REEF-DWELLING GYPIDULOID BRACHIOPODS IN THE LOWER SILURIAN ATTAWAPISKAT FORMATION, HUDSON BAY REGION

Abstract

Gypidula was a common pentameride brachiopod in the Silurian and Devonian periods and its oldest-known form, Gypidula akimiskiformis new species, occurs as a common component of a distinct Gypidula Association of the rich and diverse reef-dwelling brachiopod fauna in the Lower Silurian (upper Telychian) Attawapiskat Formation of the Hudson Bay Basin. This pioneer species has a high degree of morphological plasticity, with several infraspecific variations significant for the study of gypiduloid taxonomy and early evolution: 1) the sparsely costate shells may have strong to faint medial ribs, with some shells being quasi-smooth and 2) the inner hinge plates are commonly basomedially inclined toward the dorsal valve floor (not united to form a cruralium), but may also be subparallel or slightly basolaterally divergent toward the valve floor (similar to those in Clorinda). This implies that the basomedially inclined inner hinge plates were not a stable character at the initial stage of evolution of Gypidula during the late Telychian. Another gypiduloid from the Attawapiskat Formation, Erilevigatella euthylomatanew genus and species, is characterized by a shell lacking fold or sulcus but having Clorinda-type, basolaterally divergent inner hinge plates and double-flanged crura. Compared to Levigatella, which has been regarded as an intermediate between Clorinda and Gypidula because of its Gypidula-type dorsal sulcus and Clorinda-type inner hinge plates, the rectimarginate shells of Erilevigatella make the distinction between Clorinda and Gypidula even more blurred at the early stage of their evolution. features of the Attawapiskat gypiduloids and to explore their implications for the classification, early evolution, and paleo- ecology of the superfamily Gypiduloidea. STRATIGRAPHIC SETTING From the Late Ordovician to Late Devonian, the Hudson Bay Basin was a large intracratonic basin, with a predominantly car- bonate depositional environment over an area of nearly one mil- lion square kilometers. It is separated from the Michigan Basin to the south by the Fraserdale Arch and from the Williston Basin to the southwest by the Severn Arch. The Hudson Bay Basin (sensu lato) comprises the basin proper and a considerably smaller Moose River Basin at its southern end, with the two sub-basins separated by the Cape Henrietta Maria Arch, which is marked today by the Precambrian inliers in the Sutton Ridges area (Fig. 1). The Lower Silurian succession of the Hudson Bay region con- sists of three formations of carbonates, with the Severn River, Ekwan River, and Attawapiskat Formations spanning the upper Rhuddanian-uppermost Telychian (possibly extending to the low- ermost Wenlock) interval (Fig. 2).