Large Paleocene Girvanella Stromatolites

These Paleocene stromatolites exemplify the diminished status of stromatolite-forming bacteria during the Phanerozoic Eon. The huge microbial mats building stromatolitic reefs were long gone by the Cambrian, relegating the prokaryotic builders to the bottom of an ever more complex food chain. Oxygenation of air and water prior to, and continuing in the Cambrian also made for a much more hostile environment for the bacteria. These stromatolites from the early Cenozoic era exhibit a small Girvanella form comprising irregular, unsymmetrical spherical nodule sometimes called algal balls. In this case the nodules are generally small and many. Girvanella is more commonly found in Cambrian and younger stromatolites, when bacterial colonies were subject to predation, such as from various members of Domain Eukarya that dominated marine reef systems after the Cambrian Explosion.