Re-evaluation of the status of the dinoflagellate cyst genus Cleistosphaeridium

Although the typically Cenozoic dinoflagellate cyst genus Cleistosphaeridiu m resembles the Jurassic genus Systematophora in its epical archeopyle, sko lochorate form and the presence of penitabular process complexes, other mor phological features indicate that the two genera are not closely related. F or example, pick-like processes endings (termed "dolabrate" herein) are cha racteristic of species of Cleistosphaeridium, but not of Systematophora. Th e unrelated nature of the two genera is supported by their very different s tratigraphic ranges. Thus, Cleistosphaeridium is considered not to be a tax onomic synonym of Systematophora. Its type material is re-examined, the dia gnoses of Cleistosphaeridium and Cleistosphaeridium diversispinosum are eme nded, and three new combinations are proposed: Cleistosphaeridium placacant hum. Cleistosphaeridium ancyreum and Cleistosphaeridium polypetellum A critical review of the biostratigraphic occurrence of the four species of Cleistosphaeridium yields the following confirmed ranges: Cleistosphaeridi um diversispinosum,a - Early Eocene to Early Oligocene; Cleistosphaeridium placacanthum - Mid Eocene to Late Miocene with most records in the Oligocen e to Early Miocene; Cleistosphaeridium ancyreum - Early Eocene to Mid Mioce ne; Cleistosphaeridium polypetellum - Early to Mid Eocene. There appear to he general morphological tendencies from more to less membranous processes in the Eocene and, throughout the range of the genus, increasing developmen t of basal ridges connecting the processes within a complex.