The narrow neck of land that connects N. America and Mexico with S. America. During the Mesozoic there was an open marine connection between the Atlantic and Pacific Oceans. At the culmination of the Andean–Laramide orogeny (Late Cretaceous) a temporary connection between N. and S. America was formed, which explains the similarity between the floras of California and S. America at that time. Some early mammals also migrated southwards across the isthmus. During the early Cenozoic the land connection was broken but it became re-established in the Pliocene, thereby allowing another 14 mammal families to migrate from north to south. Some families, notably the elephants (see proboscidea), appeared to be unable to migrate southwards, perhaps being constrained by the climatic zones that had to be traversed in such a migration.