Late Ordovician Orthide and Billingsellide Brachiopods from Anticosti Island, Eastern Canada: Diversity Change through Mass Extinction

The Upper Ordovician (upper Katian to Hirnantian) carbonate succession of Anticosti Island, eastern Canada, contains an exceptionally well-preserved, unusually rich, and diverse orthide brachiopod fauna, with 21 species assigned to 15 genera and 7 families, plus Vellamo diversa (Shaler) of the order Billingsellida. Four species are described as new: Dalmanella antisculpta, Paucicrura fistulosa, Onniella tricapitata, and Mendacella megacircularis. The presence of Hirnantia in the uppermost Ordovician Ellis Bay Formation of Anticosti Island is confirmed for the first time, including two species, Hirnantia sp. nov. A and H. sagittifera in the Prinsta Member and the Laframboise Member, respectively. Multivariate analysis indicates that the brachiopod fauna of the Ellis Bay Formation of Anticosti Island is highly distinct from the pre-Hirnantian epicontinental brachiopod fauna of the North American paleocontinent (Laurentia). Locally abundant orthides in the underlying Vauréal Formation of the island, such as Plaesiomys (with trilobate cardinal process), Dalmanella, and Paucicrura, strongly suggest a Richmondian faunal affinity. A drastic brachiopod faunal turnover at the Ellis Bay – Vauréal formational boundary is marked by the disappearance of large-shelled strophomenides and rhynchonellides, and the abrupt proliferation of a new suite of orthides in the lower Ellis Bay Formation, such as Barbarorthis, Plaesiomys (with bilobate cardinal process), Hebertella, Onniella, together with the pentameride Parastrophinella and the spire-bearers Eospirigerina and Hindella. In the Prinsta Member of middle Ellis Bay Formation, the brachiopods are characterized by such exotic, short-lived, opportunistic taxa as the orthides Gnamptorhynchos and Hirnantia sp. nov. A and the rhynchonellide Orthorhynchyllion prinstanum, which are associated with aulacerid sponge biostromes. In the Lousy Cove and Laframboise members of the upper Ellis Bay Formation, orthide and other brachiopods usually occur as sporadic shell beds of high individual abundance and low species diversity. Nearly monospecific shell pavements of Mendacella are most common, dominated by either unusually large or abnormally small shells. The high diversity of orthides and other brachiopods in the lower Ellis Bay Formation was probably due to a mixture of brachiopods from Laurentia and Baltica. A relatively high diversity of brachiopods in the middle part of the formation was maintained by the invasion of opportunists. A much reduced diversity in the upper Ellis Bay Formation may have been caused by a stressed local environment (e.g., further sea-level drop, partial restriction of the Anticosti Basin, and abnormal salinity).