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Digit Loss

I've always been interested in the subject of digit loss, and this conversation 
has intrigued me.  Hans Larsson gave an excellent talk on digit loss at the 
Armour Symposium, although he focused on the how instead of the why.  

I definitely tend to agree with George that digit loss occurs for a reason!  
When theropods continued to lose digits there was obviously something, either a 
newly evolved structure or some sort of evolutionary pressure, that made 
keeping all five digits harmful.  

The primitive condition for all amniotes is five digits, and extant birds have 
from 0-3 (if I'm not mistaken).  Some birds, like the extinct Hesperornis, even 
lost their entire radius and ulna, and possessed only a humerus with a 
rudimentary stilt.  I recall reading that this may have helped Hesperornis 
glide through the water, much like pengiuns and other shore birds.  

For Hesperornis' lifestyle (the water), keeping three digits was harmful,as it 
would have possibly prevented the animal from becoming a successful predator. 
So, evolution reduced the original number of digits to this stilt.  The same is 
true in theropods, I would tend to say.  Exactly what made possessing five 
digits harmful?  Maybe it was the evolution of the wing, maybe not.  Perhaps it 
was locomotion.  What is interesting is that some theropods have lost two 
digits, and these digits never seem to re-evolve.  This may mean that the 
evolutionary pressure still existed, preventing the re-evolution of the 
primitive state.  

Digit loss is something commonly seen, so it must have been an evolutionary 
response to a commonly recurring pressure.  The answer is out there...somewhere.


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