Present evidence indicates that the Pacific Ocean is the remnant of a larger, pre-Mesozoic ocean basin, since restricted by drifting continents. Similar Paleozoic

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faunas were distributed widely throughout this basin and interconnecting seaways. Zoogeographic distinctions were few, and seem attributable chiefly to isolation in restricted seaways rather than by oceanographic dissimilarities within the basin. Currents in the basin were slow and water mass distinctions poorly developed. As the assembled continents drifted apart in the early middle Mesozoic, faunal endemism increased in response to the development of different water masses through increased circulation and a restricted basin. Since the late Mesozoic, water mass distributions and current patterns have been analogous to modern ones although not as sharply defined. By the beginning of the Tertiary the configuration, but not the size of the basin, was established. This basin configurati n, together with an increasing climatic gradient, intensified oceanic and atmospheric circulation, upwelling, and biotic provinciality and speciation. Imposed on this trend were periods of climatic amelioration (early Paleocene and Oligocene) that allowed planktonic biotas to disperse widely, suggesting decreased circulation and weakened water mass boundaries. Upwelling intensity and primary production decreased, the calcium-carbonate compensation depth increased, and extinction occurred among pelagic organisms as habitats and nutrient supplies to which they were adapted disappeared. Provincialism of plankton and shelf benthos increased during the later Tertiary as circulation intensified in response to cooling high-latitude marine climates.

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