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Archie fingers

I think part of my post about digit development may help clear up 
something that has been bothering me.

The "birds are nowhere close to dinosaurs" people are fond of flaunting 
embryological studies showing that the three digits in a bird's wing are 
II, III, and IV, whereas, they say, in theropods they are I, II, and III 
(and sometimes IV).  Even a cursory examination of the bones in question, 
however, makes it perfectly obvious that the bird and theropod hand bones 
are entirely homologous.

I think I may know where the confusion came from (although I warn you 
that this is only a guess).

I mentioned earlier that the functional digits of tetrapods are formed in 
a branching sequence arising from the ulna (or fibula).  Well, actually, the 
sequence goes like this:  humerus/femur, ulna/tibia, proximal carpal/tarsal 
IV, prox. III, prox. II, prox. I.  The radius/tibia and proximal 
carpal/tarsal V are side branches from the humerus/femur and digit IV, 
respectively.  The digits arise as linear segmentation series from the 
proximal carpals/tarsals.  Don't worry if a particular animal doesn't 
have five proximal carpals or tarsals.  There is a lot of rearranging, 
suppression, and fusion that goes on later.

What I neglected to mention, and what constitutes the crux of what I have 
to say, is that a linear segmental series also arises from the radius or 
tibia.  This is a digit, located medial to the pollex or hallux, called the 
"prepollex" (or "prehallux"); and its development is later suppressed in all 
tetrapods except a few species of frogs.

The upshot is, if you look at the hand of a tetrapod while it is still in 
the initial stages of development, the digit that will eventually become 
the thumb will look like it is the second digit, the index finger will 
look like the third, and so on.  Perhaps the scientists who made the 
observations about the development of a bird's wing failed to take this 
into account.

Nick Pharris
Pacific Lutheran University
Tacoma, WA 98447

"If you can't convince them, confuse them." -- Harry S. Truman