Mass extinction

While extinction of population, genetic lineages or entire species are a common occurrence in the history of life, mass extinctions - brief times of crisis where both the amount and diversity of life sharply drop - are few events of huge importance that shape the history of a planet.

Despite the huge capacity for adaptation displayed by life, a rapid change in environmental conditions can bring the general extinction rate far above the speciation rate. Since the vast majority of biomass and biodiversity are found in bacteria, it's likely that not even the most catastrophic event can significantly dent Earth's biosphere, but it can still have grave consequences on its most evident part, macroscopic animals and plants.

Historical mass extinctions
In the Phanerozoic Eon, distinguished by the abundant presence of large-sized animals and plants, five major mass extinctions have been identified, with different causes and severity:
 * 1) The End Ordovician extinction (450-440 Ma) involved more than a hundred families of marine invertebrates, and the total victim count is estimated about 27% of all families, 57% of all genera and 60-70% of all species. A great reduction of early reef has been observed, along with a crisis of brachiopods, bryozoans, conodonts, trilobites and graptolytes. The most likely cause is the southwards movement of Gondwana, which triggered an ice age and thus caused the sea level to fall, destroyed habitats worldwide.
 * 2) The Late Devonian extinction (375-360 Ma) destroyed 19% of families, 50% of genera and 70% of species. It saw a great reduction of trilobites, graptolytes, brachiopods, placoderms; armoured jawless fish disappeared entirely; there was, again, a crisis of reefs, composed by organisms with a calcium-rich skeleton, because of accumulation of magnesium in seawater (they'll be replaced by aragonite-rich reefs). Another global cooling episode is suspected, perhaps triggered by a meteor impact; the magnesium raise, and a strong eutrophication, could have been produced by the formation of the first forests.
 * 3) The End Permian extinction (251 Ma), the largest one, destroyed 57% of families, 83% of genera and maybe 90-96% of marine species. Trilobites and most therapsids (then the dominant vertebrates) disappeared entirely, while brachiopods, mollusks, echinoderms, corals, fish, amphibians, reptiles, insects and plants are merely heavily damaged. The formation of Pangaea (which produced desertification and ecosystem mixing) is probably responsible, along with an ice age, probably the huge lava emission from the Siberian Traps and maybe an exhalation of methane from the oceanic clathrates (see below).
 * 4) The End Triassic extinction (205 Ma) destroyed 23% of families, 48% of genera and 70-75% of species. Biodiversity had not entirely recovered from the previous mass extinction; several groups of reptiles and amphibians disappear, opening the way to dinosaurs. Its cause is still unknown: volcanic events connected to Pangaea breakup are suspected.
 * 5) The End Cretaceous extinction (65 Ma) destroyed 17% of families, 50% of genera and 75% of species. Victims include most mollusks (such as ammonites) and reptiles (plesiosaurs, mosasaurs, pterosaurs and dinosaurs, except for birds); mammals, birds, crocodiles and small-sized reptiles are not heavily damaged. This extinction has been ascribed to the Chicxulub meteor impact in Mexico, and perhaps the volcanic activity of the Deccan Traps.