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Whale genealogy gives new insight into evolution: researchers
http://www.ecns.cn/2015/04-15/161856.shtml
Apr 15th 2015, 08:02
New Zealand researchers said they have developed the world's most comprehensive family tree into one of the largest and most unusual animals ever to live.
The evolutionary history of baleen whales went back almost 40 million years and indicated periods of intensive evolution and extinction, University of Otago researchers said Wednesday.
While most other mammals feed on plants or grab a single prey animal at a time, baleen whales use the baleen in their gigantic mouths to gulp and filter an enormous volume of water and food.
Dr Felix Marx and Professor Ewan Fordyce said similar family trees had been constructed before, but theirs was by far the largest and the first to be directly calibrated using many dated fossils.
"We find that the earliest baleen whales underwent an adaptive radiation or sudden 'evolutionary burst'," Fordyce said in a statement.
The early phase of whale evolution coincided with a period of global cooling when the Southern Ocean opened and gave rise to a strong current circling the Antarctic that today provided many of the nutrients sustaining the modern global ocean.
"Rather surprisingly, many of these early whales were quite unlike their modern descendants. Although some had baleen, others had well-developed teeth and actively hunted for much bigger prey than is taken by modern species," said Fordyce.
But, after a few million years, the toothed baleen whales disappeared -- perhaps because of growing competition from other newly evolved toothed marine mammals, such as dolphins and seals -- leaving behind only their filter-feeding cousins, he said.
That extinction occurred between 30 million and 23 million years ago and was about the time that the circum-Antarctic current reached its full strength, providing more nutrients that made filter feeding a more viable option.
They found that filter-feeding whales remained successful and diverse until about 3 million years ago, when the number of lineages suddenly crashed.
"This decline was driven mainly by the disappearance of small species of baleen whale, which left behind only the giants -- ranging from 6 to as much as 30 meters -- that plough the ocean today," Marx said in the statement.
The disappearance of small whales likely resulted from the onset of the ice ages, which altered the distribution of available food, caused shallow water habitats to shift or sometimes disappear, and created a need for long-distance migration between polar feeding grounds and equatorial breeding grounds.
"This behaviour -- long distance-migration -- is still one of the hallmarks of all baleen whales alive today," said Fordyce.
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