Oncolites from the upper Burgsvik beds (Silurian), with shell fragments as nuclei. Large flower is 2 cm in diameter.
Oncolites are sedimentary structures formed of oncoids, which are layered spherical growth structures formed by cyanobacterial growth. Oncolites are very similar to stromatolites, but instead of forming columns they form approximately spherical structures. Often the oncoids form around a central nucleus, such as a shell particle, and the calcium carbonate structure is precipitated 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.
Recent/Subrecent freshwater oncoids in Bavaria