DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Fossil stromatolite. Section through a rock with layers of a fossilised stromatolite. These layers are mineralised microbial mats. Each successive layer was formed by blue-green algae (also called cyanobacteria) on top of an older layer. The older layers mineralise as the microbes secrete calcium carbonate. This forms rock-like structures called stromatolites, with a living microbial top layer. Eventually, the microbial community dies and the stromatolite (a living fossil while it was alive) may be truly fossilised as rock layers cover it. This specimen is from Bolivian pre-Cambrian rock, from over 540 million years ago when blue-green algae were among the earliest forms of life.
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