Extinctions happen continuously. If you mean mass extinctions, the generally accepted number is five. See link.
The first major extinction on Earth has been referred to as the “oxygen holocaust,” when almost all the anaerobes on Earth were wiped out by the rise of microbes who created oxygen through photosynthesis 2.5 billion years ago. The second mass extinction in the Late Ordovician occurred 445 million years ago and wiped out 50 percent of marine life, which may have been caused by shifting sea levels and plate tectonics. The third mass extinction in the Late Devonian happened 370 million years ago and was caused by global cooling. The fourth mass extinction at the end of the Permian 250 million years ago, also known as the “Great Dying,” almost wiped out life on Earth completely, and was caused by a multiplicity of factors. The most famous mass extinction, which ended the great “age of reptiles,” happened 65 million years ago at the end of the Cretaceous.
5 mass extinctions:
Ordovician-Silurian Extinction (444 million years ago) – about 100 marine families became extinct, including families of trilobites, conodonts, and graptolites.
Late Devonian Extinction (364 million years ago)– By now, both land and sea was habitable. This extinction seemed to only affect marine life, with 19% of all families and 50% of all genera becoming extinct.
Permian-Triassic Extinction (251 million years ago) – Considered Earth’s most severe extinction event, the Permian-Triassic Extinction resulted in the extinction of 90% of all marine species and 70% percent of terrestrial vertebrate species. Afterwards, fungal species became he dominant form of terrestrial life for quite some time.
Triassic-Jurassic Extinction (200 million years ago) – Half of all species on Earth at that time went extinct, leading the way for dinosaurs to become the dominant species in the Jurassic period.
Cretaceous-Tertiary Extinction (65.5 million years ago)- This is the famous mass extinction that led to the wipeout of the dinosaurs as well as 50% of all plant and animal species.
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