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Re: Digit Loss




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. It took some 50 million years of evolution for
horses to lose their digits, for example, and whole groups of dinosaurs
shared almost exactly the same fore and hindlimb digit configurations for
scores of millions of years. Humans today still have the same digital counts
as amphibians from the Carboniferous Period. Digital counts and phalangeal
counts persist mightily in the face of immense selection pressure and in
spite of sometimes drastic limb modifications. It takes an awful lot of
evolutionary work to make even a single phalanx go away in a lineage.


I disagree. In most lineages (including the ones you mention) the hands were actually *used* for something. Not always the same thing, of course, but *something*. Not in tyrannosaurids though.

Basically, what we are arguing about is whether tyrannosaurids lost their third finger before or after the forelimb was reduced (vestigialized, perhaps). I say it happened after, and that the loss of the third digit was not prompted by any selection at all. The loss of a finger made no difference at all to tyrannosaurids - as long as his teeth and toes didn't disappear, a _Tyrannosaurus_ could still bring down a hadrosaur irrespective of the number of digits on its puny forelimbs.

You claim that the loss of the tyrannosaurid's third finger was somehow connected to the aerial performance of an (as-yet-unknown) dinobird from which tyrannosaurs are supposedly descended. I would ask, how does a three-fingered basal tyrannosauroid like _Eotyrannus_ fit into this scenario?


Tim

------------------------------------------------------------

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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