FOSSIL Lagerstatten comparable to that of the Burgess Shale potentially provide the detail necessary to resolve and assess the so-called Cambrian Explosion of multicellular life(1,2); distributional and taphonomic biases, however, often limit the generalizations that can be drawn. Here I report widespread occurrences of Burgess Shale-type fossils in shallow-shelf sediments of the Lower Cambrian Mount Cap Formation, District of Mackenzie, Northwest Territories, Canada. These borehole assemblages significantly expand the known geographical and ecological range of such fossil biotas and document the early appearance of both wiwaxiid polychaetes and filter-feeding crustaceans. The ability of this latter group to exploit microplanktic primary productivity marks a fundamental shift in trophic structuring and may distinguish Phanerozoic from pre-Phanerozoic ecosystems.