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

Stromatolites 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. 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 precipitation. Finding reliable ways to distinguish between biologically-formed and non-biological or "abiotic" stromatolites is an active area of research in geology.

Stromatolites were more abundant on the Earth in Precambrian times. The Precambrian atmosphere was rich in carbon dioxide, but lacking in oxygen necessary to sustain more complex multicelluar life. Stromatolites in the geological record declined sharply in both diversity and number during the Paleozoic, and are uncommon in modern day marine environments. During the Precambrian, there were no burrowing or grazing animals to destabilize sediments and consume growing microbial mats - favoring the preservation of microbialites. During the Precambrian, changing chemical conditions in the ocean could be responsible for the precipitation of non-biological stromatolites through the growth of tiny crystals.

While prokaryotic cyanobacteria themselves reproduce asexually through cell division, they were instrumental in priming the environment for the evolution of more complex eukaryotic organisms. Cyanobacteria are thought to be responsible for increasing the amount of oxygen in earth's atmosphere through photosynthesis. Cyanobacteria use water, carbon dioxide, and sunlight to create food. The biproducts 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. An example of such a location is Shark Bay in Western Australia where excellent examples are found.

Layered spherical growth structures similar to stromatolites are also known. They are called "oncolites".

Reference

Grotzinger and Knoll; Stromatolites in Precambrian Carbonates: Evolutionary Mileposts or Environmental Dipsticks? 1999.