Revision of the Middle Cambrian trilobiteAgnostus acadicusHartt

Abstract

Agnostus acadicusHartt (inDawson, 1868) was the first agnostoid trilobite to be described from Middle Cambrian strata in North America. It was later designated the type species ofAcadagnostusKobayashi, 1939. The original description ofA. acadicuswas based on two specimens, a cephalon and a pygidium, which are illustrated for the first time with photographs. These are interpreted to represent separate species, each belonging to a different genus. The holotype cephalon is reassigned asPeronopsis acadica, which is judged to be a subjective senior synonym ofAgnostus fallaxLinnarsson, 1869, and several other species-group taxa. Consequently,Acadagnostusis suppressed as a subjective junior synonym ofPeronopsis. Agnostus fallaxis the type species ofAxagnostusLaurie, 1990, which also is suppressed as a subjective junior synonym ofPeronopsis.The pygidium on which the original description ofA. acadicuswas based is reassigned toHypagnostus parvifrons(Linnarsson, 1869).Peronopsis acadica, as emended, is a common, cosmopolitan agnostoid in open-marine lithofacies. It has an observed stratigraphic range from thePtychagnostus praecurrensInterval-zone to thePtychagnostus punctuosusInterval-zone.