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Re: Digits, Digits, Digits!!



Steve  Brusatte (dinoland@lycos.com) wrote:

<...>

  It doesn't take a an extra gene to make a copy of a digit,
just a transcription and copy of old genes into the same genome,
effectively extending the genome. This can happen normally, as
in the dewclaw of cats or the second pinkie in man, or can be
directly affected by messing with transcription in, say,
Drosophila (for whom most studies are pioneered -- don't ask me
for cites, I don't dabble in genetics :) ). Same for cervical
vertebrae in sauropods, giraffes, and some other funny critters
... :)

<P.S.: Jaime, hasn't there been a few recent studies which have
shown that the appendix may play a minor role in digestion, such
as housing a population of symbiotic bacteria.>

  I never said the appendix lacked function. Even a dewclaw can
function. That the appendix operates in this way only furthers
the issue: it serves the same function as the cecum in many
other mammals, like the rest of us archontans, and even
carnivores retain the cecum. Our digestive system as is does not
rely upon the appendix to provide our microflora, and compared
to the organs in other primates, like allouattines (howlers) who
have very large ceca, ours are rudimentary. Being omnivores with
some adaptations to primarily meat, man does not neccesarily
need a cecum, or an appendix. There is no selective advantage to
it. I cannot see it being selected for, as there is no selective
pressure to retain it.

----

  Another reason tyrannosaurs may have lost the third digit: as
the arm becomes smaller, and the fingers retain some functional
capability (as they do or they would look like *Carnotaurus*!)
space would not allow for more than two fingers, say, in
tyrannosaurids sensu stricto; thus there would be a selective
advantage in having only two for the mobility of the second
digit in a small arm, sacrificed to allow tissue to be exapted
for the overly large head and neck. -- Comments?

=====
Jaime A. Headden

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

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