By Julie Steenhuysen

CHICAGO, June 20 (Reuters) - The discovery of a primitive, shrew-like mammal fossil in Mongolia has revived the view that its modern mammal cousins arrived just as the dinosaurs made their dramatic exit about 65 million years ago, U.S. researchers said on Wednesday.

Recent studies have placed the arrival of modern mammals at anywhere from 140 million to 80 million years ago, long before an asteroid crashed into Earth and wiped out the dinosaurs.

"The fossil itself is the least interesting part of the story scientifically," said John Wible of the Carnegie Museum of Natural History in Pittsburgh, whose research appears in the journal Nature.

He said the discovery of a new shrew-like mammal in 1997 -- Maelestes gobiensis -- led to an exhaustive analysis of the fossil record that dates the emergence of modern mammals at the end of the Cretaceous period 65 million years ago.

Recent molecular studies have held that modern mammals may have lived long before the dinosaurs died out at the end of the Cretaceous period, which began 145 million years ago and ended with a bang 65 million years ago.

Placental mammals -- like dogs, cats, mice, whales, elephants, horses and humans -- give birth to live young after a long gestational period. Of the 5,416 species of living mammals, 5,080 are placentals.

The rest are marsupials like kangaroos, which nourish their offspring in a pouch, and the very rare monotremes, such as the egg-laying duck-billed platypus.

'OKLAHOMA LAND RUSH'

"We wanted to test whether there were any Cretaceous placentals," Wible said in a telephone interview.

"If the molecular dates are correct, we should be finding things that look like modern placentals in this time period and we are not."

They found that none of these Cretaceous forms of early mammals are related to any living placental mammals. "They are just extinct dead ends," he said.

Wible said his work reinforced the idea that the death of the dinosaurs created an opportunity for explosive growth of modern mammals.

"You've got all of these ecological niches that were occupied by the dinosaurs. They go extinct, and you've got wide open spaces. It's like the Oklahoma land rush," he said.

The analysis all began with the discovery of Maelestes, an unusually complete fossil discovered in the Gobi Desert of Mongolia during a joint expedition of the Mongolian Academy of Sciences and the American Museum of Natural History.

The rodent-like creature -- one of those evolutionary dead-ends -- lived 75 million years ago, about the time of the Velociraptor, Oviraptor and Protoceratops.

"It looks like road kill. It is very well preserved," Wible said.

He and colleagues classified the toothsome creature as a new eutherian mammal, a broader group that includes placentals and their extinct relatives.

"He would have been a voracious little predator," he said, but it was not a modern placental mammal.

"The beauty of this fossil it that it forced us to do the analysis."

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