New archaeocyathan occurrences in the upper Harkless Formation (Lower Cambrian of western Nevada)

Abstract

A newly discovered assemblage of archaeocyathans, dominated by the unusual genus Retilamina, occurs in outcrops of the upper Harkless Formation in Esmeralda County, Nevada. These rock units have been correlated with the Saline Valley Formation in the Bonnia–Olenellus Zone (late Early Cambrian) of eastern California, making this the youngest known occurrence of archaeocyathans in the White–Inyo region.Although the assemblage contains a variety of different organisms (including the alga Renalcis, trilobite and echinoderm debris, bivalves?, and Chancelloria spines), it is dominated by archaeocyathans. Diplocyathellus, a conical form with a complex inner wall, is the only genus of regular archaeocyathan in the assemblage. The irregular archaeocyathans Arrythmocricus and Metaldetes exhibit dendroid branching and cylindroconical morphologies, respectively. A third irregular, Retilamina debrennei n. sp., is an atypical archaeocyathan with a sheetlike or shallow convex dome morphology. Retilamina formed natural cavities that sheltered cryptobionts. Unlike coelobiont communities reported from other Lower Cambrian cavities where the diversity of the supported biota is relatively high, the Harkless cavities contain only the alga Renalcis. While the undersurfaces of Retilamina are consistently encrusted by Renalcis, the upper, exposed surfaces typically lack epibionts. Similar morphologies and associations for Retilamina have been reported from bioherms and biostromes from southern Labrador and western Newfoundland. Unlike most archaeocyathans, Retilamina had a solitary, free-lying life habit. Retilamina's broad base and low profile provided stability in episodically turbulent conditions or on biologically disturbed substrates.This occurrence demonstrates that the disappearance of archaeocyathans from the stratigraphic record of the southern Great Basin in the late Early Cambrian is a paleoecological artifact. Archaeocyathans were extant in the region in the late Early Cambrian but the habitats conducive to archaeocyathan growth or preservation are, for the most part, not represented in the preserved strata.