Calculate Percent By Mass

Second only to the Permian-Triassic extinction, the late Ordovician event 440 million years ago, saw the demise of approximately 85 percent of marine species in two distinct pulses, according to Peter Sheehan, a paleontologist with the Milwaukee Public Museum.

In his 2001 paper, “The Late Ordovician Mass Extinction,” published in the Annual Review of Earth and Planetary Sciences, Sheehan describes a glacial interval with two episodes of extinction occurring at the beginning of the glacial phase, and one at the end.

“We have a good understanding of the basic changes during the extinction and recovery,” says Sheehan.