Most mass extinctions that have afflicted life on Earth during the past 500 million years have occurred during times of major volcanic eruption and all were accompanied by major changes in climate, sea level and oxygenation levels in the ocean.  Among the five major mass extinctions, only the end-Cretaceous (KT) displays a close coincidence of four factors - an iridium anomaly (commonly assumed to represent an impact), an impact crater (Chicxulub), a large igneous province (the Deccan Traps) and major climate and sea level changes (Fig. 2). The KT mass extinction also differs in that it follows the longest period (145-65.5Ma) of low background extinction (Fig. 2). Throughout the Cretaceous, generic diversity had increased, accelerating during the Campanian and peaking during the late Maastrichtian, prior to the mass extinction.  PDF