The extinction at the end of the Permian is a major event in Earth’s history. We define it as the boundary between two Eras, the second largest division in the geologic timeline.
At that boundary, over 90% of all marine species went extinct. Some familiar life forms including the trilobites disappeared forever while other ecosystems shifted permanently; such as bivalves like clams suddenly becoming much more common in the oceans than the brachiopods that dominated the Paleozoic.
Since the scale of this mass extinction was realized in the 20th century, one of the enduring mysteries of geology has been its cause. Scientists have proposed impacts of asteroids, chemical catastrophes in the ocean or the atmosphere, and impacts from massive volcanic eruptions now called the Siberian traps.