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Ziegler's (1965) early Silurian brachiopod community zones and Boucot's (1975) Benthic Assemblages have been instrumental in determining the community structures of early–middle Paleozoic level-bottom communities, but are difficult to apply to reef ecosystems due to the mixing of level-bottom community assemblages in these systems. In this study, Shannon and Simpson diversity levels were calculated for reef-dwelling brachiopod faunas from several North American and one European localities. Reef-dwelling brachiopod diversity collapsed at the end of the Ordovician and did not recover until the mid-Aeronian when modest diversity reef faunas appeared in the higher tropics. Reef-dwelling brachiopods did not achieve high diversity levels until the Telychian when reefs invaded the paleoequatorially located Hudson Bay and Moose River basins. The Attawapiskat Formation (Telychian) contains the earliest known coral-stromatoporoid reef complex with associated high-diversity brachiopod faunas. These faunas were separated into nine community associations through clustering and principal component analyses. These associations can be further grouped into a level-bottom-type and a cryptic-type based on dominant brachiopod taxa, diversity levels, average shell size, and substrate type. Surprisingly, the large species Pentameroides septentrionalis was able to thrive in the relatively flat and open spaces of inter-reef areas and among frame-building corals and stromatoporoids within reef cores. The absence of Stricklandia within the Attawapiskat Formation is due to the high water temperature of the equatorial seas which were too warm to be invaded by this cool-water dependent genus. In addition, the presence of deep-water Clorinda and Gypidula in the shallow-water Attawapiskat reefs is explained by the lack of hurricane-grade storms in the equatorial region, which would be comparable to the quiet-water conditions below storm wave base in higher latitudes. [ABSTRACT FROM AUTHOR]

Eocoelia akimiskii sp. nov. from the Lower Silurian (upper Telychian) Attawapiskat Formation of Akimiski Island, James Bay, Nunavut, is the first and oldest Eocoelia to be described from the Hudson Bay Basin, one of three largest inland basins of North America. The new species lacks dental plates, dental cavities, and marginal deflection or lip, which indicates a post-Aeronian form of the well-known Eocoelia lineage. In rib numbers, Eocoelia akimiskii falls between E. curtisi and E. sulcata, being closer to E. sulcata. In rib strength, however, the new species is more closely allied to E. curtisi. Although Eocoelia is currently assigned to the Rhynchonellida because of its lack of spiralia, several features (particularly the lack of a septalium, the presence of a unique notothyrial platform and cardinal process, and dense, free-hanging fibrous growth frills) of the genus and other leptocoeliids are distinctly atypical of the rhynchonellides. The new species occurs in an inter-reef, shelly packstone facies within the Attawapiskat Formation, which is characterized by coral-stromatoporoid reefs with abundant, diverse, reef-dwelling brachiopods and other shelly organisms. The close association of Eocoelia akimiskii with the Attawapiskat reefs supports a shallow subtidal (BA2) setting generally assigned to the Eocoelia Community. The reefs themselves, however, host an extremely abundant brachiopod fauna dominated by Pentameroides, Trimerella, Septatrypa, and Gypidula. Four species of Clorinda are also common elements of the reef-dwelling brachiopods. This demonstrates that the concept of the classic Early Silurian level-bottom brachiopod communities cannot be directly applied to reef-dwelling brachiopod communities. [ABSTRACT FROM AUTHOR]