Stromatolites are a trace fossil of the biofilms of blue green algae that grow around an object such as shell of an oyster or other mollusc. As the algal filaments grow outwards, sediment is continually deposited and accreted onto the surface of the algae, thus increases in size forming the distinctive concentric rings that you see when the fossil is broken in cross section. They form knobbles round boulders and can reach sizes of a meter wide in the fossil record. Stromatolites grow in shallow, hypersaline pools in shore settings, inhospitable to molluscs which might otherwise graze on the algae. Modern day examples of marine stromatolites can be found in Shark Bay in Australia.
Age approx. 0.70 Million Years Digital Capture: Photogrammetry
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