635 million years ago, almost the entire planet was a frozen ball of ice. And yet mere tens of millions of years later a population explosion happened deep underwater in South China, preceding the better-known Cambrian explosion by a significant time period. In these oxygen-free waters, more than 3,000 fossils of 15 species of seaweeds and worms lived briefly, and then all died suddenly.
The Lantian Basin where the fossils were found is an environment of black shale, thought to be almost oxygen free, but these newly-discovered species reveal that there were short bursts of oxygenation in an otherwise O2-free zone. These bursts were long enough for these species to adapt and thrive, but so short that the ecosystem died suddenly, preserved in fossil form when the oxygen disappeared. The oxygen loss was so sudden that some of the fossil seaweeds still have their roots intact.