HOLARCTIC MAMMALIAN FAUNAS AND CONTINENTAL RELATIONSHIPS DURING THE CENOZOIC
A series of tables lists land mammals common to North America and Eurasia, their probable times of migration by subepochs throughout the Cenozoic, and important groups that did not migrate in given subepochs. Major faunal interchanges occurred in early Eocene, late Eocene, early Oligocene, late Miocene, middle to late Pliocene, and Pleistocene. There was little or no interchange in the middle Eocene and middle to late Oligocene. Each interchange involved migration in both directions, but there was probably more movement from Eurasia to North America than in the opposite direction. All interchanges were selective, and they became increasingly limited from Eocene to Pleistocene. The most important selective influence was probably the relatively cold climate of the land connection. This connection was probably from Siberia to Alaska throughout the Cenozoic and was in almost continuous existence with important interruptions in parts of the Eocene and Oligocene and perhaps shorter interruptions at later times. The measurement of faunal resemblance in general is discussed, and mammalian faunal resemblance between Eurasia and North America charted for the Cenozoic. Changes in resemblance are correlated with five factors: faunal interchange, extinction of autochthones, divergent evolution, local extinction of migrants, and migration from other regions.