Throughout the Ordovician, 2 well-distinguished provinces were delineated by distribution of conodonts in the northern hemisphere. One, the North Atlantic province, includes all of northwest Europe, the British Isles, and a tract in the eastern Appalachians that stretches from Newfoundland on the north to Georgia and Alabama on the south. A second, the North American Mid-Continent province, embraces all of interior North America, the western belts of the Appalachians, and at least part of the Canadian Arctic Archipelago. Although there was limited and episodic exchange between North Atlantic and North American Mid-Continent conodont faunas, and vicarism is evident between some elements, the two were strikingly different and apparently largely unrelated.

Limited information suggests that Mid-Continent faunas were also characteristic of at least parts of the Siberian platform, and elements of these faunas are known also from the Ordovician of New Zealand and Australia. Paleogeographic, paleotectonic, and paleomagnetic considerations suggest that the Mid-Continent fauna developed at low latitudes, perhaps astride the Ordovician equator, and that the North Atlantic fauna was characteristic of higher latitudes. This suggestion is reinforced by the presence of mixed or modified North Atlantic and Mid-Continent faunas in rocks that accumulated at relatively more deeply submerged sites in the Mid-Continent or Cordilleran areas. Water temperature was probably the most important factor in defining boundaries between the 2 recognizable provinci l faunas.

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