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Re: Digit loss



-----Original Message-----
From: philidor11 <philidor11@snet.net>
To: twilliams_alpha@hotmail.com <twilliams_alpha@hotmail.com>;
dinosaur@usc.edu <dinosaur@usc.edu>
Date: Saturday, June 09, 2001 10:55 AM
Subject: Re: Digit loss


><...[I]n certain cases, such as when appendages or organs no longer serve a
>function (or lose most of their original function) through selection, they
>are no longer selected for traits that improve or regain that function.
>[Me:  but they could improve or regain that function by mutation so long as
>the capability still existed.]
>
>IMHO, the reduction of tyrannosaurid forelimbs was not a random change.
But,
>I would aver that the loss of a manual digit (producing a didactyl manus)
>did persist by random chance, not by selection.
>
>I'm not necessarily saying that tyrannosaurid forelimbs were utterly
>useless.  What I am saying is that whatever function tyrannosaurid hands
>were used for, it didn't matter if they had three fingers or two fingers or
>one finger.  A loss of a finger (from three to two) was not detrimental to
>the animal's fitness.  This trait was retained not because it was good
>(beneficial) but because it was neutral (had no effect on the tyrannosaur,
>one way or the other).>
>
>Agreeing, mostly.  The consistent direction of reduced forelimbs necessary
>to make them look so vestigial does argue that there must have been
>selective pressure.  I do wonder, though, how the loss of capability would
>be a benefit.  Unless someone wants to argue 'use it or lose it', the fact
>that the forelimbs were so far reduced means that a tyrannosaur would have
>had less flexibility if the environment changed.  Therefore, not being able
>to use the forelimbs so extensively was achieved at a price less than the
>gain.  Hard to figure what such a significant gain might be.
>As an aside, 'use it or lose it' does imply a directionality in evolution.
>Unless there is a significant functional gain, the only advantage to losing
>it would be aesthetic.
>On the number of fingers, if there were no advantage to a specific number,
>and if mutation were neutral and fairly extensive, wouldn't you expect to
>see a different number within the same species coming from the same time
>period?  And among species?  You could also expect to see the digits
>disappear and reappear, much as 6 toed cats have 5 toed offspring.
>Until variation appears extensively in the record, isn't it most
appropriate
>to think that at some point in the past there was an advantage to a certain
>number of digits and that the capability of re-expressing lost digits had
>disappeared?



Well, I`d like to throw in my two cents here.....I think Tyrannosaurs got
their start as flying forms (basic BCF). They probably lost the third finger
as part of the wing structure. At any rate, this means that they started out
small. Instead of picturing the arms as "shrinking", perhaps they just did
not grow along at the same rate as the rest of the animal. Perhaps because
the manus was not all that functional as a grasping organ from the start.
Are tyranosaur digits opposable??? I don`t believe I`ve seen them pictured
that way.