A revision of the Paleogene dinoflagellate genera Areosphaeridium Eaton 1971 and Eatonicysta Stover and Evitt 1978

摘要

Northern Hemisphere specimens of Areosphaeridium diktyoplokus (Klumpp 1953) Eaton 1971, the type of Areosphaeridium Eaton 1971, and Eatonicysta ursulae (Morgenroth 1966) Stover and Evitt 1968, the type species of Eatonicysta Stover and Evitt 1978, have a gonyaulacalean sexiform tabulation pattern. Other Northern Hemisphere species of Areosphaeridium, such as Areosphaeridium arcuatum Eaton 1971, have a gonyaulacalean partiform tabulation pattern and are more closely akin to Southern Hemisphere specimens of Areosphaeridium diktyoplokus. Accordingly, we emend the diagnosis of Areosphaeridium and Eatonicysta and propose the genus Enneadocysta for those species formerly included in Areosphaeridium but with a partiform tabulation pattern. The genus Cooksonidium is proposed for forms with a sexiform tabulation pattern and both simple intratabular processes and process complexes. The morphologic reinterpretations have resulted in some revisions to the morphogenetic series defined by Eaton (1971). The stratigraphic and geographic ranges of the four genera are distinctive. Areosphaeridium and Eatonicysta have been recorded only from the Eocene of the Northern Hemisphere. Cooksonidium is restricted to the Late Eocene of the Southern Hemisphere. Enneadocysta has a confirmed Mid Eocene to Early Oligocene range and occurs in both hemispheres. We erect the following new species: Eatonicysta sequestra, Enneadocysta deconinckii, Enneadocysta harrisii, Enneadocysta robusta and Enneadocysta partridgei. Eatonicysta ursulae subsp. furensis is raised to the rank of species as Eatonicysta furensis.