Re: Hone and Benton 2007 (their second paper)

[MICHAEL HABIB <habib@jhmi.edu>]

 >  There was one study that found that the curvature of pterosaur toe 
> claws 
>  tended to be similar to the claws of walking or wading birds.  By 
> contrast, 
>  the claws on the wings were curved like the toe claws of birds that 
> use 
>  their feet for grasping or perching.  Rather than suggesting 
> arboreality in 
>  pterosaurs, the hypothesis was that pterosaurs walked bipedally but 
> used 
>  their wing claws to grasp prey while it was being chomped on by the 
> jaws.  I 
>  don't know if this study has been published yet, but it did form the 
> basis 
>  of a thesis (Atreyee Gupta) and an SVP presentation (David Krauss).  
> (I 
>  don't know if the study took the keratinous sheath into account.)


That's quite intriguing; thanks for posting on it (I must have missed that 
presentation somehow...I'll have to look up Atreyee's thesis).  In any case, 
the predation mechanic sounds like a strong conclusion to me (for what that's 
worth).  However, I have a feeling that concluding bipedal locomotion from 
those claw morphologies is slightly premature (though certainly among the range 
of rational conclusions).  In many taxa, much of the weight would not be put on 
the actual fingers during quadrapedal locomotion (metacarpal IV would be the 
main support).  In those that would use the manual digits I, II, and III when 
walking, I would not really expect the claw shape to be under particularly 
strong selection from a locomotor standpoint, because the forces would still be 
largely transmitted to mtC IV.  If the claws were used in feeding, that signal 
would (I hypothesize) be likely to swamp any weak locomotor selection on claw 
shape.  Besides, the manual claw function in locomotion w
ould largely have been to grip during leaping launch, in which case an 
"arboreal" like claw might actually be the most mechanically sound solution 
(making it doubly useful).

>>  Yes, I agree.  On my earlier point, I would say that adopting a 
> bipedal 
>  stance for handling prey is separate from bipedal progression per se. 

Good point.

>  Although there's nothing to suggest that pterosaurs couldn't walk and 
> eat at 
>  the same time.

Also an excellent point.  After all, the hind limbs in pterosaurs aren't really 
that weak, relative to their mass (and relative to the size of the thorax 
and/or abdomen).  Really, pterosaurs' startling proportions are a result of 
hypertrophy of the forelimbs, head, and neck.  As such, I do not see any reason 
why a pterosaur could not walk and eat, or move bipedally to reach for an 
object with the forelimbs.  However, running and launching would be much better 
served (ie. much more powerful) using quadrapedal progression.  Combining this 
fact with the known quadrapedal trackways, and the fact that all known 
pterosaurs could contact the substrate with the forelimbs while standing, I 
suggest that the most rigorous actions (sprints, launches, etc) were 
accomplished quadrapedally.  Basic walking probably was, as well, barring 
munching on the move.

Cheers,

--Mike H.