Mounds of together microorganisms as can form within environments.
Particularly it is photosynthetic microorganisms that form stromatolites, and especially cyanobacteria. For stromatolites to form, however, there must be some means of avoidance of disruption of their structures, such as one can see particularly absent animal presence.
Stromatolites consequently are quite rare, though still present, in the . In the fossil record, by contrast, stromatolites are relatively common and particularly so prior to rise of animals in (see, for example, the Cambrian explosion). Fossil stromatolites in fact exist that date to approximately 3.5 billion (with a "b") years ago.