The paleogeography and marine paleoclimate of the Arctic Ocean Basin have evolved considerably during the past 100 million years, since the beginning of the Late Cretaceous. The shallow-water marine faunas that are needed to understand much of this history have recently been found, so that paleontologic studies of the Arctic Ocean Basin and its margins are presently in their early stages. The paleogeographic history of the Arctic Ocean may be divided conveniently into four intervals, each characterized by different marine connections to the world ocean. The first interval extended through most of the Mesozoic, when the Arctic Ocean was a northern gulf of the Pacific Ocean and subsequently developed seaway connections to other oceans. The second interval was during the Late Cretaceous and early Paleogene, when the Arctic Ocean was more or less completely isolated geographically from the world ocean. The third interval lasted from the late Paleogene to the middle Pliocene, during which the Arctic Ocean was connected to the Atlantic but not to the Pacific; and the fourth interval, lasting to the present day, was ushered in at about 3.0–3.5 Ma by the opening of the Bering Strait. The marine paleoclimate of the Arctic Ocean during the Late Cretaceous and early Tertiary was remarkably warm and equable until the onset of gradual cooling that culminated in late Neogene and Pleistocene ice cover.