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Placental Mammals Originated On Earth 65 Million Years Ago, Researchers Assert

the left is Maelestes gobiensis; on the right is a living hairy-tailed
mole Parascalops breweri from Allegheny County. Maelestes was
comparable in size to the mole (40 to 80 grams), but had a more
shrew-like life style . Note that the skull of Maelestes has more room
for teeth that the mole does, and in fact, Maelestes had even more
teeth as the bone holding the upper incisors is missing. (Credit: John
ScienceDaily (June 21, 2007) — An early mammal fossil discovered in Mongolia 
led to researchers
asserting that the origins of placental mammals, which include humans,
can be dated to approximately 65 million years ago in the Northern
Hemisphere. These findings will be published in the June 21 issue of
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.
Adapted from materials provided by Carnegie Museum of Natural History.
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