Abstract The Late Ordovician Epoch was marked by one of the two greatest global sea-level rises and inundations of the North American paleocontinent during the Phanerozoic (last 544 million years), accompanied by rapid diversification of invertebrate faunas in shallow, epicontinental seas. Toward the end of the Late Ordovician, continental glaciation in the southern hemisphere (Gondwana landmass) caused a major sea-level drawdown and marine regression from North America, bringing about one of the five major mass extinction events in life history. The diversity of marine life in the inland seas would be particularly sensitive to global sea-level fluctuations caused by the growth and decay of the Gondwana ice cap. This monograph is part of an ongoing comparative study of the biodiversity changes of the Late Ordovician – Early Silurian brachiopods (the most abundant and diverse group of shelly benthos at that time) in continental-margin basins and inland seas of Canada. Study of the brachiopod faunas helps us understand many aspects (duration, extent, intensity, and timing) of the climatic changes and their effects on marine environments far from the site of the glaciation. The Late Ordovician carbonate deposits now preserved in the Williston Basin contain a rich and diverse benthic shelly fauna that lived in the ancient equatorial epicontinental seas just before the Late Ordovician mass extinction event, and this work deals with the taxonomy, biostratigraphy, paleoecology, and paleobiogeography of the brachiopod fauna. The authors described a total of 16 genera and 22 species and discussed their ancient living environments and faunal provincialism.