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Placental Mammals Climb a New Tree

20 June 2007 (All day)
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John Wible/CMNH

Revising history.
The skull of a newly described extinct mammal, Maelestes gobiensis (left), is comparable in size to a mole's (right).

Did the demise of dinosaurs pave the way for the great diversity of mammals seen today? Many scientists believed so. Then, 3 months ago, an extensive analysis of DNA suggested that mammals only became prominent long after dinosaurs left the scene (ScienceNOW, 28 March). Now researchers studying the teeth, jaws, and other physical features of living and extinct species have challenged these molecular data, offering new insight into when and where the world's most common furry creatures arose and began to diversify.

More than 90% of living mammals nourish their young in the womb. Thus, unraveling the story of the so-called placental mammals is central to understanding mammalian evolution. But pinning down their origin has been rough going. DNA evidence places the first placental mammals anywhere from 140 million to 80 million years ago, during the Cretaceous period. Researchers also disagree about when these early mammals diversified into the ancestors of today's placental groups. Some suggest that rabbit- or ungulate-like placental mammals existed early in the Cretaceous period, whereas other researchers push for a more recent origin, circa 65 million years ago--around the time when dinosaurs disappeared.

The latter theory gets additional support from a new analysis by paleontologist John Wible of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, and colleagues. In 1997, the team unearthed the jaw and parts of the backbone, ribs, skull, and limbs of a shrewlike species, which they named Maelestes gobiensis. By comparing 400 morphological features, such as the shapes and numbers of teeth, in the new fossil with those in 68 other specimens, the researchers have now placed the 73-million-year-old creature in the Eutherian evolutionary tree, an umbrella group that includes placental mammals.

But the new data have forced the team to redraw the tree. According to the new tree, the first placental mammals appeared around 65 million years ago, not 100 million years ago or more, as some molecular data have suggested. What's more, the new tree indicates that these mammals very quickly diversified into the modern groups, close to the downfall of the dinosaurs, Wible and colleagues report in the 21 June issue of Nature. Both molecular and fossil studies have their shortcomings, says Wible, but by analyzing lots of characters in lots of species, he is convinced his tree is trustworthy.

The analysis revises mammalian history in other ways as well. For one, placental mammals do not neatly divide into the four categories that researchers typically group them into. Additionally, the tree indicates that placental mammals arose in the Northern Hemisphere, not in the Southern Hemisphere, as some molecular biologists have argued.

The study indicates that mammalian evolution "is absolutely related to the fall of the dinosaurs," says J. David Archibald, a paleontologist at San Diego State University in California. But Stephen O'Brien, an evolutionary biologist with the National Cancer Institute in Frederick, Maryland, is not convinced the work tells the true story of placental mammals. He criticizes Wible and colleagues for not factoring in the genetic evidence. "I think their data are okay, but their global interpretation and hypotheses are anything but supported [by the data]," he says.

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