re: Gamma-ray burst linked to mass extinction...
Interesting.
<a href="http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.earth.29.1.331;jsessionid=jBWb5RLkg5Kb>Ordovician Period More here. If this ice age occured due to a Gamma ray burst, wouldn't it have also affected other planets as well? We may not have been alone in the cold during the Ordovician.
Just something to note: During the Ordovician, most of Earth's oceans (and life/sea life) were north of the Equator.