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Possible Reason for Outer Digit Loss?

George Olshevsky (Dinogeorge@aol.com) wrote:

<Note, e.g., that Carnotaurus retains four short ceratosaurian
digits in an otherwise pretty useless (whatever that means)
forelimb. I think digits are lost only when they are >positively
detrimental<, not just useless. In other words, a forelimb with
three digits will, when reduced, generally retain the three
digits. Tyrannosaurids have two-digit manus because somewhere
along the line the third digit got in the way of some function
that was better done without it.>

  I'm pretty much willing to agree with on this point. One
possible reason why digit loss occurs in vertebrates is that,
for the ability to spread one's fingers when manipulating, or
toes, the distal ends of the metacarpals must be staggered in
such a way as to allow the digits to diverge from one another.
This can be tested by finding digits with lateral ligamental
pits and ball-shaped distal metacarpals, which allow lateral
excursion to take place. Loss of outer digits is one way of
doing this to the favored inner three, so that the already small
outer two are non-essential. It would be easier to reduce a
small, barely functioning digit (especially one lacking a claw)
than to reduce a fully functional one. Lack of function can be
an even more desirable application for loss than simply an
adaptation that is harmful: such a feature tends to be
detrimental to an animal's survival from fairly early on.

<In other words, why didn't we lose digit 1 and/or digit 2 as
well as/instead of digit 3?>

  Because digit I was being selected for? Almost never in
vertebrates as I know them does the pollex disappear in favor of
other elements, unless the manus is used for terrestrial
locomotion, as explained in my last post with running
vertebrates: loss of non-essential digits in an animal that
needs to concentrate force to as little an area as possible (pes
becomes smaller, outer and inner digits reduced to favor the
inner-most 3/2/1). Since the manus in theropods is never used to
locomote or the first digit was favored in the manus of birds as
an alular digit, the first digit is very unlikely to be lost.

Jaime A. Headden

  Where the Wind Comes Sweeping Down the Pampas!!!!

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