Early silurian brachiopods and biostratigraphy of the Hudson Bay lowlands, Manitoba, Ontario, and Quebec

Abstract

Articulate brachiopods of the Llandovery rocks of the Severn River, Ekwan River and Attawapiskat formations of the Hudson Bay Lowlands comprise 35 genera and 50 species, including 18 new species: Severella pinnigera, Gypidula rudiplicativa, Rhytidorhachis guttuliformis, Rostricellula subtilicostata, Microsphaeridiorhynchus hercostomaticum, Hercotrema spissicostatum, Meifodia discoidalis, Atyrypoidea praelingulata, Atrypopsis severnensis, Parmula hemisphaerica, Merista rhombiformis, Whitfieldella pygmaea, Whitfieldella sulcatina, Howellella porcata, Alaskospira fabiformis, Dalejina striata, Eoplectodonta (Eoplectodonta) hudsonensis, and Protochonetes harricanensis. The species of Alaskospira, Atrypoidea and Merista represent unusually early appearances of these genera. The biohermal complexes of the Attawapiskat Formation provided a great diversity of environments. Some species with small shells seem to have occupied specialized and protected niches within the complexes, whereas large, thick shelled species occupied open water environments. Three brachiopod assemblage zones can be recognized. The Virgiana decussata Zone (dominantly pentamerids) is succeeded by the Hercotrema winiskensisþStegerhynchus borealis Zone (dominantly rhynchonellids). Above, the Pentameroides septentrionalisþLissatrypa variabilis Zone has a very varied fauna with abundant pentamerids and spire-bearers. Species diversity increases from 6, to 15, and to 36 species in the three zones. The changes in diversity and in taxonomic composition were probably related to the changes in environments during the Early Silurian transgression from dominantly intertidal and lagoonal to dominantly subtidal and reefal. Correlations with successions on Anticosti and Manitoulin islands indicate that the Severn River Formation is latest Rhuddanian to late Aeronian in age, the Ekwan River Formation is Telychian, and the Attawpiskat Formation is late Telychian. Comparison with the outcrop succession of southern Manitoba indicates correlation of the basal Severn River Formation with the Fisher Branch Formation; the remainder of the Severn River Formation with the Moose Lake or ``Inwood'' (sensu McCabe), Atikameg, and East Arm formations; and the Ekwan River and the Attawapiskat Formation with the Cedar Lake Formation, but with the uppermost Attawapiskat being slightly younger.