A reef trilobite fauna of five species from the Formosa Limestone Member of the Amherstburg Formation (Lower Devonian, Emsian) of southern Ontario is strongly dominated by the proetids Crassiproetus and Mannopyge; it also includes the brachymetopid Mystrocephala, the lichid Acanthopyge, and the aulacopleurid Harpidella. This is the first fully documented trilobite fauna from North American Devonian reefs. The proetid-dominated Formosa reef fauna from the Appalachian Province is more similar to Carboniferous reef trilobite faunas than it is to other Devonian reef trilobite faunas from the Old World Province in Europe, Africa, and Australia that are dominated by scutelluids. Crassiproetus Stumm is assigned to the Proetinae; Mannopyge n. gen. (type species, M. halli (Stumm)), to the Warburgellinae.

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