Fossilized stromatolites

Ancient fossilized strata of stomatolites at Steep Rock Iron Mine Steep Rock Stromatolites, Canada (right)

Originating over 3 billion years ago, stromatolites are the oldest known fossils. Fossilized stromatolites were laid down by colonial structures built by photosynthesizing cyanobacteria and other microbes. The cyanobacteria, also called blue-green algae, were likely responsible for the creation of earth's oxygen atmosphere. These organisms are prokaryotic – cells lacking a cellular nucleus – and thrived in warm aquatic environments, building reefs much as coral does today. Domed columns are typical, and the fossilized structures were quite extensive. Cyanobacteria were the dominant lifeform on Earth for over 2 billion years.

Today they are nearly extinct, living a precarious existence in only a few localities worldwide – the most famous location is Shark Bay in Western Australia. Domed columns are typical, and the fossilized structures were quite extensive.
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Close-up of Precambrian stromatolites of the 1.3 Ga Siyeh Formation in Glacier National Park (left).

NW Ontario, Canada contains the Archean Steep Rock carbonate platform, noted as one of the best-exposed and preserved platforms of this age on the planet. Stromatolite in ferruginous chert-carbonate unit, Kakabeka Falls Carbonate Chert Picture

More images of fossilized stromatolite reefs: Gunflint formation stromatolites on shore of Lake Superior / exhumed stromatolites 1.8 Ga near Great Slave Lake, Canada / close-up of Great Slave Lake stromatolites / Siyeh Formation stucture / bioherm of intertidal columnar stromatolites / domal stromatolites / Petrified Springs Reef near Saratoga Springs / Petrified Springs "Lion" / Lykins Formation / Table Head Group, Nfld / Ordovician St. George Group, Nfld / patch reef Middle Cambrian, Nfld / Durness Group, Scotland / stromatolite reef bed in Glacier National Park / Johnnie formation, California, 65 Ma / stromatolite reef Auvergne, France and close up of fossilized stromatolite / fossilized stromatolite reef in Mungok Ri, Korea / close-up of reef

"The centres of many stromatolites in the Siyeh Formation are concave downward rather than domed upward, as if they have collapsed. This may, indeed, have happened. The region lay close to the edge of the continent, where frequent earthquakes would have shaken the stromatolites, perhaps damaging them in this way." Source