Stromatolites

Stromatolites are laminated sedimentary structures formed by microbial communities, especially cyanobacteria, that trap, bind, and mineralize sediment to build layered aggregates. In calm, shallow waters, microbial mats secrete sticky polymers that trap fine grains, while metabolic byproducts promote mineral precipitation, cementing successive laminae into domes, columns, or crusts. The result is a concentric, often ribbed or mound-like structure that records a history of microbial activity and sedimentation over time.

Stromatolites are among the oldest known fossils and provide crucial evidence for the antiquity of life on Earth. Fossilized forms date to about 3.5 billion years ago, with abundant examples in the Precambrian. Their long record helps scientists study the early biosphere, the evolution of photosynthesis, and the rise of atmospheric oxygen during the Great Oxidation Event.

Today, living stromatolites survive in restricted environments where storm or grazing pressures are low and chemistry favors carbonate precipitation. Notable modern examples include Shark Bay in Western Australia, where extensive stromatolites persist, as well as other hypersaline lagoons, tidal flats, and certain hot springs worldwide.

Fossil stromatolites can preserve fine laminations and microbial textures, but distinguishing biogenic structures from abiotic analogs can be challenging. Consequently, paleontologists assess stromatolites using texture, morphology, and contextual sedimentology, along with modern analog studies.