Trilobites assigned to 14 genera and 14 species are reported from basal part of the Deadwood Formation in the Black Hills of South Dakota. Six additional taxa are left in open nomenclature. One new species, Cedarina dakotaensis, is described. These trilobites are assigned to a new zone, the Cedarina dakotaensis Zone, named after the most abundant trilobite species. Species of Cedarina and Modocia are the most abundant in the Cedarina dakotaensis Zone, accompanied by less common specimens of species of Arapahoia, Menomonia, Hardyoides, Welleraspis, Kormagnostus, and Kingstonia. Cedarina dakotaensis and Modocia centralis are the most abundant species in the nearshore sandstone lithofacies, whereas Arapahoia spatulata is the predominate taxon in the offshore limestone lithofacies. The fauna of the Cedarina dakotaensis Zone (which lacks species of Cedaria) occupies the biostratigraphic interval of the traditional Cedaria Zone of the Marjuman Stage. Trilobites from the Cedarina dakotaensis Zone can be used to correlate the basal part of the Deadwood with coeval strata elsewhere in North America.

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