Early closing between oceans The Central American Seaway, which once separated the Panama Arc from South America, may have closed 10 million years earlier than is believed. Montes et al. report that certain minerals of Panamanian provenance began to appear in South America during the Middle Miocene, 15 to 13 million years ago (see the Perspective by Hoorn and Flantua). The presence of the minerals indicates that rivers were flowing from the Panama Arc into the shallow marine basins of northern South America. One interpretation of this finding is that large-scale ocean flow between the Atlantic and Pacific had ended by then. If this is true, then many models of paleo-ocean circulation and biotic exchange between the Americas need to be reconsidered. Science , this issue p. 226 ; see also p. 186