The paper, co–authored by Carnegie Museum of Natural History curator of mammals Dr. John Wible, is the most comprehensive support to date for the traditional paleontological view that placental mammals originated after the Cretaceous–Tertiary (K/T) Boundary, when 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. Controversy has surrounded the idea of when and where placental mammals first made their appearance on Earth. Temporal hypotheses remain as varied as the species themselves ranging from the beginning of the Cretaceous period at 145 million years ago to the very end at 65 million years ago.

The controversy is debated not just among paleontologists who study the fossil record but among molecular systematists who study DNA in living mammals. Yet the DNA studies do not agree on the timing or place of placental origin, with hypotheses ranging between 140 and 80 million years ago, sometimes in the Northern Hemisphere and sometimes in the Southern. The most recent molecular study, published in late March in Nature, supported the emergence of the major groups of modern placentals 100 million years agoe.

"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."

The catalyst behind this research into the origin of modern placental mammals by Dr. Wible and his co–authors was the discovery of a new mammal, Maelestes gobiensis. Maelestes 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, also known as MAE (the root of the new mammal’s name). Members of the expedition included Guillermo Rougier of University of Louisville and Michael Novacek of American Museum of Natural History, both co–authors of the Nature paper. The final co–author of the paper is Robert Asher of University of Cambridge, United Kingdom.

After its discovery, the fossil was transported to the American Museum of Natural History in New York and later cleaned and prepared by the museum. In 2003, Dr. Wible examined it and made note of its unique dentition and realized it was a new species. This mammal is noteworthy because it is well–preserved and fairly complete, a rarity amongst early mammals. Afterwards, Dr. Wible obtained the specimen on loan from the American Museum of Natural History and brought it to Carnegie Museum of Natural History where he began work in comparing its features to other forms.

“It was the discovery of Maelestes that sparked the research to begin with,” said Dr. Wible. “If Maelestes had not been found, the broader analysis would not have been done.”

Maelestes was shrew–like with a similar diet and locomotion but larger in size. It existed during the time of well-known Late Cretaceous dinosaurs such as Velociraptor, Oviraptor and Protoceratops.

Wible and his co–authors classified the Maelestes as a new eutherian mammal, the broader group that includes placentals and their extinct relatives. To place Maelestes among other mammals, the researchers conducted a phylogenetic analysis, examining 409 morphological features across 69 living and extinct taxa.

The research from this analysis strongly suggests that no member of any living groups of placental mammals reaches back further than 65 million years ago, when the dinosaurs disappeared.

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