Sources
http://en.wikipedia.org/wiki/Cambrian%E2%80%93Ordovician_extinction_event
http://science.nationalgeographic.com/science/prehistoric
world/cambrian/http://hannover.park.org/Canada/Museum/extinction/cammass.html
http://en.wikipedia.org/wiki/Devonian_Extinction
http://en.wikipedia.org/wiki/Ordovician%E2%80%93Silurian_extinction_event
http://www.britannica.com/EBchecked/topic/1523112/Ordovician-Silurian-extinction
http://en.wikipedia.org/wiki/Carboniferous_rainforest_collapse
http://www.bbc.co.uk/news/science-environment-11870322
http://science.nationalgeographic.com/science/prehistoric-world/permian-extinction/?rptregcta=reg_free_np&rptregcampaign=20131016_rw_membership_n1p_us_se_w#
http://en.wikipedia.org/wiki/Permian%E2%80%93Triassic_extinction_event
http://science.nationalgeographic.com/science/prehistoric
world/cambrian/http://hannover.park.org/Canada/Museum/extinction/cammass.html
http://en.wikipedia.org/wiki/Devonian_Extinction
http://en.wikipedia.org/wiki/Ordovician%E2%80%93Silurian_extinction_event
http://www.britannica.com/EBchecked/topic/1523112/Ordovician-Silurian-extinction
http://en.wikipedia.org/wiki/Carboniferous_rainforest_collapse
http://www.bbc.co.uk/news/science-environment-11870322
http://science.nationalgeographic.com/science/prehistoric-world/permian-extinction/?rptregcta=reg_free_np&rptregcampaign=20131016_rw_membership_n1p_us_se_w#
http://en.wikipedia.org/wiki/Permian%E2%80%93Triassic_extinction_event