Enhancing dinoflagellate cyst classification through combined LM and SEM analysis: A reassessment of Spiniferites magnificus and Fibradinium annetorpense

Scanning Electron Microscopy (SEM) is underutilized in the taxonomic study of fossil dinoflagellate cysts, which are predominantly described through Light Microscopy (LM). However, SEM can complement LM by revealing detailed morphological features, such as ornamentation and perforations, that are often obscured under LM alone. This study applies the “single-grain method,” combining LM and SEM, to refine the taxonomic descriptions of Spiniferites magnificus and Fibradinium annetorpense. SEM observations revealed previously undocumented features in S. magnificus, including variable intergonal processes, perforated sutural crests, and robust trifurcate gonal processes. These findings refine its taxonomy and suggest environmental influences, such as salinity variations, on its morphological variability. Similarly, SEM analyses of F. annetorpense allowed the determination of the cyst tabulation and clarified the previous described fibrous outer wall structure as the result of continuous ridges and indentations across the crests formed at the edge of each paraplate. This has also highlighted the presence of nano sized indentations forming rings around the paraplates. Both species hold significant biostratigraphic value. Spiniferites magnificus serves as a key marker for the Late Danian in the North Sea Basin, with its extinction and inception coinciding with critical regional biozones. Fibradinium annetorpense has a wider stratigraphic range with reported occurrences from the Late Cretaceous to the Thanetian, but the specimens analyzed in this study are from upper Danian sediments collected in western Greenland.