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Re: Giant flightless birds



At 10:05 AM 5/11/99 -0400, Larry Febo wrote:
>OK,...here`s something else.There seems to be a trend in the number of
>digits possessed by theropods. That trend seems to be a loss of such digits.
>A good reason for lose of digits might be that they simply were not needed
>as birds evolved more sophisticated wing structures.

True.  However, there are other good reasons, too: folding ability works
better if the already reduced digit IV is lost altogether.

(That is the inherent problem with building scenarios on what "looks good"
or "feels good" to yourself might not the scenario that "looks good" or
"feels good" to others.  In that case, how does a third party choose between
them?)

>Archaeopteryx had three
>free digits (even pterosaurs had three free digits), but they don`t seem to
>be opposable.

Take a look again: digit I is just about as opposable as in _Deinonychus_,
_Troodon_, etc.  Remember that dinosaur digit opposability is achieved by
rotation of the metacarpal-phalangeal joint, rather than at the
carpal-metacarpal joint (as in, for example, humans).  Also, dinosaur
"opposability" is not a precision grip between the fingers (as in humans),
but simply bringing digit I in towards the palm at a different angle than
the others.

>Still, they were functional for "hooking" onto branches and
>clambering about in the treetops.

Sure, and probably for grabbing small prey items (lizards, sphenodontids,
mammals, small pterosaurs, etc.).

>Hoatzin chicks have two, which function in
>similar manner. Now, T_Rex also has two. Again, I can see this as a holdover
>from a previous "hoatzin-like" state.

I could see that.  I could also see that as simply as part of the other
important aspect of tyrannosaurid forelimbs: their great reduction.

>What I can`t readily see is why T_Rex (if it was always terrestrial)

Well, NOBODY thinks that the species _Tyrannosaurus rex_ was a tree climber,
nor any other tyrannosaurid species.  Perhaps the ancestors of
tyrannosaurids were, but not Tyrannosauridae itself.

>had
>lost the opposable quality of it`s digits .

Oh, but tyrannosaurids DO have about the same opposability as in other
tetanurines: digit I does come in at an angle to the rest of the hand.  The
main difference with tyrannosaurid hands is that they are proportionately so
small and they have lost the third finger.

>Again, it seems to make more
>sense that a terrestrial animal would always be in need of good grasping
>ability of its forelimbs for ther procurement of prey.

Tell that to a dog, a hyena, or other jaw-based hunter!!  These animals do
not use the forelimb to aquire prey: at best they use them to stabilize the
victim while the jaws do their work.  Interestingly, these predators also
have very powerful jaws and neck muscles, claws which are not well curved
(compared to cats), and very long and slender legs: these same features are
also seen in tyrannosaurids.

>The only reason for
>loss of such ability, (that I can visualize) would be that the condition was
>inherited from an Avian ancestor that itself had lost the third digit.

And yet I can see another.  What we must now do is find some way of choosing
between these models other than our own particular preferences.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661