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stromatolite
Stromatolites or stromatoliths (; from Greek στρώμα, ''strōma'', mattress, bed, stratum, and λίθος, ''lithos'', rock) are layered bio-chemical accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms (microbial mats) of microorganisms, especially cyanobacteria. Fossilized stromatolites provide ancient records of life on Earth by these remains, which might date from more than 3.5 billion years. Lichen stromatolites are a proposed mechanism of formation of some kinds of layered rock structure that is formed above water, where rock meets air, by repeated colonization of the rock by endolithic lichens.〔Lichen Stromatolites: Criterion for Subaerial Exposure and a Mechanism for the Formation of Laminar Calcretes (Caliche), Colin F. Klappa, Journal of Sedimentary Petrology, Vol. 49 (1979)No. 2. (June), Pages 387-400, ()〕〔Paleobotany: The Biology and Evolution of Fossil Plants, Edith L. Taylor, Thomas N. Taylor, Michael Krings, page ()〕
==Morphology==
A variety of stromatolite morphologies exists, including conical, stratiform, branching, domal,〔(【引用サイトリンク】title=Two-ton, 500 Million-year-old Fossil Of Stromatolite Discovered In Virginia, U.S. )〕 and columnar types. Stromatolites occur widely in the fossil record of the Precambrian, but are rare today. Very few ancient stromatolites contain fossilized microbes. While features of some stromatolites are suggestive of biological activity, others possess features that are more consistent with abiotic (non-biological) precipitation. Finding reliable ways to distinguish between biologically formed and abiotic stromatolites is an active area of research in geology.〔
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