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Pre-Cambrian stromatolites in the Siyeh Formation, Glacier National Park.

Stromatolites (from Greek strōma, mattress, bed, stratum, and lithos, rock) are defined as "attached, lithified sedimentary growth structures, accretionary away from a point or limited surface of initiation." A variety of stromatolite morphologies exist including conical, stratiform, branching, domal, and columnar types. Stromatolites are commonly thought to have been formed by the trapping, binding, and cementation of sedimentary grains by microorganisms, especially cyanobacteria (formerly known as blue-green algae). However, very few ancient stromatolites actually contain fossilized microbes. While features of some stromatolites are suggestive of biological activity, others possess features that are more consistent with "abiotic" (non-organic) precipitation. Finding reliable ways to distinguish between biologically-formed and abiotic (non-biological) "stromatolites" is an active area of research in geology.

Stromatolites were much more abundant on the planet in Precambrian times. While older, Archean fossil remains are presumed to be colonies of single-celled blue-green bacteria, younger (that is, Proterozoic) fossils may be primordial forms of the eukaryote chlorophytes (that is, green algae). One genus of stromatolite very common in the geologic record is Collenia.

Prior to 2.4 billion years ago, the earth's atmosphere was rich in carbon dioxide. However, the Precambrian air lacked the oxygen that sustains the complex multicellular life that has evolved since the "Cambrian explosion" 540 million years ago. Stromatolites in the fossil record decline sharply in both diversity and number during the late Proterozoic eon, although they are present, but not common, in Paleozoic era strata. Today, stromatolites are quite uncommon in marine environments, and thus are called "living fossils."

Their former abundance may be because there were no burrowing or grazing animals back during the Precambrian to destabilize sediments and consume growing microbial mats, thereby favoring the preservation of these microbialites. Also, changing chemical conditions in the ocean during this time could be responsible for the precipitation of non-biological stromatolites through the growth of tiny crystals.

Modern stromatolites in Shark Bay, Western Australia.

Stromatolites at Lake Thetis, Western Australia.

While prokaryotic cyanobacteria themselves reproduce asexually through cell division, they were instrumental in priming the environment for the evolutionary development of more complex eukaryotic organisms. Cyanobacteria are thought to be largely responsible for increasing the amount of oxygen in the primeval earth's atmosphere through their continuing photosynthesis.

Cyanobacteria use water, carbon dioxide, and sunlight to create their food. The byproducts of this process are oxygen and calcium carbonate (lime). A layer of mucous often forms over mats of cyanobacterial cells. In modern microbial mats, debris from the surrounding habitat can become trapped within the mucous, which can be cemented together by the calcium carbonate to grow thin laminations of limestone. These laminations can accrete over time, resulting in the banded pattern common to stromatolites. The domal morphology of biological stromatolites is the result of the vertical growth necessary for the continued infiltration of sunlight to the organisms for photosynthesis.

Modern stromatolites are mostly found in hypersaline lakes and marine lagoons where extreme conditions exclude animal grazing. One such location is Hamelin Pool Marine Nature Reserve, Shark Bay in Western Australia where excellent specimens are today observed, and another is Lagoa Salgada, state of Rio de Janeiro, Brazil, where modern stromatolites can be observed as bioherm (domal type) and beds. Fresh-water stromatolites can be found in Cuatro Ciénegas, a unique ecosystem in the Mexican desert.

Layered spherical growth structures similar to stromatolites, named "oncolites," are also known from the fossil record.

• Grotzinger and Knoll; Stromatolites in Precambrian Carbonates: Evolutionary Mileposts or Environmental Dipsticks? 1999.
• Allwood et al. (2006) Stromatolite reef from the early Archean era of Australia; Nature, 441 714-718.

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