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Anther occurred when the Cretaceous period became the Tertiary about 65 million years ago, when 75 percent of species disappeared. This article is about the first mass extinction at the end of the Ordovician geological period 440 million years ago, when about 86 percent of species were killed.
Mass Extinction in the Fossil Record. Mass extinctions are apparent in the fossil record wherever some sort of organism virtually disappears from the geologic column above a certain layer. One of the most well-known mass extinctions is the disappearance of the dinosaurs and a number of other organisms at the K-T boundary.