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I always have had a problem with climatic extinctions of major taxonomic
groups. Like dinosaurs. I know they couldn't migrate very successfully,
for reasons Joe Daniel brought up.
Say it gets a lot colder, fast. What about the dinosaurs (or whatever) that
already live close to the equator? I am always impressed by the daily high
temperature readings in Freetown, San Salvador, Quito, Nairobi, etc. during
the middle of the winter, when it might be zero degrees here in Indiana. If
our temperature regime were to suffer a further decline, I'll bet just about
everything already living in equatorial zones would survive there. However,
the tropical climatological zone would surely shrink latitudinally. But
some dinos or whatever (e.g., banana trees, etc.) would hardly be affected,
if at all.
So, during climatic changes, some species or genera should survive. Later
on, those could repopulate areas (probably very slowly) that may indeed have
been vacated due to the climatic downturn. The fossil record would likely
show a diversity decrease that slowly recovered. But total extinction of
every member of the group, everywhere? I wonder.