A review of the areoligeracean dinoflagellate cyst Cyclonephelium and morphologically similar genera

Abstract

A group of mainly Cretaceous areoligeracean dinoflagellate cyst genera, which we call the ‘Cyclonephelium group’, has proved difficult to classify. The group comprises Aptea, Canningia, Canninginopsis, Cassidium, Cauveridinium, Cerbia, Circulodinium, Cyclonephelium, Doidyx, Senoniasphaera and Tenua. As a group, they also converge morphologically with ceratiaceans. Cyclonephelium-group taxa show considerable morphological diversity and gradation. However, the most important criteria for discriminating dinoflagellate cyst-based genera – tabulation and archaeopyle type – are uniform among areoligeraceans and ceratiaceans and so are not useful in this case. Any subdivision of the Cyclonephelium group will break apparently natural continuities; nevertheless, any resolution must involve a hierarchy of morphological criteria. In developing a ‘best-fit’ hierarchy of morphological criteria for the Cyclonephelium group and its separation from ceratiaceans, we consider historical concepts, morphological variation, illustrations in the literature (especially of types) and taxonomic stability. We conclude that the most pragmatic distinguishing feature of ceratiaceans in contrast to the Cyclonephelium group (and areoligeraceans in general) is that the former possess a lateral horn or distinct prominence on the inner body or wall. The hierarchy we favour within the Cyclonephelium group (in decreasing importance) is: (i) wall structure; (ii) whether the ornament is linear or free standing; and (iii) the distribution of the ornament. As a consequence, we propose one new genus (Trimuridinium), one new species (Aptea cassis), two new names (Canningia glomerata for Senoniasphaera rotundata and Circulodinium vectensis for Pseudoceratium distinctum), 49 new combinations and one new status. We emend the descriptions/diagnoses of Aptea, Circulodinium, Cyclonephelium, Pseudoceratium, Senoniasphaera, Tenua and Tenua hystrix. Cyclonephelium-group taxa predominate in neritic marine palaeoenvironments, and their use in palaeoecological analyses should be improved by a more cohesive and consistent taxonomy. The group may have been the root stock for ceratiaceans in the Late Jurassic, the two families becoming more clearly separate from the Late Cretaceous onwards. Cyclonephelium-group areoligeraceans are sparse in the Paleogene and confirmed species are absent in the Neogene.

Keywords:

• Areoligeraceae
• biostratigraphy
• Cretaceous
• Cyclonephelium group
• dinoflagellate cysts
• evolution
• taxonomy

Acknowledgements

We are greatly indebted to Ian Harding and Martin Pearce for their detailed and insightful reviews of this paper, which led to significant improvements; and also to Peta Mudie for her review on behalf of the Geological Survey of Canada, which led to important refinements. Bill MacMillan assisted with drafting. Susanne Feist-Burkhardt, Uli Heimhofer and an unnamed librarian at Hannover University, Germany, provided literature. Jennifer Galloway, Thomas Hadlari and Andrew MacRae provided graphics on which some of the diagrams in the Supplemental data are based. Thomas Hadlari, Danielle Thomson and Claudia Schröder-Adams collected samples from the Hume River section, Thomas Hadlari collected samples from the Imperial River section, and Jennifer Galloway collected samples from the Glacier Fiord section. We are grateful to all these individuals. The publication of this paper was entirely financed by the British Geological Survey and the Geological Survey of Canada (Natural Resources Canada). SELW and JBR publish with the permission of the Executive Director, British Geological Survey (NERC). This is NRCan Contribution number 20180002.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Notes on contributors

Robert A. Fensome

ROBERT A. FENSOME works for the Geological Survey of Canada (GSC) in their Atlantic Division at the Bedford Institute of Oceanography, Dartmouth, Nova Scotia. During his 35-year tenure at the GSC, Rob has focussed mainly on the Mesozoic and Cenozoic dinoflagellate cysts from off Canada’s east coast, but is now venturing into projects involving assemblages from north-western and Arctic Canada. While biostratigraphical and palaeoenvironmental studies justify his paycheque, he also has a strong interest in dinoflagellate evolution.

Graham L. Williams

GRAHAM L. WILLIAMS’s main field of interest is the Mesozoic–Cenozoic palynostratigraphy of offshore eastern Canada, including the Arctic. His long-standing interest in dinoflagellate cysts has led to studies of these palynomorphs in both hemispheres, including the deep ocean basins. After 7 years at Pan American Petroleum Corporation (for anyone who remembers that far back), Graham joined the Geological Survey of Canada in 1971. He has spent the past 48 years at the Bedford Institute of Oceanography in Nova Scotia, where he has the good fortune to work with some outstanding colleagues.

James B. Riding

JAMES B. RIDING is a geologist/palynologist with the British Geological Survey (BGS) in Nottingham, UK. He undertook the MSc in palynology at the University of Sheffield, and several years later Jim was awarded a PhD by the same institution. During 2004, Jim gained a DSc from the University of Leicester, where he completed his bachelor’s degree in geology. His interests include the Mesozoic–Cenozoic palynology of the world, palaeoenvironmental palynology, palynomorph floral provinces, forensic palynology, preparation techniques, the history of palynology, and the morphology, systematics and taxonomy of dinoflagellate cysts. Jim is a past President of AASP – The Palynological Society, and became Managing Editor of Palynology in 2004.