Vesiculation in the dinoflagellate cyst Cannosphaeropsis franciscana Damassa, 1979 across the K/Pg boundary (Vancouver Island, Canada) with implications for spiniferate gonyaulacacean taxonomy and ecophenotypy

Abstract

Wall structure and process type have long been essential taxonomic characters used in generic and specific determinations among spiniferate gonyaulacacean dinoflagellate cysts. We observe solid to fully vesiculate wall structure as well as a range of surface texture and process development all within a suite of over 400 specimens of the species Cannosphaeropsis franciscana recovered from the sedimentary rocks of the upper Maastrichtian–lower Selandian Oyster Bay Formation, Vancouver Island (British Columbia, Canada). The genus Cannosphaeropsis is herein provided with an emended diagnosis and description to accommodate this variability as is the species Cannosphaeropsis franciscana which represents a plexus of conspecific morphology unparalleled within the fossil Gonyaulacaceae. In addition, we propose three subspecies of Cannosphaeropsis franciscana: Cannosphaeropsis franciscana subsp. franciscana (autonym), Cannosphaeropsis franciscana subsp. vacuoseptata subsp. nov., and Cannosphaeropsis franciscana subsp. vesiculata subsp. nov. Most vesiculate forms of the species are stratigraphically constrained to within localized Dinoflagellate Cyst Zone D3 situated immediately above the K/Pg boundary. Vesicles are suggested as an adaptation prolonging cyst buoyancy in the water column. Within C. franciscana, prominent vesiculation occurs during an interval characterized by unstable, stratified marine conditions associated with the post-Cretaceous transgressive phase. Assemblage data throughout the formation also indicate that members of the vesiculate genus Hafniasphaera increase abundances in nutrient-rich and likely stratified coastal waters when Spiniferites species are also abundant. The paleoecology of Cannosphaeropsis and the value of these dinoflagellate cysts as paleoenvironmental indicators is considered as well as all currently accepted members of the genus and their chronostratigraphic and geographic distributions.