As species have appeared and others have disappeared, the overall trend in diversity has been strongly upward. Note also that this trend has periodic interruptions, called mass extinctions. The average rate over the past 200 my is 1-2 species per year, and 3-4 families per my. The average duration of a species is 2-10 million years (based on last 200 million years). Mass extinctions. Roughly 20 are recognized, of which 5 are major and the largest is the Permean, in which 50% of all marine families and 80+% of all marine species went extinct.
http://www.snre.umich.edu/~dallan/nre220/outline3.htm
Ordovician–Silurian extinction events (End Ordovician or O-S): 450–440 Ma at the Ordovician-Silurian transition. Two events occurred that killed off 27% of all families, 57% of all genera and 60% to 70% of all species. Together they are ranked by many scientists as the second largest of the five major extinctions in Earth's history in terms of percentage of genera that became extinct.
https://en.wikipedia.org/wiki/Extinction_event
Triassic–Jurassic extinction event (End Triassic): 201.3 Ma at the Triassic-Jurassic transition. About 23% of all families, 48% of all genera (20% of marine families and 55% of marine genera) and 70% to 75% of all species became extinct.
https://en.wikipedia.org/wiki/Extinction_event
Two events occurred that killed off 27% of all families and 57% of all genera. Together they are ranked by many scientists as the second largest of the five major extinctions in Earth's history in terms of percentage of genera that went extinct. Source(s): http://en.wikipedia.org/wiki/Extinction_...
https://answers.yahoo.com/question/index?qid=20111205174220AAKhpfb
The K-T Extinction event. 1  Dinosaurs, many land plants, ammonites, much marine life perished. 2  The explanation: a huge meteor, perhaps 150 km in diameter, collided with Earth.  Alternative explanations: dual meteors, or massive volcanic activity triggered by a meteor.
http://www.snre.umich.edu/~dallan/nre220/outline3.htm
Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as major, and the data chosen to measure past diversity.
https://en.wikipedia.org/wiki/Extinction_event
At the end of the Frasnian Age in the later part(s) of the Devonian Period, a prolonged series of extinctions eliminated about 19% of all families, 50% of all genera and 70% of all species. This extinction event lasted perhaps as long as 20 Ma, and there is evidence for a series of extinction pulses within this period.
https://answers.yahoo.com/question/index?qid=20111205174220AAKhpfb
Best Answer: There have been 5 Mass Extinction events in Earth's history. They are. 1.Cretaceous–Tertiary extinction event (End Cretaceous or K-T extinction): 65.5 Ma at the Cretaceous.Maastrichtian-Paleogene.Dania... transition interval. The K–T event is now called the Cretaceous–Paleogene (or K–Pg) extinction event by many researchers.
https://answers.yahoo.com/question/index?qid=20111205174220AAKhpfb
The most recent and debatably best-known, the Cretaceous–Paleogene extinction event, which occurred approximately 66 million years ago (Ma), was a large-scale mass extinction of animal and plant species in a geologically short period of time.
https://en.wikipedia.org/wiki/Extinction_event
Extinction is a natural event  perhaps 95% of all the species that have ever lived have gone extinct. By late 1700s, as more fossil species were unearthed, catastrophism became the dominant explanation.
http://www.snre.umich.edu/~dallan/nre220/outline3.htm
how many extinctions have occurred
Five to more than twenty extinctions have occurred. There have been five to more than twenty extinctions occurred.