The layered fossilized structures formed by ancient microbial mats are called stromatolites.
Stromatolites develop when microbial communities—especially photosynthetic cyanobacteria—trap, bind, and cement sediment in shallow water. As the microbes grow upward toward sunlight and new sediment accumulates, the structure builds distinctive layers, or laminae. Many stromatolites are therefore geological records of microbial activity rather than fossilized bodies.
They are especially important because ancient stromatolites occur widely in Precambrian rocks and provide evidence of some of Earth’s earliest life. Some formations date back more than 3 billion years, although scientists still debate whether certain ancient structures formed biologically or through non-biological mineral processes.
A common mix-up is with thrombolites. Stromatolites are characteristically layered, while thrombolites have a clotted, poorly laminated or non-layered texture. Oncolites are another related structure, usually rounded and formed by concentric microbial coatings.