Re: thoughts on Pterasaur feet

[Adam Yates <zooamy@zoo.latrobe.edu.au>]

On Thu, 3 Apr 1997, Bettyc wrote:

> Adam Yates wrote:
> > Interesting thought, perhaps it was a nonfunctionalgenetic consequence of 
> > digit
> > elongation in the forelimb. Much like the enlarged but unused tibial
> > sesamoid of Pandas (the homologous forelimb bone, the radial sesamoid, is
> > enlarged for use as a "thumb" - see Gould's "The pandas thumb"). For this
> > to be true though, the didgits would have to be homologous ie, both
> > fourth and fifth, unfortunately I don't think this is the case.
> 
> wouldn't it be likely this fusing of the 5th and 4th finger have occured
> AFTER it was used functionally as a wing?  To give strength?
> If that's the reason why the fingers fused, why would this fusing
> invalidate the homologous-mess of the long-toe?
> (I don't understand the full implications of 'homologous' in this
> instance)
> > -- 

I'll have to apologise for my original post it was written in a hurry and 
upon rereading it, is as about as clear as Adelaide tap water (Australian 
In-joke). The point I was trying to make was that the some of the genetic 
instructions used for limb making are used in both fore and hind limbs. 
In this sense pairs of elements eg, humerus and femur, thumb and big toe 
etc. are in a sense homologous. It is an interesting point that sometimes 
modifications that have adaptive significance on one limb occur on the 
other limb where there is no adaptive benefit. An example is the enlarged 
thumb like  radial sesamoid bone of the Pandas' wrist, wich is used in 
manipulating its bamboo food. Interestingly the homologous element in the 
ankle, the tibial sesamoid, is also enlarged (though not to the same 
degree) but is non functional. It struck me that if what you say about 
the long toe of the pterosaur foot is true then it could be an example of 
the same phenomenon. However the fact that the elongated toe is the fifth 
digit of the foot while the elongated finger is the fourth digit of the 
hand, so they are not "homologous" in the sense that I am using the word. 
Unless of course our interpretation of the pterosaur manus is incorrect 
and the wing finger is not the fourth....

Cheers 

Adam Yates