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.
"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