Lower Cambrian Harkless Formation

Esmeralda County, Nevada

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.

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