Stromatolites, formerly defined as: "laminated organo-sedimentary structures formed by the trapping and binding, and/or precipitation of minerals by microorganisms", can be thought of as microbial mats in which a rock-like layer of either sand or precipitated minerals is also present. Fossil stromatolites constitute our earliest and most pervasive record of life on Earth. Although many stromatolites are fossils, there are a number of locations on the modern-day Earth where stromatolites are still forming.

Examples of Stromatolites in the Fossil Record

Stromatolite, Rossport Group, 1300 million years old, Ontario, Canada

This flat laminated structure was constructed approximately 1300 million years ago by a cyanobacterial microbial mat community which grew in a shallow, hypersaline coastal marine environment. Higher concentrations of silica in the darker layers have protected the organic matter from oxidation.

Stromatolite, Chuar Group, 760 million years old, Arizona

This dome-shaped, laminated structure was constructed approximately 760 million years ago by a cyanobacterial microbial mat community which grew in a shallow, subtidal coastal marine environment. Subtle differences in mineral content cause the individual growth layers to stand out when the rock is subjected to weathering.

Stromatolite, Morrison Formation, 150 million years old, New Mexico

This is a vertical cross section of a stromatolite that was constructed by cyanobacteria and green algae in a continental, alkaline lake. This 150 million-year-old stromatolite was found in the Mesozoic-age Morrison Formation, which is better known for its dinosaur remains. Note the laminated texture of the stromatolite, each layer of which marks the growth site of an ancient microbial mat.

Modern Stromatolite, Alkaline Evaporitic Lake, Nevada

This structure consists of numerous internal columns which were constructed by cyanobacteria growing in an evaporitic lake. The growth habit of the microbial communities has been imparted upon the carbonate structure, thereby creating a "biogenic fabric" which is a characteristic of this particular bacterial community. Stromatolites still grow today in alkaline and saline lakes where the adverse chemistry of the lake waters have suppressed the populations of algae and grazing animals.

Stromatolite, Belt Supergroup, 1350 million years old, Glacier Park, Montana

This stromatolite once grew in a shallow carbonate platform in water depths of a few meters, about 1350 million years ago. It was part of an extensive subtidal and intertidal reef and mudflat complex that, despite the absence of plants and animals, bore striking resemblance to modern coastal environments.

Stromatolite, Transvaal Supergroup, 2500 million years old, Barberton Mountains, Transvaal, South Africa

This stromatolite developed as part of an enormous 2500 million-year-old carbonate platform complex that included extensive reef, forereef and backreef components. This platform complex reveals that, despite the absence of corals or other multicellular reef-forming organisms, these ancient stromatolitic reefs resembled their modern counterparts in virtually all other aspects

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