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The Permian extinction—killing 95 percent of marine life—was by far the most devastating of Earth’s periods of mass extinction, and yet scientists are only now starting to move past simple theories of why it occurred.

By examining fossils of extinct marine animals on a cellular level, scientists from Stanford and the University of California, Santa Cruz, have begun to notice patterns that point to the cause of the Permian extinction and may even inform us on marine life today.

They discovered that marine animals with skeletons or shells made from limestone or calcium carbonate were more likely to die than were animals with other types of shells or skeletons. This was true across the planet and did not correlate to how the animal moved or what it ate.

Based on this evidence, scientists now believe that the conditions that led to this were most likely caused by a giant volcano eruption, which would cause altered acidity levels, higher ocean temperatures, and areas of oxygen depletion in the ocean.

This left gastropods like snails and bivalves like clams and scallops to become the dominant marine life after the Permian extinction and that change remains to this day. “Modern marine ecology is shaped by the extinction spasms of the past,” said scientist Matthew E. Clapham.

Read more about the study in the New York Times article.