The World Before Worms
For three billion years, the floor of Earth's shallow seas was a different world. It was a firm and stable landscape coated in microbial mats. These extensive biofilms, composed mainly of cyanobacteria, bound the sediment together, creating a tough, leathery surface often compared to elephant skin. This created a sharp boundary between the seawater and the anoxic, hydrogen sulfide-rich sediment just millimeters below the surface. On this firm "matground," the enigmatic Ediacaran biota lived—strange, multicellular organisms like Dickinsonia and Charnia that largely remained on the sediment surface. For eons, this two-dimensional world of surface-dwelling life was the standard.
An Agronomic Revolution
The start of the Cambrian Period, around 541 million years ago, introduced a radical new way of life: burrowing. This innovation, termed the "Cambrian substrate revolution" or "agronomic revolution," saw animals for the first time digging vertically into the sediment to find food or seek protection. This churning of the seafloor is known as bioturbation. The culprits were likely early worms and arthropods whose new behaviors permanently altered the planet.
The evidence for this revolution is not in body fossils, but in trace fossils—the preserved tracks, trails, and burrows left behind. The main marker for this event is the trace fossil Treptichnus pedum, a complex, branching burrow pattern. The first appearance of T. pedum in the rock record is so significant it officially defines the boundary between the Ediacaran and Cambrian periods.
These new "ecosystem engineers" destroyed the microbial mats that had defined the seafloor for billions of years. The firm matgrounds were churned into soft, watery "mixgrounds," similar to modern seafloors. This process of digging allowed oxygen and water to penetrate the sediment, flushing out the toxic hydrogen sulfide and making the subsurface habitable for a wider range of organisms. While this opened up immense new ecological space within the sediment, it was a catastrophe for the Ediacaran biota. Their stable, firm world disappeared, leading to a mass extinction and leading to the three-dimensional ecosystems we know today.