WARM weather may have pushed the landlubber ancestors of some marine animals back to the sea.
Vertebrates ventured from water onto land only once, about 365 million years ago. But since then some 31 groups have gone back the other way, including prehistoric plesiosaurs and present-day whales and dolphins.
Fossils and estimates of past temperatures show that 26 of the groups took the plunge when the world was warmer than today, says at the University of California, Davis.
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Warm water would help cold-blooded animals to remain active as they adapted to the oceans, he says. This would be less of a problem for warm-blooded mammals and birds, but even they were more likely to take to the water during warm spells, Motani told the meeting in Las Vegas, Nevada, last week.