Pre-Caradoc graptolite faunas of North America and Europe are believed to indicate a significant divergence into 2 major facies associations: a "Pacific" fauna which is characteristic for Australia and most of North America, and an "Atlantic-Baltic" fauna which is documented in British and Scandinavian sequences, and which has been reported recently from Newfoundland.

Pelagic graptolites have not received much attention for their potential as facies indicators, but studies of Appalachian and European graptolite occurrences have demonstrated that two contrasting faunal developments existed during the Early Ordovician with both facies occurring on both sides of the Atlantic.

North American circumcratonic pelagic seas apparently provided a prevalent west-to-east current pattern which is indicated by periodic waves of immigrant Pacific graptolites in the northern Appalachians of Quebec and western Newfoundland, as well as in western Ireland and the Atlantic Norwegian Caledonides. This facies is in contrast with penecontemporaneous graptolite congregations of epicratonic black shales in the Oslo region, southern Sweden, Wales, eastern Ireland, and northernmost Newfoundland.

The juxtaposition of both facies in both North America and Europe suggests the existence of a continuous physical barrier during part of the Early Ordovician. The axis of this barrier extended from Newfoundland to central Norway. Only a pre-drift continental plate margin association of North America and Europe makes this assumption tenable.

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