Silurian Girvanella Stromatolites from Ohio

The oval to circular structures in these images are bacterial fossil nodules, referred to as Girvanella sp (note that while commonly called algae balls, they were actually formed by prokaryotic, photosynthetic bacterial, most likely cyanobacteria, the primitive organisms that largely produced Earth's atmospheric oxygen). It has been cut and polished perpendicularly to the individual colonial nodules and thus beautifully exhibits the multiply colored growth rings. The reverse affords a view of the structures in their natural state.

These stromatolites dates to the Upper Silurian ~ 420 million years ago when stromatolite was no longer prevalent and abundant. By the Cambrian, photosynthetic bacteria responsible for the biogenic formation of stromatolite structures no longer had the earth to themselves. The oxygenated atmosphere had become toxic to some bacteria, and they had to compete with other organisms, some of which would have been predaceous to this most ancient of life forms. No wonder this that these stromatolites show the Girvanella spheres that where small independent colonies, never reaching the large domes of the Proterozoic. The Salina Formation derives its name from the vast deposits of rock salt which it contains, a resource mined in several regions throughout eastern North America.