The dinoflagellate cysts Thalassiphora subreticulata n.sp. and Thalassiphora balcanica: their taxonomy, ontogenetic variation and evolution

Abstract

Thalassiphora and other large ‘winged’ dinoflagellate cysts common in Oligocene–Pliocene stratified epicontinental seas display morphological variation greater than the plasticity of extant taxa, thereby raising questions about causes. This variation has been attributed either to directed ontogeny in response to salinity or oxygen gradients or to evolutionary development in response to special environmental conditions. Some authors have grouped certain taxa that mark the closing phases of European Paratethyan basins into an intergradational plexus including species of Thalassiphora, Galeacysta, Nematosphaeropsis and cruciform Spiniferites. Spiniferites (previously Thalassiphora) balcanicus and Galeacysta etrusca were considered end members of this plexus, despite large differences in morphology. We re-evaluate interpretations of the plexus through comparison primarily with a new north-western Atlantic Eocene species Thalassiphora subreticulata and new Croatian material of Thalassiphora balcanica, and we comment on differences from other Thalassiphora species. The large Eocene species Thalassiphora subreticulata (up to 148 µm maximum dimension) is camocavate, and has a coarsely reticulo-fibrous, irregularly perforate periphragm forming a shallow, bowl-shaped structure, as in Thalassiphora pelagica. Electron microscopy shows the perforations are crossed by fibrils in accord with a proposed ‘stretched net’ model of periphragm development. The smaller Late Miocene Paratethyan species Thalassiphora balcanica (maximum dimension to 115 µm) is also camocavate, with a similar fibrous periphragm which encloses about half the ventral surface and has smooth-edged and open perforations. Scanning electron microscope images show this species lacks the branched spinous processes used to justify its transfer from Thalassiphora to Spiniferites by Sütő-Szentai. In both Thalassiphora species, morphological variations do not support either the benthic–planktonic stage ontogenetic model or the oxidation-state model previously proposed for Thalassiphora pelagica. Among 30 species currently assigned to Thalassiphora, no correlation was found between cyst size and age. However, the range of morphology in this genus points to the need for taxonomic re-assessment, which might help reveal evolutionary trends.

Keywords:

• dinoflagellate cyst morphology
• Paratethys
• Pannonian
• palaeosalinity
• species complex

Acknowledgements

We gratefully acknowledge the help of Viktória Baranyi (University of Oslo) and Mária Sütő-Szentai for providing literature sources (including some translations from Hungarian) and light microscope images of Spiniferites balcanicus. We also thank Haifang Yu for the Chinese–English translation of He Chenquan’s descriptions of Thalassiphora taxa. We thank Lori Campbell and Owen Brown, Geological Survey of Canada Atlantic (GSCA), and Guang Yang (Dalhousie University Earth Sciences) for help in processing the sample of Thalassiphora balcanica used for SEM study. Bill McMillan (GSCA) is thanked for drafting figures 1 to 3, and Graham Williams (GSCA) kindly reviewed a first draft of the paper. The careful reviews of Kasia Śliwińska (GEUS) and Przemysław Gedl (Polish Academy Sciences) were also very helpful. This paper is ESS Contribution no. 20180742 of NRCan Geological Survey of Canada.

Additional information

Notes on contributors

Peta J. Mudie

PETA J. MUDIE has studied the Pliocene–Recent palynology of Mediterranean seas, including the Arctic Ocean, first as a post-doctoral student at Dalhousie University, then as a research scientist with the Geological Survey of Canada, combined with joint adjunct appointments at Memorial University of Newfoundland and Dalhousie University, for 35 years. Her field work includes four seasons on an ice island in the Canadian Arctic, work on Arctic icebreakers and ocean drilling ships, and International Geological Correlation Program excursions to Neogene–Pleistocene type sections around the Black Sea.

Robert A. Fensome

ROBERT A. FENSOME is a research scientist with the Geological Survey of Canada in their Atlantic Division at the Bedford Institute of Oceanography, Dartmouth, Nova Scotia. During his 34-year tenure at GSC, Rob has focussed mainly on the Mesozoic–Cenozoic dinoflagellate cysts from offshore eastern Canada, but is now venturing into projects involving assemblages from north-western and Arctic Canada. While biostratigraphic and palaeoenvironmental studies justify his paycheque, he also has a strong interest in dinoflagellate evolution.

André Rochon

ANDRÉ ROCHON has studied marine palynology of Pleistocene, Holocene and modern sediments of the North Atlantic and Arctic Oceans for the last 25 years, and collaborated on studies of the Marmara Sea. He also recently began working on dinoflagellate cyst assemblages in the South Atlantic Ocean along the Argentinian coastline and in Brazilian waters. His expertise includes the use of dinoflagellate cysts as proxy indicators of sea surface conditions and as invasive species in ballast water, cyst–theca relationships, algal culturing and distribution of dinoflagellates in Arctic phytoplankton.

Koraljka Bakrač

KORALJKA BAKRAČ is a research scientist with the Geological Survey of Croatia in their Geological Department. She has focused mainly on the Cenozoic dinoflagellate cysts from the central Paratethys Sea – their stratigraphy and ecology. Most recently she has worked on projects involving Holocene palynomorph assemblages from the eastern Adriatic Sea and coastal lakes. This work includes spores and pollen as well as non-pollen palynomorphs.