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RE: Digit Loss (kiwis and tyrannosaurs)




-----Original Message-----
From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
Dinogeorge@aol.com
Sent: Friday, June 08, 2001 5:35 PM
To: dinosaur@usc.edu
Subject: 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. <<
Another tact is why didn't more tetrapods loose digits? The most primitive
digit count, by the by, is 7 or eight, so from the beginning there has been
digit lose in some respect.

Why didn't hadrosaurs? They lost the claw on digit 3 but kept the phalanges.

Tracy L. Ford
P. O. Box 1171
Poway Ca  92074