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 late Precambrian, about 2.2 to 2.4 billion years ago.