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At 14:10 14/11/96 -0500, you wrote:
>To all:
>My sister -- who is, BTW, dinosaur-ignorant -- tells me that, as a pure fact,
>birds ARE NOT a kind of dinosaur.  For example, Stegosaurus and Deinonychus
>-- very dissimilar anatomically -- are both dinosaurs, according to her
>"fact", but Rhea -- a bird that Deinonychus has more in common with than
>Stegosaurus -- is NOT a dinosaur at all.  I would appreciate it if someone
>could, in simplified terms, explain why birds are included in the Dinosauria
>(in most cases, anyway).  Don't e-mail me telling me why birds AREN'T
>dinosaurs ... that's not what I'm asking for!
>Rachel Clark

If you accept (and not everybody does) that birds evolved from an ancestor
that was a dinosaur, then by what is called cladistic classification birds
must be dinosaurs too.  That is because cladistics defines "dinosaurs"
(and, folks, I'm trying to be simple here) as any animals that are
descended from the common ancestor of the ornithischians and the
saurischians - in other words, if you followed Triceratops' evolutionary
history backward, and did the same for Tyrannosaurus, at some point you
would come to the same animal.  Everything descended from that animal is a
dinosaur.  Everything not descended from it is not a dinosaur.  So if you
think birds were derived from something like Deinonychus, then since
everyone agrees that Deininychus is a descendant of the common ancestor of
Triceratops and Tyrannosaurus - in other words, it's a dinosaur - then
birds are dinosaurs.  If, however, you think birds descended from (say)
early crocodile relatives, birds would not be dinosaurs because crocodiles
are not descended from that Triceratops-Tyrannosaurus ancestor.

This kind of classification, you will notice, depends entirely on who an
animal's ancestors were - it says nothing about what it looks like or how
much it has changed.  And, of course, by this system birds are also fishes,
and so are we, since we are descended from the common ancestor of (say)
herrings and crossopterygian fishes.
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court
Mississauga, Ontario, Canada L5L 3W2          Internet: ornstn@inforamp.net