The Taxonomic, Biogeographical, Palaeogeographical and Palaeoecological Significance of the Late Ordovician Ostracod Fauna of the Ellis Bay Formation, Anticosti Island, Eastern Canada

Ostracods form a significant component of the Late Ordovician fossil fauna of the Ellis Bay Formation, Anticosti Island, Canada. Sixty-two ostracod species are identified, belonging to 36 genera. These include 14 species described as new, and 13 species described in open nomenclature. The ostracod fauna reveals new and important data bearing on the biostratigraphical, palaeogeographical and palaeoecological significance of the Ellis Bay Formation. The stratigraphic distribution of the ostracods permits the recognition of four successive intervals: (A) the Longiscula subcylindrica Biozone, (B) the Eurychilina erugoface Biozone, (C) the Tetradella anticostiensis Biozone and (D) an Interregnum. Stratigraphical intervals A, B and D can be related to Copeland’s (1973) ostracod biozonation for the Ellis Bay Formation. Biozone C is equivalent to three of Copeland’s (1973) ostracod sub-zones. Presence/absence analysis of the ostracod distribution patterns in the Ellis Bay Formation identifies two palaeoecologically discrete assemblages: an open marine assemblage characterises most of the formation (lithostratigraphical members 1 to 6); whilst a lower diversity assemblage typifies the uppermost Member 7. Biogeographically, the Ellis Bay Formation ostracod fauna is typically North American (Laurentian), but also shares affinities with the Late Ordovician ostracod assemblages of palaeocontinental Baltica and Avalonia.