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RE: Digit Loss (kiwis and tyrannosaurs)



Tracy Ford (dino.hunter@home.com) wrote:

<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.>

  It seems that once the digits were downgraded to five, the
ability to shift on land between ectaxonic, mesaxonic, and
entaxonic positions, the innermost and outermost digit were
still being utilized for locomotion -- anyfurther would be to
ustilize excess flesh to move, and this was not advantageous; to
be ectaxonic and still have a leverage digit laterally suggests
the utility of this digit was selected for, as it persisted from
basal Tetrapoda onward, and persists in nearly all tetrapod
groups, extant or extinct. Once the digit was disengaged from
terrestrial locomotion, variability in count was not repressed
(we see this in polydactyl ichthyosaurs, which are secondarily
aquatic) and less mechanical restraits are offered. Digits are
less important for a manus not being used to engage prey or the
substrate, so may no longer be selected for. Birds no longer
need extraneous manal digits because they are selected against,
as interfering with locomotion, adding to the skeletal frame,
etc. Pedal digits are selected for because they still strongly
engage the substrate (stance, perching, etc.; there is
remarkable variability in digital evolution and relationship
among birds).
 
<Why didn't hadrosaurs? They lost the claw on digit 3 but kept
the phalanges.>

  My theory is that the digit retained a sort of nail, but that
hadrosaurs were largely faculatively quadrupeds, so did not need
the manus as much as iguanodontids, or even dryosaurids, who may
have manipulated prey with the manus, dug with claws, etc.
Hadrosaurs went elsewhere. They still retain large hooves on the
other digits, and a mitt-like manus, enough to satisfy
manipulation of the substrate (digging, mound-building, etc.) or
plants -- or mates :).

  Every good lady likes to keep her hand in :) [and I'm not the
least sexist!].

=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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