finely-layered fossilized microbial mats (in columns or mounds) that are found in rocks up to 3500 million years (My) old. Their origin is controversial, yet studies of modern stromatolites indicate that they are formed by the activities of cyanobacteria, in shallow marine environments. Stromatolites form the earliest-known reefs (2000 My old), some of which are up to 450 m thick. They are abundant in the Proterozoic (2500 - 900 Ma) but decline sharply in the Late PreCambriam (900 Ma).