Oceanic dispersal is a type of biological dispersal that occurs when organisms transfer from one land mass to another by way of a sea crossing. Often this occurs via large mats of floating vegetation, such as are sometimes seen floating down major rivers in the tropics and washing out to sea, occasionally with animals trapped on them. Dispersal via such a raft is sometimes referred to as a "rafting event."
Rafting has played an important role in the colonization of isolated land masses, such as Madagascar, which has been isolated for ~120 million years (Ma), and South America, which was isolated for much of the Cenozoic. Both land masses, for example, appear to have received their primates by this mechanism. According to genetic evidence, the common ancestor of the lemurs of Madagascar appears to have crossed the Mozambique Channel by rafting between 50 and 60 Ma ago. Likewise, the New World monkeys are thought to have originated in Africa and rafted to South America by the Oligocene, when the continents were much closer than they are today. Madagascar also appears to have received its tenrecs (25–42 Ma ago), nesomyid rodents (20–24 Ma ago) and euplerid carnivorans (19–26 Ma ago) by this route and South America its caviomorph rodents (over 30 Ma ago). Simian primates (ancestral to monkeys) and hystricognath rodents (ancestral to caviomorphs) are believed to have similarly dispersed from Asia to Africa about 40 Ma ago.
Among reptiles, several iguanid species in the South Pacific have been hypothesized to be descended from iguanas that rafted 10,000 kilometres (6,200 mi) from Central or South America (an alternative theory involves dispersal of a putative now-extinct iguana lineage from Australia or Asia). Similarly, skinks of the related genera Mabuya and Trachylepis apparently both floated across the Atlantic from Africa to South America and Fernando de Noronha, respectively, during the last 9 Ma. Skinks from the same group have also rafted from Africa to the Cape Verde islands, Madagascar, the Seychelles, the Comoros and Socotra. (Among lizards, skinks and geckoes seem especially capable of surviving long transoceanic journeys.)
Colonization of groups of islands can occur by an iterative rafting process sometimes called island hopping. Such a process appears to have played a role, for example, in the colonization of the Caribbean by mammals of South American origin (including caviomorphs and monkeys).
However, oceanic dispersal of terrestrial species may not always take the form of rafting; in some cases, swimming or simply floating may suffice. Tortoises of the genus Chelonoidis arrived in South America from Africa in the Oligocene; they were probably aided by their ability to float with their heads up, and to survive up to six months without food or fresh water. The dispersal of anthracotheres from Asia to Africa about 40 Ma ago, and the much more recent dispersal of hippos (descendants of anthracotheres) from Africa to Madagascar may have occurred by floating or swimming.
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