Stromatolite

Stromatolites are carbonate rich layered structures built up by bacteria. Modern stromatolites are rare. They are found mostly in hypersaline lagoons in warm climates such as that of western Australia where excellent examples are found at Shark Bay. Stromatolites are common as fossils and are found in rocks up to several thousand million years old. The oldest known probable stromatolite fossils are so called "egg carton" structures from western Australia thought to be about 3.5 billion years old.

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