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In a message dated 95-12-02 17:28:51 EST, longrich@phoenix.Princeton.EDU
(Nicholas R. Longrich) writes:

>This could also be interpreted as the idea that large predators 
>tended to evolve from the small theropods, rather than from other large 
>predators. Big animals, I am guessing, do pretty well in relatively 
>stable situations, while small animals clean up when things go haywire. 
>Big animals go extinct more easily, possibly (look at the last ice age), 
>just because A) there are fewer of them B) because there are fewer of 
>them, the number of different genes they might have would be different 
>C)they take longer to reproduce, so evolution cannot procede as quickly 
>amng elephants as, say, mice which can run through a couple generations 
>per year. Of course, big animals have a lot of advantages too. They can 
>be very efficient, migrate huge distances, and eat low quality food. 
>The general impression I often seem to get from land animals is 
>that small animals evolve into big animals and more small animals, then 
>the big animals go extinct (repeat as necessary). Anyways, just some 
>vague thoughts on the subject.

I go into this a bit in one of the introductory sections (on dinosaur
diversity and extinction) in MM #2. Your vague thoughts are not as vague as
you think.