Stromatolites are pretty important organisms, seeing as they where the dominant reef builders for most of the Precambrian period.
Stromatolites are made of Cyanobacteria as well as a few other prokaryotes.
Their structures vary from laminar sheets to domes and columnar structures. All of these contain CaCO3.
Stromatolites are a major constituent of the fossil record for about the first 3.5 billion years of life on earth, though their diversity started to decline by the start of the Cambrian due to grazing and burrowing organisms.
Stromatolites also occurred after the Precambrian in the Phanerozoic and Lower Triassic as well as modern times, although usually very restricted by their environment.
The reason for this may be due to a lack of grazing organisms after a mass extinction event or due to hyper salinity of the marine environment.
Also, most of the Post-Precambrian Stromatolites are formed from eukaryotic bacteria such as blue-green algae cemented together by CaCO3 instead of being formed by the Cyanobacteria.
Sources:http://study.plymouth.ac.uk/Module/EOE1 ... 0RT-KP.pdfhttp://en.wikipedia.org/wiki/Stramatolites
Found this on my forum today and I thought it might be relevant or useful to people here.
Originally from:http://geo-center.heavenforum.com/palae ... 68.htm#345
This link just takes you to the page the information was posted in, no discussion has taken place so far.
I supplied the link solely for those that might have an interest in the subject so they can look at the rest of the forum.
If this is against the rules I will remove the link.