| The circular to oval specimens are cross-sections of an extinct variety of calcareous sponge called an archaeocyathid, from the Lower Cambrian Poleta Formation, Esmeralda County, Nevada. In image at top, the elongated dark-colored specimen below the oval to circular archaeocyathid cross-section is a longitudinal cross-section of an archaeocyathid. Photographs courtesy of an individual who goes by the cyber-name "wayoflife." |