Abstract
As one moves up or down through the stratigraphic column, the types of fossil organisms often change abruptly. The boundary dividing Permian and Triassic sediments is an example. An estimated 96% of all fossil species found below this boundary are not found above it (Raup 1979, Sepkoski 1989). The sudden change from the presence of a particular type of fossil to its absence in overlying strata is called an extinction. Extinction of many species at approximately the same stratigraphic boundary is termed a mass extinction. The greatest mass extinction occurred at the boundary between the Permian and the Triassic, and is used to divide Paleozoic from Mesozoic sediments.