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Paleoworld and extinction theories

I saw another Paleoworld recently that talked about the K/T
extinctions.  It brought up some interesting points that as a
geologist, I have never really given much thought to.

They mentioned three theories for the extinctions: meteorite 
impact; climate change; and disease. 

The meteorite theory seems clear enough why things would become
extinct- there is a rapid change in the environment, and things can't
cope with rapid change. The point was made that the mammals, being
more abundant and smaller, were more suited to survival. On the other
hand, what about some of the oldest living relatives of the dinosaurs-
the turtles and alligators? Why are they still around? Did the fact
that they were (in part) aquatic help them survive?

Similarly, if climate change were responsible, why did the mammals
survive, and SOME of the reptiles and amphibians?

The last theory relates to the land bridges and the movement of 
creatures over them. The theory goes that with the transported 
animals came new diseases that killed off the dinosaurs. Is there
any fossil evidence to support this? I would imagine that some
diseases would affect the bones, and this could bee seen by some
foresnsic examination. Has this been documented?
Dana Naldrett  (naldret@cc.umanitoba.ca)   
Environmental Earth Science Associates Inc.
P.O. Box 157, St. Norbert Post Office      
Winnipeg, Manitoba, Canada  R3V 1L6