Ammonoid faunas representative of every major part of Triassic time occur at one place or another in the marine Triassic strata of western and arctic North America. Though some intracontinental provincialism is evident, particularly among Lower and Middle Triassic ammonoid faunas, various local sections where parts of the faunal sequence are preserved—sections in southeastern Idaho, western Nevada, northern California, northeastern Oregon, northeastern British Columbia, southwestern British Columbia, southeastern and southern Alaska, and the Arctic Islands of Canada—can be pieced together in overlapping fashion into a unified sequential framework of biostratigraphic units presented in an annotated zonal chart. At least 35 zonal units of demonstrably different age are now recognized; these units are apportioned among the Griesbachian, Dienerian, Smithian, and Spathian Stages of the Lower Triassic; the Anisian and Ladinian Stages of the Middle Triassic; and the Karnian, Norian and Rhaetian Stages of the Upper Triassic. This zonation based on North American ammonoid faunas for the first time provides an objective and reasonably detailed standard of reference for expressing the age and correlation of the marine Triassic rocks of Canada and the United States.
