Trilobites from the 2,350-foot type section of the Sekwi Formation, Mackenzie Mountains, were collected from 47 horizons and placed in three zones. The oldest is the Fallotaspis Zone (new), recognized in western Canada, California, and Morocco and correlated with the oldest trilobite assemblage in Siberia. Next is the Nevadel/a Zone (new), which can be recognized at various localities in western North America and probably represents the approximate equivalent of the Cal/avia Zone of the Atlantic Fauna! Province. The youngest zone, the Bonnia-0/enel/us Zone, has been defined by Rasetti (1951) and is in concept the same as the Olenellus Zone. For the past 80 years, this zone has been equated generally and incorrectly with the whole of the Lower Cambrian in the Pacific Fauna! Province. The Bonnia- 0/ene/lus Zone is, at least in part, equivalent to the Protolenus Zone of the Atlantic Fauna! Province. Twenty-four genera are present including the following which are new: Bradyfallotaspis, Ho/miella, Kee/easpis, Nehanniaspis, Parafallotaspis, Sekwiaspis, and Variope!ta. F ifty-seven species are described, of which 5 were previously known, 29 are new, and 23 are unnamed.

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In the past an ‘explosion’ in diversity and abundance of small shelly fossils and of trace fossils has served to mark the base of the Cambrian. However, no evidence has been presented to prove that the ‘explosions’ of the two groups were synchronous. We describe small shelly fossils and trace fossils from the same phosphatic limestone beds that indicate that the two events were separate in time. The small shelly fossils are Anabarites trisulcatus, Hyolithellus cf. H. isiticus, Microcornus? sp., Protohertzina anabarica, P. unguliformis, P. sp. A, Pseudorthotheca sp. A, Rushtonia? sp. A, four types of tuberculate plates and one type of reticulate plate. These fossils represent a restricted, ‘pre‐explosion’ fauna and are assigned to the Anabarites‐Circotheca‐Protohertzina Assemblage Zone, an uppermost Precambrian zone in the Meishucun Stage, Yunnan Province, China. A point at the top of this zone has received strong international endorsement for future designation as the base of the Cambrian. Associated with the small shelly fossils are the trace fossils Cruziana sp. A, Cruziana? sp. B, Rusophycus sp. A, Palaeophycus rubdark and arthropod scratch marks. If found in isolation, this trace fossil assemblage would be considered as post‐Precambrian because it includes large, highly organized arthropod traces that are traditionally accepted as occurring above the trace fossil ‘explosion’. We therefore conclude that the trace fossil ‘explosion’ predates the small shelly fossil ‘explosion’. If the proposed location of the base of the Cambrian in Yunnan is accepted, the small shelly fossil ‘explosion’ concept and its relationship to the boundary would not be greatly modified. The trace fossil ‘explosion’, however, would no longer indicate the base of the Cambrian and the ranges of some trace fossils would be extended into the Precambrian.

In 1913 Walcott described well preserved olenellid trilobites from the Mount Robson area that "make up a subfauna in the upper portion of the Lower Cambrian that has not been found elsewhere." Recent field studies indicate the "unique" 61k collection came from a glacial moraine containing a mixture of fossils from the upper Nevadella Zone and lower Bonnia-Olene/lus Zone. The source horizons for most of the 61k taxa were found in nearby sections, permitting an enlarged description of the fossils and a presentation of their ranges. Below the productive 61k source horizons are (in ascending order) Nevadia weeksi Walcott, Nevadia parvoconica sp. nov., and Nevadella mountjoyi sp. nov. First in the source horizons is Nevadella eucharis (Walcott) followed by Nevade/la perjecta (Walcott). Above N. perjecta is a narrow (0.3 m) interval containing the boundary between the Nevadella and Bonnia-Olenellus zones. Found above the boundary interval is Mummaspis occidens (Walcott) gen. nov., followed by Olenellus truemani Walcott, Laudonia bispinata Harrington, Olenellus truncatooculatus sp. nov., and Mummaspis oblisooculatus gen. et sp. nov. Olenellus mura/ensis sp. nov. was not found in outcrop, but is known to coexist with 0. truncatoocu/atus on some 61k slabs. It is presently only a guess that Laudonia amputata sp. nov. came from the productive interval. It was not found in outcrop and is not associated with other taxa on 61k slabs. Gabriellus lanceatus gen. et sp. nov. occurs in the medial Bonnia-0/enellus Zone, well above the main producing horizons. Numerous locations in the North American Cordillera can be correlated with the beds containing the 61k taxa. The abrupt faunal change at the boundary between the Nevadella and Bonnia-Olenellus zones is briefly discusse

Seventeen species of trilobites are described from the Mackenzie Mountains. They belong to nine new species, one established species, and seven unnamed species. One new genus, Ekwipagetia, is described. All of the trilobites are from strata that correlate with the NevadeZZa Zone and are from the Sekwi Formation. At least part of the fossils are from the outer detrital belt, a belt of strata ringing the craton and lying seawards of a middle carbonate and an inner detrital belt. Few of the trilobites are similar to those described by Fritz (1972) from the nearby middle carbonate belt, trilobites that we r e used to establish a Lower Cambrian zonation for the North American Faunal Province. Since this zonation seems to work well in the middle and inner detrital belts, a major alteration of the zonation to include the present fossils is not recommended. Instead, it seems that a new zonati on for the Lower Cambrian fossils in the outer de trital belt might be more useful, but the description of more fossils is necessary before this idea can be either confirmed or rejected.