![]() ![]() Occasionally, biodiversity on Earth undergoes a rapid reduction in the form of a mass extinction in which the extinction rate is much higher than usual. A large extinction-event often represents an accumulation of smaller extinction- events that take place in a relatively brief period of time. Researchers have identified five major extinction events in earth's history since: The first known mass extinction in earth's history was the Great Oxygenation Event 2.4 billion years ago, which led to the loss of most of the planet's obligate anaerobes. End of the Ordovician: 440 million years ago, 86% of all species lost, including graptolites. ![]() Late Devonian: 375 million years ago, 75% of species lost, including most trilobites.End of the Permian, "The Great Dying": 251 million years ago, 96% of species lost, including tabulate corals, and most extant trees and synapsids.End of the Triassic: 200 million years ago, 80% of species lost, including all of the conodonts.End of the Cretaceous: 66 million years ago, 76% of species lost, including all of the ammonites, mosasaurs, ichthyosaurs, plesiosaurs, pterosaurs, and nonavian dinosaurs.(Dates and percentages represent estimates.) Smaller extinction-events have occurred in the periods between these larger catastrophes, with some standing at the delineation points of the periods and epochs recognized by scientists in geologic time.
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