The great dying's silent seas
Around 252 million years ago, Earth experienced its most severe extinction event, the end-Permian mass extinction, often called "The Great Dying." Massive volcanic eruptions from the Siberian Traps released staggering amounts of carbon dioxide, triggering catastrophic climate change. This event eliminated an estimated 81% of all marine species. Among the casualties were the dominant reef-building organisms of the Paleozoic Era. For millions of years before the extinction, ocean reefs were constructed by rugose and tabulate corals, various sponges, and bryozoans. These groups were completely wiped out.
Their disappearance created a void in the world's oceans. For a period lasting from 8 to 10 million years, and possibly as long as 14 million years, complex metazoan (animal-built) reefs vanished from the planet. The Early Triassic oceans that followed the extinction were hostile to life. Geochemical evidence from marine limestone shows widespread anoxia (low oxygen) and ocean acidification. Seawater oxygen levels may have dropped by a factor of 100. These harsh conditions, punctuated by volatile temperature spikes, suppressed the recovery of complex marine ecosystems. Life that did survive was dominated by "disaster taxa"—hardy, opportunistic organisms that thrived in the stressful environment. These included the bivalve Claraia and the brachiopod Lingularia, which formed widespread, low-diversity communities. The first structures to resemble reefs after the extinction were not built by animals, but by microbes, forming structures called microbialites.