Mammals' Molars Worlds Apart?

The specialized teeth that enabled ancient, shrew-like creatures to flourish and gave rise to all modern mammals evolved independently in two animal groups living continents apart, a study suggests.

Scientists said the finding could dramatically alter theories about the pace of early mammals' global advance in the waning days of the dinosaurs more than 65 million years ago.

For the study, three paleontologists analyzed fossilized teeth and bones of mammals that lived between 65 million and 200 million years ago.

By comparing characteristics of the teeth, they concluded that the advanced molars that ensured the small creatures' success evolved in two mammal groups that arose separately on northern and southern continents.

Previous theories had suggested that these specialized tribosphenic molars -- which can simultaneously shear food and crush it in mortar-and-pestle fashion -- had evolved only once, in the Northern Hemisphere.

However, recent finds in Madagascar and Australia cast doubt on that idea, suggesting a southern origin for the dual-function molars.

The latest study was published in Thursday's issue of the journal Nature. It was led by Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh.

The animals' advanced teeth gave them an evolutionary advantage by allowing them to eat a wider range of foods than other more primitive mammals.

Other scientists hailed the new study but cautioned that more fossils are needed to support or refute its conclusions.

"This is a bold hypothesis that takes the dental evidence about as far as it can go," said Leonard Krishtalka, a professor of ecology and evolutionary biology at the University of Kansas.

Luo and two colleagues scrutinized fossils of 21 early mammals ranging in size from shrews to small raccoons.

The scientists said the dual north-south evolution of the advanced tribosphenic teeth occurred after the supercontinent of Pangea split into southern and northern continents about 160 million years ago.

The scientists' revised mammal evolutionary tree suggests that placental and marsupial mammals originated in the northern continents, while three different types of mammals appeared in the southern continents.

Of the three southern lineages, only one remains -- the monotremes, which are represented today by the egg-laying platypus and echidna, the scientists said.

One of the recent fossil finds that challenged mammal evolution theories was a jaw fragment found in Madagascar in the late 1990s that contains three of the advanced molars. It is believed to be 167 million years old, or 25 million years older than all previous tribosphenic fossils.

The co-discoverer of that fossil, John J. Flynn, a curator at the Field Museum of Natural History in Chicago, said the new study is "the first volley of what will be a very interesting debate" over the evolution of mammals.

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