Extinction

Extinction
The extinction of organisms is an expected event in the Earth’s history. They occurred subtly and constantly without causing devastating outcomes. There have, however, been important periods where more than 50% of the earth’s species were completely wiped out over a few million years. When this happens, it is known as Mass Extinction. Scientists are not exactly sure of how many mass extinctions there have been, but are positive that there have been at least Five Major Mass Extinctions that have, so far, occurred.
The first mass extinction called the Ordoviciansilurian Extinction occurred about 439 million years ago. It was caused by the drop in sea levels as glaciers formed. Then, again, by the rising sea levels when the glaciers melted. The second mass extinction (Late Devonian Extinction) occurred roughly 346 million years ago. Scientists are still befuddled by the cause of this ghastly event. The third and most devastating mass extinction occurred more or less 250 million years ago and is believed to have been caused either by an asteroid or comet hitting the earth, or by volcanic floods. Almost 90% of all species, (53% of marine families, 84% of marine genera and 70% of land species) became extinct during this time and it was named the Permian-Triassic Extinction. The fourth mass extinction, titled the Triassic Extinction, was caused by either gigantic floods or by larva from volcanoes that lead to global warming. It occurred between 199 million to 214 million years ago.
The Cretaceous Tertiary Extinction (fifth mass extinction) was the most recent and most famous as it was that of the dinosaurs. It occurred approximately 65 million years ago and, even though scientists are not entirely sure what caused it, they have certain evidence of the conditions of Earth at that time. There was a sharp drop in temperatures and the already shallow sea levels began to severely ebb. Many plants that served as food for other organisms such as the dinosaurs died... [continues]

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