Archaeocyathids from the Lower Cambrian Montenegro Member of the Campito Formation, Inyo County, California. Archaeocyathids are an extinct variety of calcareous sponge that never made it beyond early Cambrian times roughly 510 million years ago.

Scientifically polished cross-sections (to facilitate definitive identifications) of the genera Cambrocyathus (top) and Pyncoidocyathus (bottom). Images courtesy a specific scientific publication.