"Mark Godden's Little Bit Of Cyberspace Mk.II"

Section through a Jurassic stromatolite  from the Lower Purbeck, Bowers Quarry, Portland.

Stromatolites are some of the oldest fossils known, dating back as far as 3500,000,000 years. Stromatolites formed by the action of cyanobacteria (blue green algae) which precipitated layered calcium carbonate while living in thick algal mats in a sea-shore environment. Early stromalolites probably produced the gaseous oxygen which went on to form the first thinly oxygenated atmosphere around the Earth. 

By absorbing harmful rays from the Sun, this early oxygenated atmosphere "paved the way" for terrestrial plants which, because of less hostile atmospheric conditions, were gradually able to colonize early land surfaces. These early plants were then able to simultaneously reduce the levels of atmospheric carbon dioxide and contribute more oxygen to the atmosphere, ultimately producing conditions suitable for terrestrial animal life. We probably owe our very existence to the ancestors of the fossil in the picture! 

Modern stromatolites can be found in certain places where the environment is hostile to organisms which would normally keep bacteria numbers down, such as in the very salty water of Shark Bay, Western Australia.

The word stromatolite is Greek for "layered stone" and the laminated structure of the specimen pictured (at actual size) is clearly visible. Un-cut these mushroom shaped fossils look like calcareous "brains" or "cow pats" and are extremely common in certain horizons.