Late Ordovician and early Silurian strophomenid brachiopods from Anticosti Island, Quebec

Thirty-three species of strophomenid brachiopods, seven of them new, belonging to 23 genera are used to establish six biozones on Anticosti Island, two in the Vaureal Formation, one in the Ellis Bay Formation, a barren zone through much of the Becscie Formation, a biozone encompassing the uppermost Becscie Formation, Merrimack and Gun River formations, and one in each of the Jupiter and Chicotte formations. These faunas fall into two biogeographic assemblages. Species present in the Vaureal Formation are part of a high-diversity fauna endemic to North America. Species in the Ellis Bay to Chicotte formations are found only on Anticosti, or have Baltic affinities, and are part of a fauna with geographically widespread genera. Maximum faunal replacement occurred at the base of the Ellis Bay Formation, and not at the Ordovician-Silurian boundary.;Fibrous microstructure occurs in the plectambonitids, laminar microstructure in all other strophomenids. Granular calcite occurs in coarse taleolae, wedge-shaped socket plates, muscle scars, and stropheodontid denticles. Shell microstructure varies with other morphological features. Those species with coarse, blocky calcite, taleolate pseudopunctae always have wedge-shaped socket plates. Those species lacking coarse pseudopunctae, but with either fine taleolae, or lacking pseudopunctae altogether, always have curved, concave-up socket plates. It is herein proposed that those groups with wedge-shaped socket ridges (Rafinesquinidae, Leptaenidae, chonetids, and the stropheodontids) had a single common ancestor, and that there was a common ancestor for the Strophomenidae and Orthotetoidea. The fibrous shelled plectambonitids form a group to themselves, with no clear link to other strophomenids.;Muscle attachment pads in the delthyrial cavity correspond to the positions of neither the adductor nor diductor muscle scars; pedicle adjustor muscles in modern brachiopods occupy this position. The circular gap between the median fold of the pseudodeltidium and groove on chilidium is proposed as the point of emergence of the pedicle. The tiny foramen, commonly sealed early in growth, is suggested to be part of a water-intake system used to open the valves, active early in ontogeny before the growth of the cardinal process. Once the cardinal process appeared, the foramen was sealed.