Palaeoecology of the archaeocyathan reefs from the lower Cambrian Harkless Formation, southern Nevada, western United States and carbon isotopic evidence for their demise

Archaeocyath sponges, the first group of reef-building organisms, have a globally distributed albeit restricted stratigraphic occurrence. Archaeocyathan reefs are well known from the western United States, particularly in units like the Montenegro, Poleta, and Harkless formations of the White-Inyo ranges and Esmeralda County, Nevada. While it is recognized that these organisms disappear at some time above the last occurrence of their reefs in the Harkless Formation, the precise timing of their disappearance is unknown. Here, we show that the last archaeocyathan reefs in the upper Harkless Formation near Gold Point, Nevada, are overlain by 20–60 m of interbedded siltstone and carbonate that preserve a negative carbon isotopic excursion consistent with the AECE event (archaeocyath extinction carbon excursion). Point counts of reef thin sections and analysis of acid-insoluble residues of reef and reef-adjacent samples illustrate that these last reefs also harbor previously undescribed diversity and abundance of animals living in and among the reefs, including an array of small shelly fossil taxa. We confirm previous work that the reefs of the early Cambrian were ecologically diverse and functioned in many ways like reefs of the later Paleozoic, even just prior to their global demise in the late early Cambrian, and that the extinction of archaeocyaths is recorded in strata of the Harkless Formation.