Permian Triassic Extinction
The Permian Triassic extinction event is undoubtly the largest extinction event. Nienty percent of life died. Many call it "the great dying". So much life died that it took millions of years for our planet to recover. There are many theories of how a horrendous event like this could happen. They range from acid rain to meteorite impact. Regardless, if it was not for a few resilient species, we would not be here today.
Theories
Volcanic Eruption- One leading theory is that a series of volcanoes released toxic gasses into the atmosphere. Over a series of time these gasses may have polluted water and become acid rain. Today, areas that must withstand large amounts of acid rain are being used to investigate the reliability of this theory.
The Forming of Pangaea- When the two main Paleozoic continents formed Pangaea, it drastically changed the geography of earth's oceans, upsetting ecosystems. Animals who were perfectly adapted to shallow seas suddenly found themselves in a hostile environment.