Living stromatolites are composed of multiple layers of various sediments and mats of algae and bacteria. These structures can be found mainly near the edges of large oceans, which allow for constant sediment mixing from different parts of the surrounding environment due to the constant currents they experience. Stromatolites are usually found in dome-shaped structures that can get a couple of meters tall, from layers that range in thickness of millimeters to inches (Paul et al. 2000). The layered minerals may be quite hard, due to the diverse amount of combined carbonates that, when combined, can turn into a concrete-like consistency (Grotzinger et al. 1999). Perhaps because the form such hard mineral structures, they fossilize well. In fact, some of the oldest fossils on record, dating to 3.5 billion years, are fossilized stromatolites. Stromatolites are scientists’ best records for examining the life and environment of the earth at that time (Berkley). Because oxygen did not begin to accumulate in the atmosphere until approximately 2.0 billion years ago, it may be that the bacterial mats in the earliest fossil stromatolites are either non-photosynthetic or used non-oxygenic, sulphur-based photosynthesis.

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Grotzinger, J.P., and A. H. Knoll. STROMATOLITES IN PRECAMBRIAN CARBONATES: Evolutionary Mileposts or Environmental Dipsticks? Accessed 1/17/2010.

Paul, J., and J. M. Peryt. 2000. Kalkowsky's stromatolites revisited (Lower Triassic Buntsandstein, Harz Mountains, Germany).Palaeogeography, Palaeoclimatology, Palaeoecology, 161: 435-458.