Stromatolites and their close cousins the Thrombolites are rock formations of microbial mats that form predominantly in Limestones and Dolostones (Dolostone is a sedimentary carbonate rock that contains a high percentage of the mineral Dolomite). They are most commonly described as biogenically-produced structures formed by colonies of photosynthesising cyanobacteria. Cyanobacteria is a general term given to a large group of photosynthetic Eubacteria. These are often referred to as "blue-green algae", though they are Eubacteria, and not algae.
Stromatolites are very interesting fossils because they give us a visual glimpse of the development of life when the world was very youngn. A small piece of Stromatolite encases biological activity spanning perhaps thousands of years. Indeed, the oldest known fossils are cyanobacteria from the Archaean rocks of western Australia, dated 3.4 billion years old. Given that the oldest known rocks (3.8 billion years old) are only a little older than the Australian Stromatolites we could assume that life on our planet started very early in Earth's history.
However, Stromatolites are not completely clear-cut in the eyes of paelontologists. Indeed some believe that Stromatolites are not formed by bacteria at all, but are the result of chemical precipitation. This theory is strongly disputed by the recent studies of Dr Allwood and her colleagues. They have studied a large number of Archaean rocks and demionstrated different patterns of Stromatolite formation. In an article in this month's Nature they argue that the diversity, complexity and environmental associations of the Stromatolites describe patterns that—in similar settings throughout Earth's history—reflect the presence of organisms. What's more, Stromatolites show that even very early in the Earth's history there existed biological diversity.
Abigail C. Allwood, Malcolm R. Walter, Balz S. Kamber, Craig P. Marshall and Ian W. Burch. (2006) Stromatolite reef from the Early Archaean era of Australia, Nature 441, pp714-718.
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