Tuesday Nov 16, 2021, 6 pm GMT+1

Sandy MCLACHLAN^1*, Vera POSPELOVA^2,1 and Elaine C. HUMPHREY¹ 

Morphological variance within the spiniferate gonyaulacacean dinoflagellate cyst Cannosphaeropsis franciscana across the K/Pg boundary; the case for vesiculation as an ecophenotypic character with consideration for the genus Hafniasphaera

¹ School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, Canada
² Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis, MN, USA
^* sandyymcl@uvic.ca 

The recent discovery of an Upper Cretaceous–lower Paleogene (upper Maastrichtian–lower Selandian) succession within the Oyster Bay Formation on eastern Vancouver Island, British Columbia, Canada, has been made based on dinoflagellate cyst biostratigraphic controls (McLachlan and Pospelova 2021). This recent work has presented the opportunity to analyze an unprecedent range of variation within an enigmatic spiniferate gonyaulacacean genus and species, Cannosphaeropsis franciscana (McLachlan et al. 2021). Using both transmitted light and scanning electron microscopy, we observe forms across a broad morphological plexus which encompasses differences in process type, autophragm geometry and most significantly, vesiculation. The presence and extent of cyst vesiculation is established as a key diagnostic character resulting in the erection of three subspecies: Cannosphaeropsis franciscana subsp. franciscana (autonym), Cannosphaeropsis franciscana subsp. vacuoseptata subsp. nov. and Cannosphaeropsis franciscana subsp. vesiculata subsp. nov. The onset and decline of vesiculate forms within the lowest Danian interval is grounds for the interpretation that the subspecies reflect an ecophenotypic response to changing paleoenvironmental factors above the K/Pg boundary, specifically those of unstable, stratified marine conditions associated with the post-Cretaceous transgressive phase. Assemblage data throughout the formation also reveal members of the vesiculate genus Hafniasphaera increase their abundances in nutrient-rich and likely stratified coastal waters when Spiniferites species are also abundant. The development of vesicles is suggested as an adaptation to prolong cyst buoyancy and slow descent in the water column. An extensive literature review considering all illustrated reports of formally recognized species belonging to the genera Cannosphaeropsis and Hafniasphaera is also presented alongside taxa chronostratigraphic ranges.

References

McLachlan, S.M.S., Pospelova, V. 2021. Dinoflagellate cyst-based paleoenvironmental reconstructions and phytoplankton paleoecology across the Cretaceous–Paleogene (K/Pg) boundary interval, Vancouver Island, British Columbia, Canada. Cretaceous Research 126, 104878, doi: 10.1016/j.cretres.2021.104878.

McLachlan, S.M.S., Pospelova, V., Humphrey, E.H. 2021. 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. Review of Palaeobotany and Palynology 292, 104452, doi: 10.1016/j.revpalbo.2021.104452.