Trilobites of exhumed reefs of the Late Llandoverian Attawapiskat Formation of northern Ontario occur in accumulations that have undergone extensive taphonomic sorting. Most spectacular are monospecific to low-diversity "pockets" of the large scutelluine Ekwanoscutellum. These accumulations, which cover areas of up to a square meter, are nested stacks of mostly inverted (concave-up) large sclerites, the majority of which are pygidia. They may have formed by passive settling of sclerites in "current-shadows" in the lee of obstacles in the reef system. Excellent preservation of the sclerites, absence of epibionts, and minimal internal sediment with in the "pockets" suggest rapid accumulation. Aggregation of individuals for molting and reproduction may explain the availability of large numbers of Ekwanoscutellum sclerites as raw material for the accumulations. A second, less common assemblage is a more diverse association dominated by the scutelluine Meroperix and the illaenid Stenopareia, and is often associated with abundant calciate brachiopods. Although some sorting has taken place, more variable dorso-ventral orientations and lower sclerite abundances indicate a different origin from the Ekwanoscutellum Association. The data underscore the taphonomic complexity of patchy environments such as Silurian reefs and indicate that extensive sorting of skeletons is a significant source of bias in the paleoecologic analysis of non-framework components of reef biofacies.

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