The Late Ordovician mass extinction was a significant planetary biodiversity crisis that occurred during the transition between the Ordovician and Silurian periods, marking one of Earth's "Big Five" mass extinction events. The extinction is generally attributed to a combination of environmental stressors, primarily massive global cooling and subsequent dramatic changes in sea level. Initial cooling led to the deposition of large amounts of ice, which drastically lowered global sea levels, thereby restricting shallow marine habitats critical for many marine life forms. Subsequent fluctuations in climate and chemistry further stressed the surviving biota, contributing to the massive die-off that dramatically reshaped marine ecosystems and global faunal composition.