February 1998

A large boulder of 900-million-year old stromatolite was collected by Ed Mathez and Heather Sloan in Mauritania, in Western Africa. Stromatolite boulders contain fossilized evidence of early marine life. The photosynthetic processes of these early life forms played a key role in increasing the oxygen in the atmosphere and oceans.

A close-up view of a stromatolite boulder in the field in the desert — the Atlantic coast appears far in the background.

photo credit: Craig Chesek, © American Museum of Natural History