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.
A new dynasty of reef builders
The long process of recovery began in the Middle Triassic period. The first true animal reefs to reappear were not built by corals, but by sponges, algae, and bivalves. The ancestors of modern corals, the Scleractinia, first appear in the fossil record around 240 million years ago. These pioneering scleractinian corals were small, solitary, or lived in loosely-branched colonies, and did not form major reef structures. It took another 20 to 25 million years after their first appearance for corals to become significant reef builders.
The evolutionary origin of these new corals is a subject of ongoing research. They are fundamentally different from the Paleozoic rugose corals, especially in their skeletons, which are made of aragonite instead of calcite. One leading hypothesis suggests that scleractinians evolved from soft-bodied, anemone-like ancestors that survived the extinction event and later developed the ability to build skeletons. This theory helps explain the lengthy gap in the fossil record. By the time complex reefs re-established, the world had changed. The new scleractinian corals formed the foundation of the marine ecosystems we know today, a recovery that took millions of years to complete after the near-total destruction of the Great Dying.