Carnegie Museum of Natural History The new mammal, Maelestes gobiensis, was discovered in July 1997. The lower photo includes a paper clip to give an idea of its size. Click photo for larger image.

An early mammal fossil discovered in Mongolia lends support to the theory that placental mammals emerged and began thriving after dinosaurs died out, according to a paper co-authored by a researcher at the Carnegie Museum of Natural History in Pittsburgh.

The new mammal, Maelestes gobiensis, was discovered in July 1997 in the Gobi Desert of Mongolia during a joint expedition of the Mongolian Academy of Sciences and American Museum of Natural History.

Carnegie curator of mammals John Wible studied the fossil and helped write an article for tomorrow's edition of the British journal Nature.

Dr. Wible said the specimen supports the traditional view that placental animals, which include humans, originated after dinosaurs became extinct.

Of the 5,416 species of living mammals, 5,080 are placentals. The remainder are marsupials -- pouched mammals -- and monotremes -- egg-laying mammals. According to a news release from the Carnegie, "controversy has surrounded the idea of when and where placental mammals first made their appearance on Earth." Estimates for the time frame range from as early as 145 million years ago to as recent as 65 million years ago.

"Our research gives credence and weight to the traditional paleontological view of placental mammals appearing 65 million years ago when the dinosaurs died off," said Dr. Wible. "When dinosaurs became extinct, ecological niches emerged that gave modern placental mammals opportunities to thrive and diversify."

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More details in tomorrow's Pittsburgh Post-Gazette.