Re: Can we tell if there were ever periods of extreme heat on earth.

There are many ways that scientists attempt to study the paleoclimate
Here is a very good site from the university of Michigan that covers much 
of that:

 http://www.sprl.umich.edu/GCL/Notes-1998-Fall/climate_rec.html

Another site that uses some neat graphics is http://www.scotese.com/paleocli.htm

It does not seem that the heat was so great to have caused mass 
extinctions, but you be the judge after reviewing these sites!