DIVERSE NOSTOCERATID AMMONITE ASSEMBLAGES FROM THE TURONIAN - CAMPANIAN (UPPER CRETACEOUS) NANAIMO GROUP OF VANCOUVER ISLAND, BRITISH COLUMBIA, CANADA: REVISED BIOSTRATIGRAPHY AND SIMILARITIES TO OTHER PACIFIC HETEROMORPH ASSEMBLAGES

The Upper Cretaceous strata of the Nanaimo Group exposed on southeastern Vancouver Island and the Gulf Islands, British Columbia, preserve a diverse assemblage of molluscan fauna, including a wide array of ammonites representing several families. We herein focus on early Turonian–early Campanian heteromorph ammonites in the Family Nostoceratidae Hyatt, 1894, of which only Eubostrychoceras Matsumoto, 1967 and Amapondella Klinger and Kennedy, 1997 were previously known from the region, represented by E. elongatum (Whiteaves), E.? japonicum (Yabe) and A. sp. cf. amapondense (van Hoepen). We greatly expand the known diversity of nostoceratids from the Georgia basin with upper Santonian occurrences of E. indopacificum Matsumoto, E. otsukai (Yabe), and Ainoceras spp. Matsumoto and Kanie, 1967, all of which are also represented in Northwestern Pacific assemblages of similar age. Our findings indicate a greater biogeographic affinity between heteromorphs across the North Pacific in the Santonian, with Japanese and British Columbian fauna showing remarkable similarity. Northeast Pacific endemism is retained in the occurrence of five novel morphotypes, most of which occur above the Santonian-Campanian boundary. The appearance of new early Campanian nostoceratids in the Nanaimo Group coincides with the disappearance of all Eubostrychoceras and Ainoceras from the same sections, creating a unique taxonomic composition compared to other Pacific regions, save for the cosmopolitan genus Amapondella. The end-Santonian disappearance of Eubostrychoceras and Ainoceras is not shared elsewhere in the Pacific, indicating that Santonian-Campanian nostoceratid turnover may have been restricted to the northeast Pacific. Based on our observations, we emend the Eubostrychoceras elongatum Zone of the northeast Pacific region to the Eubostrychoceras spp. assemblage Zone to reflect the diversity of taxa associated within the upper Santonian and the substantial taxonomic turnover which occurred thereafter.