Biogeographic shifts in a transgressive succession: the Cambrian (Furongian, Jiangshanian; Latest Steptoean–earliest Sunwaptan) agnostoid arthropods Kormagnostella Romanenko and Biciragnostus Ergaliev in North America

Abstract

The first records of the upper Cambrian agnostoid genera Kormagnostella , E. Romanenko, in Romanenko and Romanenko, 1967, and Biciragnostus F. Ergaliev, in Eraliev and Ergaliev, 2001, in Laurentian North America are from a narrow stratigraphic interval in the Steptoean–Sunwaptan boundary interval (Furongian, Jiangshanian) of Nevada and Utah. In Nevada, both genera occur in a condensed bioclastic lag below a major flooding surface, and Kormagnostella also appears in a transgressive interval in Utah. Immigration of these genera is associated with sea level rise, and also with faunal turnover. Biciragnostus is confined to the latest Elvinia Zone, immediately below the onset of a trilobite and agnostoid extinction event at the base of the Irvingella major Zone (basal Sunwaptan). Kormagnostella is present in the latest Elvinia Zone, and has its highest occurrence in the I. major Zone. Stratigraphic data from the Karatau-Naryn Terrane, Kazakhstan indicate that both genera disappear near the local extinction of Irvingella , suggesting that faunal turnover in that region may have been broadly correlative with the more profound extinction in Laurentia. New species are Kormagnostella advena , K. insolita and Biciragnostus viator .