| Summary: | Cambrian strata of the eastern Mackenzie Mountains, Northwest Territories, Canada, were deposited in the semi-enclosed, Colville Basin along the eastern flank of the Mackenzie Arch. The Mount Clark Formation is composed of nearshore sandstone and is overlain by deeper water siltstones, mudstones, and carbonates of the Mount Cap Formation. The contact between these formations is interpreted as recording a flooding event. Trilobite biostratigraphy indicates the presence of the traditional Bonnia–Olenellus through Glossopleura walcotti zones (Cambrian Series 2, Stage 4–Miaolingian, Wuliuan) and the diachronous nature of the flooding surface, spanning the late Dyeran–early Delamaran boundary. Above the Olenellus Zone, the Amecephalus arrojosensis–Eokochaspis nodosa Zone, the new Zacanthoidid n. gen. 1, n. sp. 1 Zone, the new Albertelloides mischi Zone, and Glossopleura walcotti Zone are recognized. Trilobite faunas from the Albertelloides mischi and Glossopleura walcotti zones are dominated by zacanthoidids and dolichometopids in contrast to other areas in Laurentia. They have a lower diversity of ptychoparioids and oryctocephalids and lack agnostoids, eodiscoids, dorypygids, and ogygopsidids. This suggests that zacanthoidids and dolichometopids were able to tolerate conditions that were unfavourable to the other groups, probably related to the semi-restricted nature of the basin. Four new, endemic species exhibit characteristics which are considered paedomorphic. This developmental process took place in three separate lineages, suggesting that heterochrony was also environmentally controlled. Five new genera and ten new species are described.
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