Fossil decapod crustaceans from the Gammon Ferruginous Member, Pierre Shale (early Campanian), Black Hills, South Dakota

Abstract

AssTRArcT-Four hundred twenty-five fossil decapods, preserved in apatite concretions, were collected from the Early Campanian Gammon Ferruginous Member of the Pierre Shale in Butte County, South Dakota. The fauna, confined to a 4.3 meter interval of rock and mapped over eight square kilometers, is dominated by the mud shrimp Protocallianassa russelli n. sp. (74.5% of decapod fauna), the crab Necrocarcinus davisi n. sp. (14.6%), and the lobster Hoploparia mickelsoni n. sp. (9.5%), but includes four other crabs: Dioratiopus hearttailensis n. sp. (1.2%), Raninella oaheensis Bishop, 1978 (0.5%), Xanthosia elegans occidentalis n. sp. (0.5%), and Rugafariusfredrichi n. gen., n. sp. (0.2%). This group of decapods is considered to be an assemblage, the Protocallianassa-Necrocarcinus-Hoploparia Assemblage, because of similarities to the Dakoticancer Assemblage in faunal characteristics, mode of preservation, and geographic and stratigraphic distribution. This assemblage predates the migration of Dakoticancer overanus Rathbun into the Western Interior and the consequent development of the Dakoticancer Assemblage, which contains descendents of taxa in the Protocallianassa-Necrocarcinus-Hoploparia Assemblage, repeatedly present in the Upper Pierre Shale.