Cambrian Girvanella Stromatolites from California

The oval to circular, black to brown with splashes of orange structures in the two 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, e.g., likely cyanobacteria, the primitive organisms that largely produced Earth's atmospheric oxygen). This stromatolite was collected more than a decade ago from a site now closed. It has been cut and polished perpendicular to the individual colonial nodules and thus beautifully exhibits the multiply colored growth rings.

Coming from the Chambliss Limestone, Marble Mountains, California, this stromatolite dates to the Lower Cambrian more than 500 million years ago when stromatolite was no longer prevalent and abundant (see discussion below). In particular, by the Cambrian, photosynthetic bacteria responsible biogenic formation of stromatolite structures no longer had the earth to itself. The oxygenated atmosphere has become toxic to some bacteria, and they had to compete with other organisms ranging from eukaryotic microorganisms (bacterial, plants and animals), some of which would have found also been predaceous heterotrophs to this most ancient of life forms. No wonder this that this stromatolite shows the Girvanella spheres that where small independent colonies never reaching the large dome sizes as stromatolite structures did in the Proterozoic.