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Environment

A Bridge to Madagascar

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There are no chimps, lions, or antelopes on Madagascar. But there are lemurs, hedgehoglike tenrecs, and the five-foot-long feline-looking fossa, found nowhere else in the world. The island’s isolation accounts for its peculiar fauna, but the curious thing is that Madagascar has been 250 miles from Africa for 120 million years--before the ancestors of lemurs, tenrecs, and other Madagascar denizens had even evolved. So how did these mammals get to the island? The best explanation biologists have offered is that a few fearless animals floated over from East Africa on rafts of vegetation. But Robert McCall, a doctoral student in zoology at Oxford, is skeptical. Madagascar is almost like a snapshot of evolution, taken 30 or 40 million years ago, he says. If we had all these rafts then, why don’t we see any rafts in the last 20 million years?

McCall turned to geology, his second love, for an explanation. He dug up a number of little-known studies on the geology of the Mozambique Channel, which divides Madagascar from Africa. French geologists had found some interesting rock in core samples from an ancient fault on the channel floor. Below layers of typical seafloor sedimentary rock was a layer of limestone, laid down between 45 and 26 million years ago, that showed distinct signs of rain and wind erosion, meaning the rock was once dry land.

McCall believes a land bridge once spanned the Mozambique Channel, a bridge created and then destroyed by the shifting of continents. When Madagascar broke off from Africa 130 million years ago, it started moving south, plowing a large fault down the middle of the Mozambique Channel--a weakness that would later be exploited when India banged into Asia. That 43-million-year-old collision sent a geologic shock wave back across the Indian Ocean.

Madagascar would have been squashed toward Africa by the shock, says McCall. And that squashing forced up the floor of the Mozambique Channel, creating a land bridge at just the right time for all these lemurs and everything to cross. Twenty million years later, the buckling and fracturing that created East Africa’s Rift Valley pulled on the land between Mozambique and Madagascar, stretching it until the bridge vanished beneath the sea.

McCall isn’t sure how far the bridge extended. The only way to tell would be to bring up more rocks from the bottom of the two-mile-deep channel. I’d love to get out there with a bloody great big drill, says McCall. If the animals couldn’t walk all the way across, he says, at least they wouldn’t have had to swim quite as far.

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