Additions and revisions to the inflated Pachydiscidae from the Campanian (Upper Cretaceous) strata of Denman and Hornby islands, British Columbia, Canada; taxonomic implications and insights into mode of life

Abstract Anapachydiscus ( Anapachydiscus ) haegerti n. sp., an ammonite belonging to the Pachydiscidae, is reported for the first time from the upper Campanian Northumberland Formation of Hornby Island, British Columbia. Thirty-five specimens have allowed for the complete ontogenetic reconstruction of the taxon as well as thanatocoenosic (death assemblage) observations with paleoecological implications for ammonoid early life. Although isolated juvenile specimens occur, instances of concretions containing a multitude of individuals are typical. One concretion yielding twenty-six early-stage juveniles lends support to a mode of preservation reflective of life association through a taphonomic process of capture and burial. The new species presents age-diagnostic utility as a distinct fossil within the molluscan assemblage of the Nostoceras ( Didymoceras ?) adrotans regional subzone; a highly constrained interval of the upper Campanian in the eastern North Pacific. A specimen attributed to Anapachydiscus ( Anapachydiscus ) cf. A. ( A. ) fascicostatus from the upper Campanian of the Cedar District Formation on Denman Island is also described, and the genus Anapachydiscus is retained with an emended diagnosis. Additional collections have enabled revised conceptualizations of Pachydiscus ( Pachydiscus ) hornbyense and Pachydiscus ( Pachydiscus ) ootacodensis accounting for the spectrum of variance within these species. Recognition of an upper Campanian P. ( P. ) ootacodensis – Pachydiscus ( Pachydiscus ) suciaensis Concurrent-range Zone is proposed for the eastern North Pacific to assist broader efforts of interregional biostratigraphic correlation given the endemic limitation of the latter taxon. UUID: http://zoobank.org/d8eb27f6-7221-4efb-9548-ec2001986b19