Fossilized worm burrows in a limestone block 插畫檔

This illustration shows a slab of sedimentary rock from the Upper Silurian period of Canada, specifically from the Clinton Group. The surface is covered with numerous smooth, cylindrical, and winding casts known as Planolites vulgaris. These structures are trace fossils representing the ancient burrows of marine worms that once moved through the soft mud of the seafloor. As the worms tunneled to find food, their passages were eventually filled with sediment that hardened over time, preserving the path of their movement. The overlapping and intertwining nature of these casts provides a clear record of the high biological activity within the seafloor sediments during this early geological era.