FOSSIL Lagerstatten comparable to that of the Burgess Shale potentiall
y provide the detail necessary to resolve and assess the so-called Cam
brian Explosion of multicellular life(1,2); distributional and taphono
mic biases, however, often limit the generalizations that can be drawn
. Here I report widespread occurrences of Burgess Shale-type fossils i
n shallow-shelf sediments of the Lower Cambrian Mount Cap Formation, D
istrict of Mackenzie, Northwest Territories, Canada. These borehole as
semblages significantly expand the known geographical and ecological r
ange of such fossil biotas and document the early appearance of both w
iwaxiid polychaetes and filter-feeding crustaceans. The ability of thi
s latter group to exploit microplanktic primary productivity marks a f
undamental shift in trophic structuring and may distinguish Phanerozoi
c from pre-Phanerozoic ecosystems.