Girvanella Stromatolite

Nodule, complete, of Girvanella Stromatolite (Oncolite Nodule) measuring ± 13.5 cm x 12 cm.
*Oncolites are sedimentary structures composed of oncoids, which are layered structures formed by the growth of cyanobacteria. Oncolites are very similar to stromatolites, but, instead of forming columns, they form roughly spherical structures. Oncoids often form around a central core, such as a shell fragment, and encrusting microbes deposit a calcium carbonate structure. Oncolites are indicators of warm waters in the photic zone, but are also known from contemporary freshwater environments.

Paleocene, Flagstaff Limestone

Near Thistle, Utah (USA)