Evolution of the Early Silurian rhynchonellid brachiopod Fenestrirostra in the Anticosti Basin of Quebec

Abstract

Among the abundant, well-preserved rhynchonellid brachiopods in the Upper Ordovician-Lower Silurian rocks of Anticosti Island, Fenestrirostra Cooper, 1955, one of the earliest camarotoechiid genera to appear in the fossil record, is probably of oligorhynchiid ancestry and seemingly endemic to the Anticosti Basin. Three successive species of Fenestrirostra, found in micritic mudstones and calcareous shales of the middle and upper Merrimack and Lower Gun River Formations, provide an example of gradualistic evolution. Four major morphoseries distinguish the lineage: 1) change in the proportions and size of the shells; 2) increase in the number and concomitant loss of the costae from the anterior halves of the shells; 3) change in the orientation and reduction in the size of the hinge plates and hinge fossettes; and 4) change in the height of the median septum. Biostratigraphically, the Fenestrirostra lineage provides the basis for three interval or range zones that straddle the Rhuddanian-Aeronian interstage boundary in the Anticosti Basin. -from Authors