GASTROPODS IN THE ORDOVICIAN AND SILURIAN SUCCESSIONS ON ANTICOSTI ISLAND, EASTERN CANADA

Anticosti Island is located in the Gulf of St. Lawrence, Quebec, in eastern Canada. Relatively undisturbed Ordovician and Silurian sediments are preserved, with surface exposures ranging from the Ashgill (Rawtheyan) to the Llandovery (Telychian). Depositionally, they represent a shallow marine carbonate environment where Anticosti lies roughly parallel to the ancient shore line.

Gastropods are an important and prominent faunal component in the Ordovician and Silurian successions, with 58 species described. The current study, which is aimed at revising systematics, biostratigraphy, and biogeography of the gastropod fauna, has recognized several genera and species new to the island. The study is based on several large museum collections, but primarily on the unique material assembled by professor Paul Copper, Laurentian University, Canada.

Gastropods are found throughout the succession but their spatial distribution and ecological biofacies have yet to be mapped. Gastropod biogeography of Anticosti largely follows a pattern similar to that of trilobites and brachiopods. In the Ordovician Ellis Bay Formation especially, strong affinity with faunas from Baltica and eastern and marginal Laurentia is evident (for instance Tritonophon, Phragmolites, Pterotheca, Brachytomaria, and Arjamannia). Rare specimens of Pilina occur in the La Framboise reefs, a genus otherwise found sparsely in eastern Laurentia and more commonly in Baltica during the late Ordovician; Silurian forms of the genus are missing in Anticosti.

The Silurian fauna comprises widespread genera like Sphenosphaera and Salpingostoma, while a Phanerotrema species in the Jupiter Formation may be conspecific with forms found in Baltica. However, there is also a conspicuous near absence of otherwise common and widespread Silurian taxa such as Oriostoma and Euomphalopterus, which on Anticosti are only rarely seen in the reefal build-up of the Chicotte Formation. Similar patterns in trilobite distributions have been discussed as a function the pervading biofacies at deposition.