The so-called „big five“ (also Big Five) include: 

• the Ordovician mass extinction 444 million years ago
• the Kellwasser event in the Upper Devonian 372 million years ago
• the event at the Permian-Triassic boundary 252 million years ago
• the crisis period at the Triassic-Jurassic border 201 million years ago
• the mass extinction at the Cretaceous-Paleogene boundary 66 million years ago

For the Big Five, the respective species declines were likely between 70 and 75 percent, or in some cases higher. Although a revision of this classification or a reassessment in favor of other mass extinctions is occasionally proposed (especially regarding the Kellwasser event), the Big Five remain the unofficial guideline in depicting the most severe biological crises during the Phanerozoic. This is not only reflected in the relevant specialist literature, but is also frequently addressed in popular science books and articles.