for National Geographic News
The asteroid that finished off the dinosaurs 65 million years ago opened up niches for the majority of today's living mammals, according to a new study.
The finding is the latest volley in a long-simmering debate over when and where the direct ancestors of everything from whales to rats to humans first arose.
Such mammals are called placentals, which give birth to live young after a long pregnancy, as opposed to pouched marsupials like kangaroos, which give birth after a short pregnancy, and monotremes like platypuses, which lay eggs.
The new report contradicts a DNA study of living mammals published this March that argued the lineages that led to modern placentals began to arise millions of years before the dinosaur die-off and continued well after.
(Related: "Dino Die-Off Didn't Cause Rise of Mammals, Study Says" [March 28, 2007].)
The new study compares fossils from the past 150 million years with living mammals. Its findings support the view that modern placental lineages first appear around 65 million years ago in the Northern Hemisphere.
"This is related to the demise of the dinosaurs," said John Wible, the curator of mammals at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.
"The dinosaurs go extinct, and these various sorts of niches that were occupied by the dinosaurs all of a sudden become open, and it was into those niches that opportunistic placentals evolved."
Wible is the lead author of the new study published today in the journal Nature.
Wible and colleagues performed their analysis after discovering a well-preserved shrewlike creature among fossils collected in the Gobi desert of Mongolia.
The fossils were discovered in 1997 and stored at the American Museum of Natural History in New York City. In 2003 Wible noted the creature's unique teeth and realized it represented a new species.
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