"Stromatolites consist of multiple rock layers (or "stone blankets," as the Greek name implies) that formed in shallow, intertidal and sub-tidal environments. Most, if not all, of these rock formations are the remnants of ancient microbial mats that grew on top of each other in successive generations. Stromatolite-building goes on today, but very rarely. In fact, stromatolites practically dropped out of the geologic record after the Cambrian explosion (about 530 million years ago), when animals appeared that eat the mat-forming bacteria. "They've gone to extreme environments where the animals that eat them can't live," Due to this predation, microbial mats have retreated to saline lagoons and hot geothermal regions." (see Yellowstone link)