 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.