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Re: bird evolution



On Thu, 18 Mar 1999, KARI LYNN BAKER wrote:

> Hello everybody!!
> 
> I am giving a senior seminar presentation and have a question about the
> bird/dinosaur debate.
> And I don't recall this question being asked about the birds and dinosaurs
> before....so here it goes...
> 
> How did the arms of a theropod dinosaur evolve into the wings of an ancient
> bird if birds are thought to be the descendants of dinosaurs?  Anatomically,
> I cannot picture how this evolution took place.  Is anybody able to help
> explain this to me?

If you look at the forelimbs of a dromaeosaurid (_Velociraptor_,
_Deinonychus_, etc.) and compare them to the forelimbs of _Archaeopteryx_,
you'll see very little difference at all.

There's a very clear transition in the fossil record from the basal
dinosaurs/dinosauriforms to basal birds. Basal dinosaurs had five manual
digits, then primitive theropods like coelophysoids had only four, and no
claw on the fourth. Advanced theropods (tetanurans) lost the fourth digit
as well, leaving them with three digits, like birds today.

Of course, it's kind of hard to see the digits in modern bird hands, since
the first digit is fairly reduced and the 2nd and 3rd are fused together,
but they are there. Next time you eat some chicken, examine the wing bones
and you'll see them.

Many theropod illustration show the hands in an incorrect, palms-down
position. In reality, the palms probably faced inward more, like those of
birds.

I've seen many illustrations showing the transition. I myself have one
(just of the wings) at <http://dinosaur.umbc.edu/pics/baroque.htm>.
(They're underneath the _Brachiosaurus_' neck).

--T. Mike Keesey                                    <tkeese1@gl.umbc.edu>
WORLDS                                  <http://www.gl.umbc.edu/~tkeese1>
THE DINOSAURICON                               <http://dinosaur.umbc.edu>