Thrombolites are ancient forms of microbial communities that photosynthesize. They are clotted accretionary structures formed in shallow water by the trapping, binding, and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria. They are now only found in a few places in the world. Stromatolites are similar but consist of layered accretions. Hundreds of millions of years ago the ancestors of thrombolites and stromatolites produced the oxygen in the atmosphere that is required for life as we know it.