Swept Away: New Modeling Buoys Raft Theory for Origin of Madagascar’s Mammals

The African island of Madagascar, situated some 430 kilometers off the coast of Mozambique, is famous for its unique fauna, particularly its charismatic primates, the lemurs. But how the lemurs and other land mammals got there has proved an enduring mystery. To that end, new evidence supports a theory that some experts once considered unlikely: namely, that the forerunners of Madagascar’s modern mammals reached the island millions of years ago by drifting from the African mainland across the Mozambique Channel on giant rafts of vegetation ripped from the shore and launched out to sea by violent storms.

Reconstructing ancient dispersal routes is a complex exercise. On Madagascar this puzzle is complicated by the fact that the fossil record of mammals from the past 65 million years is meager. Based on the paltry available clues, some researchers thought the ancestral mammalian stock arrived via a landbridge that later disappeared with the shifting of landmasses. But geologic evidence of such landbridges is weak at best. Moreover, this theory cannot account for why the island’s many endemic terrestrial mammal species represent only four of Africa’s broader mammal groups called orders. And all of Madagascar’s land mammals are relatively small–no elephants, lions or giraffes there. If landbridges existed, critics argued, why did only small mammals belonging to these four orders make the trip over?

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