Progressive faunal migration across the Iapetus Ocean

Abstract

DURING the Lower Palaeozoic, there was a gradual increase in the similarity of the faunas between North America to the west of the Appalachians, western Newfoundland, northwestern Ireland and Scotland on the one hand, and coastal New England, southern New Brunswick, Nova Scotia, eastern Newfoundland, England and northern Europe on the other hand. The best explanation for this is that the lapetus (or Proto-Atlantic) Ocean was wide enough to separate two faunal provinces in Cambrian times, and that there was progressive migration of the more mobile components of the faunas as the old ocean closed (Fig. 1). The pelagic animals crossed first, followed later by animals (trilo-bites and brachiopods) with pelagic larval stages; but animals without a pelagic larval stage (such as benthic ostracods) were not able to cross until the ocean had closed at one point, though not necessarily everywhere along its length. Finally, faunas limited to fresh water or brackish water (like many Devonian fish) did not cross until there were non-marine connections between the continents on either side of the closing ocean.