Re: The Peters Strikes Back (pterosaurs)

["James R. Cunningham" <jrccea@bellsouth.net>]

Jaime Headden wrote:
> 
> Jim Cunningham (jrccea@bellsouth.net) wrote:
> 
> <But the shoulder girdle isn't fixed....>
> 
>   
> thus there is no need to elongate the arm to match the leg in length. In 
> pterosaurs, the fixed shoulder prevents this.

But in pterosaurs, the shoulder girdle isn't fixed medially.  At least
not in the ones I work on.  It's quite free to pivot fore and aft (with
a rather cocked axis on the inboard side), and obviously does so.

> Solution?
> Allow the effective arm (arm sans wing finger) to be equal enough to match 
> the hindlimb in length (reduction of the humerus to
> horizontal also shortens the aspect of the arm), and thus shorten the arm.

This does happen in a way that doesn't involve shortening of any of the
bones themselves.  In Quetz, for which the inner arm is longer than the
leg by roughly about the length of the humerus, both the glenoid and the
proximal humerus have matching shelves that allow the humerus to be
directed upward while still supporting the weight of the body, and the
distal humerus and and elbow do as well when fully flexed (to roughly
160 degrees), so that the metacarpal and r/u can support the humerus in
the upended position while the upended humerus supports the body. 
Synovial sac markings also indicate that that range of motion is
supported by the animal.  This effectively makes the length of the front
leg very similar to that of the back, but in order to use the consequent
similarity in front and rear leg length while walking or running, the
animal would have to reverse the direction of motion at the shoulder
when using this technique.  Consequently, I think it is more likely to
have developed for other purposes to allow a greater range of motion
when on the ground, primarily for launch (when it is absolutely
essential in the most massive animals), and also to help reduce peak
final deceleration of the upper body, head, and neck late in the landing
process by spreding the deceleration over a longer distance.  There are
enough similarities between the proximal humerus in Quetz and Hatz that
I think this process is going on in Hatz as well, and is likely to be
present in several related species (wish someone would look at this,
hint, hint).

Jim