evolution:dinos., birds, elephants, us...

Thu Sep 12 19:25:22 EST 1996

  On a recent trip to the NYC museum of natural history, I was struck
by a puzzling feature of the dinosaur display: the theme of the exibit
was one of evolution and how scientists use common physical traits in
different species to help determine common ancestry.  My problem is
as follows:  I have always been told, not only by watching Jurassic Park
, that birds are decendent of certain dinosaurs and that most mammals
are decendent of small, scurriing rodent-like creatures that appeared
just before the dinos.dissapeared.  Now almost all of the dinosuars I
saw at the museum walked on hind legs with joints pointing forward,
i.e. knees, whereas birds walk on legs with joints that point backwars,
i.e. elbows.  When did the first dinosaur that walked on elbows appear?
Who were its ancestors?  Also on the mamalian front, elephants, or so
I have been told by the good people at trivial pursuit, and the only
animals with four knees.  How then can we and the elephants have any
common ancestor?  No slow, evolutionary change could have brough about
a transition from knees to elbows or vice versa.  If we are related
would the link have to be some creature without limbs?  Is it possible
that several mammals appeared around the world without any relation
to one another?

  Any light that could be shed on the subject would be much appreciated.
Thank you very much.

                                          David Brown,
                                          Montreal, Quebec

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