Small organisms that drifted through the sea 424 million years ago, are important to paleontologists as they help unlock the secrets of the Silurian period.

The Silurian period was previously thought to be a time of stability with life recovering and flourishing after mass extinctions. However, graptolites show a different story of how the oceans became poisonous and suffocating before eventually clearing up again, unlocking extinctions and recoveries that scientists didn't see, and showing how unpredictable the Silurian period could be.

Graptolites first showed up in the fossil record during the Cambrian period over 500 million years ago and are flattened tubes with many openings that once hosted colonies of tentacled polyps. Towards the end of the Cambrian, graptolites gained the ability to move around, which allowed them to become widespread, capturing plankton as they floated in the open water.

Changes in graptolite populations matched changes in the rock record, suggesting something big was happening with the ocean. These changes were replicated in other small specialized marine creatures of the Silurian period. The Primo and Cundo episodes were named different ocean environment types and were caused by changes in how water circulated. During the Cundo episodes, the ocean became stratified with different layers that did not mix much.