Ediacaran fossils from the Sekwi Brook area, Mackenzie Mountains, northwestern Canada

Ediacaran body fossils and trace fossils occur sporadically throughout more than one kilometre of strata in the upper part of the Windermere Supergroup in the western Northwest Territories of Canada. The oldest fossiliferous unit, the Sheepbed Formation, contains body fossils (Beltanella gilesi, Charniodiscus? sp., Cyclomedusa plana, Cyclomedusa sp., Eoporpita sp., Kullingia sp. and Medusinites asteroides) and very rare trace fossils (Planolites montanus). The Blueflower Formation contains rare body fossils (Chamiodiscus? sp., Ediacaria sp., Inkrylovia sp., Pteridinium sp. and Sekwia excentrica) and abundant trace fossils (Aulichnites ichnosp., Helminthoida ichnosp., Helminthoidichnites tenuis, Helminthopsis abeli, Helminthopsis irregularis, Helminthopsis? ichnosp., Lockeia ichnosp., Neonereites ichnosp., Palaeophycus tubularis, Planolites montanus, Torrowangea rosei and a knotted burrow). Overlying dolostones and thick-bedded sandstones of the Risky Formation contain only simple trace fossils (Palaeophycus tubularis, Planolites montanus). The Sekwi Brook biota lived in a deep-water, basin slope setting below storm wave-base. Most of the body fossils probably represent benthic polypoid and frond-like organisms. The body fossil assemblage is broadly similar to that described from correlative shallow shelf deposits in the Wernecke Mountains, Flinders Ranges and Russian Platform. The trace fossil assemblage is dominated by simple and irregularly meandering burrows, but contains some patterned meanders typical of the Nereites ichnofacies. The occurrence of this relatively diverse, mainly deep-water assemblage of benthic body fossils and infaunal burrows indicates that the initial radiation of metazoans extended to the deep sea, and that some aspects of Phanerozoic-style marine ecosystems were initiated during the earliest stages of metazoan evolution