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Re: Digit Loss (kiwis and tyrannosaurs)



In a message dated 6/8/01 1:17:25 PM EST, twilliams_alpha@hotmail.com writes:

<< My guess is that tyrannosaurid didactyly was also brought about by a 
genetic 
 switch(frameshifts and homeoboxes etc) that had no effect at all on the 
 creature's survival - rather than any adaptive drive to whittle down the 
 number of manual digits. >>

Given a choice between an explanation that provides a relationship between a 
cause and an effect and another explanation that asserts only the action of 
random chance, I prefer the former. In my explanation, the third digit was 
lost in improving the function of the wing of a flying animal, and this 
condition persisted in the giant cursorial descendants of that animal. In 
your explanation, the third digit was lost for no reason other than it was 
just "useless." Why don't we have more such causeless digital losses among 
terrestrial vertebrates, in particular theropods? Why did it just hit 
tyrannosaurids? Your explanation raises more questions than it answers, and 
in fact doesn't even provide any answers.

Note that in the kiwi, the third manual digit is lost in a >wing<. Perhaps 
the directly ancestral tyrannosaurid lost its third digit for the same 
reasons that the kiwi lost its third digit. Maybe the ancestral tyrannosaurid 
was a ground-dwelling kiwi-like bird. Who knows? We need more data, of course.