| Scientific cross-sections of archaeocyathids (extinct calcareous sponge) from the upper (youngest) portions of the lower Cambrian Harkless Formation, Esmeralda County, Nevada--a stratigraphic interval immediately above which (in younger strata) archaeocyathids permanently disappear from the geologic record; they go extinct. Left--genus Diplocyathellus. Right--genus Metaldetes. Photograph courtesy a specific scientific publication. |