Ordovician fossil faunas are characterized by a marked biogeographic differentiation that results in a minimal similarity between most North American faunas and those of major Ordovician areas elsewhere in the world. This provincial distribution of most fossils has led to establishment of different schemes of fossil-based regional stages in, for instance, North America, Baltoscandia, China, and the British Isles.

Because these chrono-stratigraphic units have been largely based on shelly fossils with distributions restricted to a particular region, it has been impossible in most cases to establish a precise international correlation of these regional stages. Furthermore, some general terms, such as the "Middle Ordovician;' have a vastly different stratigraphic scope in different parts of the world causing confusion among stratigraphers and non-stratigraphers alike.

Indeed, in view of the fact that many of today's geology studies are of more than regional nature, there has been an urgent need for an international chronostratigraphic classification.