaurus, Sharovipteryx and Longisquama all retained a curved claw on digit 4, the digit which elongated to become the wing in pterosaurs. A basal bat retained unguals on all the wing digits, but other bats don’t.
A wing claw might be as useful, like a bumper on a car, to protect the wingtip from damage. With a keratin sheath, any sustained damage would sooner or later be shed during nail growth.
Dr. Chris Bennett (2008) envisioned the origin of the pterosaur wing with the palmar side of the wing finger facing anteriorly, with former flexors becoming wing extensors and vice versa. In Bennett’s (2008) hypothesis the wing claw point would have been oriented anteriorly originally. But that orientation would have tended to hook on passing obstacles in Bennett’s view, so it disappeared. Only those pterosaurs who failed to develop a wing ungual survived, because they would not have hooked themselves on tree trunks. Unfortunately, Bennett (2008) did not present any real ancestors, just hypothetical and rather cartoonish line drawings. So his hypothesis is based solely on imagination and its execution presents many evolutionary hurdles.
Evidence for a Wing Claw
Here you’ll see evidence of articulated wing unguals in several pterosaurs. On pterosaurs that apparently lack wing unguals you’ll see the ungual displaced from the tip, overlooked nearby.
Here’s a Great Example
In his blog Archosaurmusings.com, Dr. David Hone published the above image of the distal phalanx of an unidentified pterosaur wing on June 29, 2010. Dr. Hone reported phalanx 4 tapers to a point, which it does not (unless one considers the claw tip the point). Actually phalanx 4 tapers to a trochlear interphalangeal joint. Dr. Hone wrote, “Finally it often has a slight pathology (as seen here) of the tip curving to point slightly (or occasionally, very) posteriorly. [And more here]”
Dr. Hone did not realize that his purported “pathology” was actually just a very healthy fifth phalanx, the ungual or claw, which is traditionally considered missing in pterosaurs. As you can see, it is very much present here. It’s not big. It’s not dangerous. It’s just there. Often it is disarticulated, hence its apparent absence.
Other examples of wing claws can be seen here using mouse rollovers to see both the specimen and the interpretation. Below is a Shenzhoupterus wingtip with ungual. If you can’t find it, a guide can be found here.
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.
Bennett SC 2008. Morphological evolution of the forelimb of pterosaurs: myology and function. Pp. 127–141 in E Buffetaut and DWE Hone eds., Flugsaurier: pterosaur papers in honour of Peter Wellnhofer. Zitteliana, B28.