Final answer:
The most severe mass extinction occurred about 250 million years ago during the Late Permian period. It was the most devastating, leading to the extinction of a majority of species, and is part of the five major mass extinctions recorded in Earth's history.
Step-by-step explanation:
The greatest mass extinction happened approximately 250 million years ago. This event is known as the Late Permian mass extinction, or the "great dying," and it was the most severe extinction event. During this period, 57 percent of all families, 83 percent of all genera, and a staggering 96 percent of all species went extinct. This event had an enormous impact on the biodiversity of the planet, changing the course of evolutionary history.
History has recorded five major mass extinctions, with the most recent occurring 65 million years ago. This event is known as the end of the Cretaceous period. It led to the disappearance of many species including most dinosaurs, except for birds, and is widely attributed to the impact of a large asteroid, which caused significant environmental changes.