Earth has experienced five great mass extinctions—events when more than half of living species died out within a million years. The end-Permian extinction, 252 million years ago, was by far the biggest to date. Massive volcanic eruptions in Siberia spewed lava over 2.7 million square miles and triggered rapid climate change as greenhouse gases escaped from burning coal deposits. Acid rain altered ocean chemistry and killed off 90 percent of marine life, including most ammonites, echinoids, and brachiopods—plus the last of the trilobites. Complex coral reefs gave way to much simpler ecosystems for a few million years. Diversity on land also nose-dived dramatically, and most dicynodonts vanished.

Extensive reefs built by rugose and tabulate corals, including Cladopora tabulata, perished in acidic oceans during the end-Permian extinction. Brachiopods peaked in the Permian, never recovering their diversity.

Living in colonies much as coral does, filter-feeding bryozoans like Fenestella had a rigid, fan-shaped mesh skeleton. A few thousand bryozoan species still exist today.

Lucky end-Permian extinction survivor Miocidaris connorsi, a sea urchin, was among the species that gave rise to today’s urchins. Modern sand dollars also descend from a few Permian survivors.

Bottom-dwelling brachiopods, such as Edriosteges multispinosus, were abundant on ancient reefs.