Palaeozoogeographical provincialism in the Ordovician Period is well portrayed by brachiopod, conodont, graptolite and trilobite faunas. The provinces defined for each taxonomic group show a remarkable degree of correspondence in geographical extent and temporal expression, so much so as to suggest the operation of a common causal mechanism. A convincing body of evidence is available in support of the view that latitudinal variation in surface water temperature was primarily responsible for the provincialism displayed by the epipelagic graptolites. In the partially or exclusively benthic brachiopods, conodonts and trilobites, however, the primary temperature effect is masked, in varying degrees, by secondary factors, which may have included the influence of the seabed environment, geographical separation (isolation) of coeval faunas, and physical barriers to migration.