120 Million Years Old, Fossil Shows Divergence of Platypus and Anteater
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The platypus tops many people’s oddest mammal list, what with its ducklike bill and beaverlike tail. Its closest relatives, the echidnas, don’t get the press the platypus gets, but they are pretty weird, too, and are the only other monotremes, or egg-laying mammals, around.There is not much of a fossil record of monotremes, so it has never been clear when the platypus and the echidnas (also known as spiny anteaters) diverged. Most estimates using genetic analysis — looking at the rate of mutations — suggest that they split perhaps 20 million to 30 million years ago, although a few stretch that from 80 million years.
Now Timothy Rowe of the University of Texas and colleagues report in The Proceedings of the National Academy of Sciences that the divergence may have occurred long before that. They used X-ray computer tomography to examine a 120-million-year-old fossil, Teinolophus trusleri. It is an ancestral platypus, they say, with some similar morphological features, in particular a canal in the jaw that the researchers say is evidence that Teinolophus had a duckbill.
The researchers conclude that the platypus and echidna branches of monotremes were already distinct at that early date. The findings also suggest that monotremes diversified at a slower rate than the other mammals.
credited to nytimes.com
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