Wednesday, 26 August 2020


Stromatolites are a major constituent of the fossil record of the first forms of life on earth. They peaked about 1.25 billion years ago and subsequently declined in abundance and diversity so that by the start of the Cambrian they had fallen to 20% of their peak. 

The most widely supported explanation is that stromatolite builders fell victim to grazing creatures — the Cambrian substrate revolution — implying that complex organisms were common over a billion years ago. Another possible culprit are the protozoans. It is possible that foraminifera were responsible for the decline.

Proterozoic stromatolite microfossils (preserved by permineralization in silica) include cyanobacteria and possibly some forms of the eukaryote chlorophytes — green algae. One genus of stromatolite very common in the geologic record is Collenia.

The connection between grazer and stromatolite abundance is well documented in the younger Ordovician evolutionary radiation; stromatolite abundance also increased after the end-Ordovician and end-Permian extinctions decimated marine animals, falling back to earlier levels as marine animals recovered. Fluctuations in metazoan population and diversity may not have been the only factor in the reduction in stromatolite abundance. Factors such as the chemistry of the environment may have been responsible for changes.