Thrombolite

Thrombolites are ancient types of microbial communities that photosynthesize. They are similar to stromatolites, but they are formed differently.
Stromatolites are clearly layered, but thrombolites are not. They are "clotted accretionary structures". They formed in shallow water by the trapping, holding, and cementing sedimentary grains by microorganisms, especially cyanobacteria. They were common in the Cambrian and early Ordovician, and few examples still exist today.