<P> The fossil record is strongly biased toward organisms with hard - parts, leaving most groups of soft - bodied organisms with little to no role . It is replete with the mollusks, the vertebrates, the echinoderms, the brachiopods and some groups of arthropods . </P> <P> Fossil sites with exceptional preservation--sometimes including preserved soft tissues--are known as Lagerstätten - German for "storage places". These formations may have resulted from carcass burial in an anoxic environment with minimal bacteria, thus slowing decomposition . Lagerstätten span geological time from the Cambrian period to the present . Worldwide, some of the best examples of near - perfect fossilization are the Cambrian Maotianshan shales and Burgess Shale, the Devonian Hunsrück Slates, the Jurassic Solnhofen limestone, and the Carboniferous Mazon Creek localities . </P> <P> Stromatolites are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria . Stromatolites provide some of the most ancient fossil records of life on Earth, dating back more than 3.5 billion years ago . </P> <P> Stromatolites were much more abundant in Precambrian times . While older, Archean fossil remains are presumed to be colonies of cyanobacteria, younger (that is, Proterozoic) fossils may be primordial forms of the eukaryote chlorophytes (that is, green algae). One genus of stromatolite very common in the geologic record is Collenia . The earliest stromatolite of confirmed microbial origin dates to 2.724 billion years ago . </P>

How does a living organism become a fossil