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Re: Digit Loss (kiwis and tyrannosaurs)

George wrote:

I don't think digital loss is >anything like< this trivial an occurrence.
Digital development is not governed by one or two genes that can be turned on
and off or deleted or suppressed by evolutionary whim; it is governed by
entire systems of genes. [snip] It takes an awful lot of
evolutionary work to make even a single phalanx go away in a lineage.

Well, look at the kiwis. This is another group of theropods in which the forelimbs are not used for very much. The exact number of manual phalanges appears to be quite labile...


"Actually, in kiwis the finger (with ulna) develops on only the medial most
metacarpal, and indeed only two metacarpals form (not three as in most
birds).  What is even more remarkable is that kiwi wings develop in two
alternative fashions: one in which the medial metacarpal is longer and has a
digit with three phalanges (an ungual and two non-unguals); the other in
which the medialmost metacarpal is shorter and the digit has only two
phalanges. (see, for example, Parker, T. J. (1891) Philos. Trans. R. Soc.
London B 182: 26-134)."

(Thanks to Tom Holtz for the info.)

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.



Timothy J. Williams

USDA/ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163

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