Trilobites at the base of the Middle Ordovician, western United States

Abstract

Sections in the Basin Ranges provide stratigraphic standards for the Ordovician of the U.S., from which zones based on trilobites have been widely employed in regional correlation. This paper describes new trilobite faunas from a poorly known part of the succession, at the base of the Middle Ordovician (Whiterockian) in western Utah and eastern Nevada, and including the type Ibexian section near Ibex, Utah. The faunas are from the topmost Wahwah, and overlying Juab Limestone Formations, in strata equivalent to Zone L. Although well-preserved, none of the trilobites are silicified. All are typical of inshore carbonate, Bathyurid biofacies. The majority are new species congeneric with, but different from species from the overlying Kanosh Formation. The dominant bathyurid is the hitherto little-knownPsephosthenaspis, after which the new Zone is named. A threefold subdivision of thePsephosthenaspisZone is based on successive species of the genus. The lower two of these are probably equivalent to the Valhallan Stage, described from more offshore biofacies from Spitsbergen. Eighteen species are described, of which nine are formally named as new including:Goniotelina ensifer, Petigurus inexpectatus, Psephosthenaspis microspinosa, M. glabrior, Pseudoolenoides aspinosus, Ectenonotus progenitor, Kanoshia reticulataandPseudomera arachnopyge.The type species of a remarkable new illaenid-like bathyurelline,Madaraspis magnificagen. et sp. nov., is described.