So What Really Happened? -- April 6, 2020

Elevator speech for each mass extinction. Ranked in order of severity; in parentheses, chronological order):

Number five, end-Devovian (2, second oldest):

Overall, 19% of all families and 50% of all genera became extinct. A second, distinct mass extinction, the Hangenberg event, closed the Devonian period.

By the Late Devonian, the land had been colonized by plants and insects. In the oceans were massive reefs built by corals and stromatoporoids.

Number four, end-Ordovician (1, oldest):

Generally attributed to two factors: the first wave of extinction may be related to rapid cooling at the end of the Ordovician Period, and the second phase is widely regarded as having been caused by the sea-level fall associated with the glaciation.

All of the major animal groups of the Ordovician oceans survived, includign trilobites, brachiopods, corals, crinoids and graptolites, but each lost important members. Widespread families of trilobites disappeared and graptolites came close to total extinction.

Number three, end-Triassic (4, dare we say penultimate?)

This was a biggie; it resulted in the demise of some 76 percent of all marine and terrestrial species and about 20 percent of all taxonomic families.

On land, all archosaurs other than crocodylomorphs and Avemetatarsalia (pterosaurs and dinosaurs), some remaining therapsids, and many of the large amphibians became extinct.

Statistical analysis of marine losses at this time suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions.[5] 

Number two, end-Cretaceous (5, most recent, hopefully the last)

Perhaps the most famous, for many reasons; K–T extinction, abbreviation of Cretaceous–Tertiary extinction, also called K–Pg extinction or Cretaceous–Paleogene extinction, a global extinction event responsible for eliminating approximately 80 percent of all species of animals at or very close to the boundary between the Cretaceous and Paleogene periods, about 66 million years ago.

Most amazing:With the exception of some ectothermic species such as the leatherback sea turtle and crocodiles, no tetrapods weighing more than 25 kilograms (55 pounds) survived. It marked the end of the Cretaceous period, and with it the end of the entire Mesozoic Era, opening the Cenozoic Era that continues today.

The mother of all mass extinctions, number 1, end-Permian, 3, smack-dab in the middle of the five):

It was so bad, it was the only mass extinction of insects. It is the Earth's most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. Some 57% of all biological families and 83% of all genera became extinct.