The present study reviews the chronological distribution of Upper Ordovician benthic marine macroinvertebrates from the most complete Upper Ordovician exposure in the St. Lawrence Lowlands. Pelecypods and brachiopods, associated with lesser numbers of bryozoans and gastropods, are preserved in fine-grained terrigenous clastics that comprise a thick (800 m) and almost continuously exposed section of Lorraine flysch in the Nicolet River Valley, Quebec. I was able to identify 122 species-level taxa within 407 discrete bedding plane samples, and here publish the teilzones for 70 of the most abundant species. These species are preserved within a flysch sequence of the tectonically parautochthonous domain.After the earliest colonization of the basin by a mud-bottom brachiopod and pelecypod fauna, episodic introductions are most often associated with the development of bryozoan thickets. Each of the major disappearances from this portion of the Nicolet Basin followed soon after a sudden introduction of new species into the area; determination of whether or not these introduction–disappearance couplets are cause and effect requires further, more detailed bed-by-bed studies of species replacements.

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