Mass extinction events

Earth has undergone mass extinctions, during which more than 75% of existing species disappear, exactly five times in the past 540 million years.

• Ordovician, 443 million years ago.
  86% of all species lost.
  Some possible causes: Waxing and waning ice ages; rise of the Appalachians affecting atmospheric and ocean chemistry.
• Devonian, 359 million years ago.
  75% of all species lost.
  Some possible causes: Global cooling, followed by global warming, tied to new plants evolving and decrease in carbon dioxide; possibly meteor strikes as well. 
• Permian, 251 million years ago. 96% of all species lost.
  Some possible causes: Siberian volcanoes; increased carbon dioxide levels; global warming; an asteroid or meteor strike. 
• Triassic, 200 million years ago. Lasted 5 million years
  80% of all species lost.
  Possible cause: Volcanic activity caused by the breakup of the mega-continent Pangaea led to a rise in carbon dioxide levels and increased global temperatures.
• Cretaceous, 65 million years ago.
  76% of species lost, including the dinosaurs.
  Possible causes: Asteroid strike in the Yucatan led to global cataclysm and rapid global cooling.