Examining the Cretaceous-Paleogene Extinction Event Over 98% of all organisms that have ever lived on Earth are now extinct. A mass extinction event occurs when a large number of species die in a short amount of time (relative to the age of the Earth). Mass extinctions are studied extensively for both cause and effect, as there is usually room for debate about the catalysts preceding the extinction and the massive influx of new biological species that follows. There were five major mass extinctions, nicknamed the “Big Five,” which wiped out at least 50% of living species in that period. The best-known mass extinction of the Big Five, with the decimation of every non-avian dinosaur species, is the Cretaceous-Paleogene (K-Pg) extinction event. The K-Pg extinction occurred during ongoing natural disasters followed by an asteroid collision with Earth, which affected plant life, animal life, and hydrology (Cobb, n.d.). Some paleontologists believe the global ecosystem was already in decline before the final blow, an asteroid, destroyed much of life on Earth. Continental drift and seafloor spreading may have shifted ocean circular patterns, causing dramatic changes in sea levels and Earth's climate.