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Re: DINO EXTINCTIONS



In a message dated 98-08-12 03:03:52 EDT, Tetanurae@aol.com writes:

<< No doubt a really really big rock from the sky would be devistating and
kill
 everything, but it cannot be so selective as to kill say enantiornithines and
 not neornithines, or ammonites and not nautiloids.  Baking skies and burning
 forests do not selectively kill of basal ornithopods instead of basal
 primates, or avisaurids and hesperornithids instead of anseriformes: they
kill
 everything. >>

We cannot talk meaningfully about selectional effects in the aftermath of the
asteroid impact until we can obtain good data on population sizes for the
species. A species with a small, restricted population would seem to be a more
likely candidate for extinction than a species with a large population spread
over a wide territory, simply because the former has fewer individuals that
need to be destroyed to render the species extinct. Wide ranges and
territories are probably less important in a global mass extinction event;
what really counts is numbers of individuals. Large animals tend to have lower
populations (numbers of individuals) than small animals, so an asteroid impact
would extinguish them preferentially. Small, quickly maturing animals like the
early placental mammals could rapidly repopulate a region from just a few
surviving individuals once the chaos had died down, but surviving large,
slower-to-mature animals would have more difficulty doing this.

In any event, the reasons for why a particular species or group of species
survived the asteroid impact are probably almost as varied and diverse as the
species themselves. One might be forced to examine each species case by case
to come up with the right answer.