Oncolytic Stromatolites

These oncolytic stromatolites are from the lower Cambrian Period. They were found on Marble Mountain, California in the Chambless Limestone. The oncolites are the dark round or oblong shapes. They are sedimentary structures formed by cyanobacteria growing in layers around a fragment of shell or other small object. 

Oncolites are sedimentary structures composed of oncoids, which are layered structures formed by cyanobacterial growth. Oncolites are very similar to stromatolites, but, instead of forming columns, they form approximately spherical structures. The oncoids often form around a central nucleus, such as a shell fragment, and a calcium carbonate structure is deposited by encrusting microbes. Oncolites are indicators of warm waters in the photic zone, but are also known in contemporary freshwater environments. These structures rarely exceed 10 cm in diameter.