The distribution of plant and animal species in the world today reflects the interplay between the physical environment, which, for example, restricts polar bears to the high latitudes, and geographical barriers to migration, which, for example, have kept polar bears from invading the Antarctic and keep the marine snakes of the Pacific side of Panama from invading the Caribbean Sea. Such barriers divide the world into bio-geographic provinces. The fossil record enables paleontologists to reconstruct such provinces for the past and to study their history. South America, for example, was joined to Africa during most of the Mesozoic Era, as its Triassic and Jurassic reptile faunas demonstrate. When it broke away, in mid-Cretaceous time, with the opening of the South Atlantic Ocean, the dinosaurs of South America succumbed to the Cretaceous crisis and the mammals took their place as the dominant land animals, forming a very different kind of mammalian fauna that evolved independently of the Eurasian-American fauna. Australia underwent similar changes except that it remained isolated, and its indigenous mammalian fauna (a marsupial one) has evolved to its modern state. South America became linked with North America during the Pliocene Epoch, and the two very different mammalian faunas invaded each other's territories, pitting species against species for existence. Eventually, North America assimilated some South American mammals (ground sloths, armadillos, opossums, and porcupines), but the majority of South American species became extinct.
The fossil record also shows that the Caribbean and eastern Pacific marine faunas were essentially identical in Miocene times, diverging only since the Panamanian land bridge closed in Pliocene time. The question of whether to build a Panamanian sea-level canal has focused attention on what would happen to present faunas if communications were to be reestablished. Would the Pacific sea snakes, for example, invade the Caribbean to the detriment of fisheries there?