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End of the Cretaceous65 Million Years Ago
Up first, the most famous (and recent) of all the Extinction Events... the one that killed all but a small few of the dinosaurs (the ones that ended up being today's birds). This die-off killed more than just big land reptiles, however, it also obliterated 17% of all living things on Earth. Asteroid impact off today's Yucatan Penninsula is the most well known theory as to the cause, but scientists think it may have been more complicated and less sudden than a single thing. Two other terrible things were happening at that time -- the breakup of a super-continent and the monstrous multi-millennial eruption of the Deccan Traps in India.
Not a good time to visit when we finally invent that time machine.
End of the Triassic200 Million Years Ago
Next down the line, we come to the end of the Triassic Period... and this one, we know very little about. According to the fossil record, 1/5th of all families of marine life were killed. There is speculation that this die-off happened more slowly, over many thousands of years and might be partially attributed to the introduction of very few new species. But the most likely cause of all the death is the eruption of the Central Atlantic magmatic province. 2 million cubic km of lava spilled out over a few hundred years... but worse, 2 quadrillion kg of sulfur was released along with twice as much C02. Yikes.
End of the Permian250 Million Years Ago
This is the big one... the one that is easily the closest life has ever come to being eradicated from Earth. Hold on to your hats... 83% of all genera on land and sea went extinct during this period. That is 96% of all marine life and 70% of all land vertebrates. Of all the Extinction Events, its the only one that affected insects and it almost wiped out land plants entirely. Something like this can't be traced back to just one thing, or even two. It was likely a combination of many, many bad things happening all around the same time. The supercontinent of Pangea was just forming, for one, but the immense Siberian Traps also erupted at this time. There's some speculation that there was also a meteor impact... but this has yet to be confimed.
The Late Devonian370 Million Years Ago
This is a difficult event to pin down. Some scientists think it is more like two or more events over the period of 25 million years. However many awful, awful things were happening we do know that 70% of all marine species died off. Theories about anoxia (oxygen depletion) are high on the list, as less oxygen in the water would explain the death rate of marine life. At this same time, trees were really coming into their own and could have contributed to the anoxia. Most of the evidence, however, points to global cooling although the cause of it is still unknown.
End of the Ordovician445 Million Years Ago
The oldest, and the 3rd largest, this Event lost 57% of all genera. Because its so long ago, it's very difficult to resolve what happened here. Most life was in the ocean at this point on the timeline, and we can only know of the ones that actually left fossils. Ancient creatures like the jellyfish tend not to leave fossil records, so who really knows how many animals actually died. Climate change is the most likely culprit - and speculation about the location of Pangea at the time leads to a theory that it its location over the South Pole would have created the largest glacial landmass in the history of the planet. All that ice locked on land would have lowered sea levels dramatically. This would have made living very difficult for all the shallow marine life --- which was pretty much all of it at this time.