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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
Pacific Lutheran University
Tacoma, WA 98447
"If you can't convince them, confuse them." -- Harry S. Truman