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Invertebrate faunas in the shale, siltstone, and sandstone of the Nicolet River and Pontgrave River Formations in the St. Lawrence lowlands, Quebec, represent the northernmost expression of Late Ordovician Appalachian clastic-facies, benthic marine communities. Strata exposed along the axis of the Chambly-Fortierville syncline have bedding, texture, and primary structures characteristic of a regressional sequence, a pattern that is mirrored in well-defined onshore-to-offshore faunal assemblages.
About 50 species dominate the Nicolet-Pontgrave fauna, mainly articulate brachiopods, bellerophontid gastropods, bivalve mollusks, crinoids, and trilobites. There are benthic marine communities, recognized on the basis of species, which consistently occur together. The Dalmanella-Cryptolithus community is widespread and lived on or in muddy silt and mud of the outer infralittoral zone. The community has high faunal diversity, especially nuculoid bivalve mollusks and trilobites, but normally has low density. The Leptaena-Sowerbyella community has patchy distribution. This dominantly strophomenid brachiopod fauna, sporadically associated with diverse praecardioid bivalve mollusk populations, lived on fine silt bottoms of the inner and outer infralittoral zone. The Ambonychia-Modiolopsis community has low faunal diversity but many epifaunal, suspension-feeding, bivalve mollusks, especially pterioids and modiomorphids. The community lived on fine sand and silt of the inner infralittoral zone and has patchy distribution. The Rhynchotrema-Catazyga community, which has very patchy distribution, lived on muddy silt of the inner infralittoral zone, has very low faunal diversity, and contains coquinites of mono-specific character, dominantly rhynchonellid.
The pattern of high-diversity, low-density offshore communities transitional shoreward into low-diversity, high-density communities may be explained most simply by differences in the degree of environmental fluctuations.
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