Side view of Maelestes anterior skull and jaw.
Illustration: Paul Bowden/Carnegie Museum of Natural History
The fossilized remains of an extinct, shrew-like animal found in Mongolia have helped pinpoint the origin of most modern mammals, according to new work by Michael J. Novacek from the American Museum of Natural History and colleagues. Their report in the upcoming issue of the journal Nature places the rise of placental mammals, including humans, near the Cretaceous-Tertiary (K/T) boundary 65 million years ago in the Northern Hemisphere. This finding contradicts recent gene studies that put the origin between 129 and 78 million years ago in the Southern Hemisphere.
The fossil was unearthed in 1997 in the Gobi Desert by members of the Mongolian Academy of Sciences-American Museum of Natural History Expeditions (MAE) and later named Maelestes gobiensis. The remains included parts of the animal's skull, lower jaw, backbone, ribs, shoulder blade, collar bone, and legs. From these, the paleontologists were able to identify it as a previously unknown and extinct species of Eutheria, the ancient relatives of modern placental mammals, that lived more than 70 million years ago during the Cretaceous Period.
"This is another of recent discoveries from central Asia that gives us a much better picture of the pattern and timing of the great evolutionary radiation of modern mammals," said Dr. Novacek, Senior Vice President and Provost of the American Museum of Natural History, Curator in the Division of Paleontology, and coleader of the MAE. "For over 140 million years mammals lived alongside dinosaurs, but these were mainly archaic lineages. It wasn't until nonavian dinosaurs went extinct at the end of the Cretaceous that modern mammals, like the placentals, the group to which we belong, started to really diversify."
Placentals make up more than 90 percent of living mammals. The rest are egg-laying monotremes, such as the platypus, and marsupials, such as the kangaroo, that carry their embryo to term outside the womb. The vast majority of mammals that have ever lived are now extinct, and hypotheses describing the timing and nature of the diversification events that led to modern mammal lineages are nearly as numerous as the animals themselves.
Analysis of Maelestes led the MAE team, which also included members from the University of Louisville (Kentucky) and Cambridge University, to compare 408 anatomical features across 69 species, including living mammals and well-preserved early mammal fossils. The genealogical tree of living and extinct mammals they produced traces the ancestry of today's placentals to the K/T boundary. It also supports the idea that the disappearance of non-avian dinosaurs, which occurred at roughly the same time, permitted mammals to diversify explosively over a relatively short period of time.
"Our research gives credence and weight to the traditional paleontology viewpoint of placental mammals appearing 65 million years ago when the dinosaurs died off," said Dr. Wible, lead author on the study. "When dinosaurs became extinct, ecological niches emerged that gave modern placental mammals opportunities to thrive and diversify."
The study was supported by the National Science Foundation, the Carnegie Museum of Natural History, and the American Museum of Natural History.
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