Re: Hone and Benton 2007 (their second paper)

[Tim Williams <twilliams_alpha@hotmail.com>]

Michael Habib wrote:

> 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).

Having not seen the presentation either, or read the entire thesis, I don't 
know how strongly held that conclusion (bipedal walking) was.  The hand-held 
predation hypothesis is certainly appealing: fish can be slippery!

> 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.

Yes, that makes perfect sense.

> If the claws were used in feeding, that signal would (I hypothesize) be 
> likely to swamp any weak locomotor selection on claw shape.

Exactly.  Predatory birds (raptors) can walk on the ground quite 
comfortably, even though their claws are specialized for predation.

> 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).

So what would the manual claws actually be gripping during a launch?

> 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.

Agreed.  I guess the 'take-home' message is that, when it comes to 
non-aerial functions (terrestrial locomotion, feeding, mating, etc) the 
forelimbs and hindlimbs were employed opportunistically.  It probably also 
varied between different pterosaurian taxa and/or with ontogeny.  But for 
rapid progression on the ground (or as 'rapid' as a pterosaur could get) 
quadrupedality was probably the general rule.

Cheers

Tim

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