PALEOGEOGRAPHIC AND STRATIGRAPHIC DISTRIBUTION OF UPPER SUNWAPTAN AND BASAL IBEXIAN SPECIES OF THE CAMBRIAN TRILOBITE GENUS PLETHOPELTIS

Evaluation of trilobite material assignable to the genus Plethopeltis from more than forty productive horizons in 15 measured sections that include uppermost Cambrian strata in the central Appalachians (Maryland and Virginia), Texas, Oklahoma, New Mexico, Wyoming and Montana reveal that inadequate taxonomic resolution in earlier studies has distorted the stratigraphic and paleogeographic ranges for many species of the genus. The new data that allow for finer species discrimination significantly expand the known geographic range of P. stitti and support its use as an index for the base of the Upper Sunwaptan Substage in microbial reef-rich facies, at least along the southern margin of Laurentia. Also, P. obtusus is shown to have a much more restricted distribution, perhaps occurring only in the central Appalachians, and a much reduced stratigraphic range to the medial Upper Sunwaptan. Aside from P. glaber in Alberta, species of the genus from the Upper Sunwaptan (Prosaukia serotina Subzone) are poorly known owing to limited recovery. In contrast, P. concavus, previously reported only from Newfoundland, has proven to be a ubiquitous species in the basal Ibexian (Eurekia apopsis Subzone), having now been recovered from Maryland (along with P. pulveris), Texas, Oklahoma, Utah and Wyoming. The geographic range of P. arbucklensis, a key species restricted to the overlying Missisquoia depressa Subzone, has been expanded to include Texas, New Mexico and Utah. Material from coeval strata in Wyoming and Montana currently is under study to determine the nature of the lateral replacement of that species northward by P. hastatus in Alberta. A new species, perhaps assignable to Plethopeltis, occurs in the basal part of the still younger Missisquoia typicalis Subzone in northern Wyoming, extending the total known range of the genus.