Mid-Lower to lower Middle Ordovician graptolite faunas of Canada and the United States are basically alike. Failure to recognize the Tetragraptus fruticosus and Didymograptus bifidus Zones in western Canada is attributed to subprovincial differences in the faunas. The faunas of North America, Australia, and China are coprovincial and can be correlated with some assurance. At species level, these faunas differ from those of Britain and northern Europe, making interprovincial correlations difficult.

First appearances of Amplexograptus, Climacograptus, Cryptograptus, dependent Didymograpti, Glossograptus, Glyptograptus, Isograptus, and Trigonograptus are essentially contemporaneous in both provinces. The Didymograptus bifidus Zone of the United States is considerably older than that zone in Britain where it would correlate with the mid-Arenig Didymograptus nitidus Subzone. The equivalent of the British D. bifidus Zone lies wholly within the Paraglossograptus etheridgei Zone of North America; the Lower-Middle Ordovician boundary in the United States should be raised accordingly.

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