Collenia

Collenia is genus of fossil cyanobacteria that form a particular type of stromatolites.

Description
Collenia are stromatolites made up of convex layers flattened in the center, forming columnar colonies. The microorganisms involved were likely photosynthetic bacteria expiring oxygen.

Fossil record
Collenia stromatolites were very common in the Paleoproterozoic era during the Siderian and Rhyacian periods, about 2.2 to 2.4 billion years ago.